Table of Contents
Kerberos Fundamentals
PAC Privilege Attribute Certificate
1. PAC Structure
2. PAC Credential Information
3. PAC Signature
4. KDC Validates PAC
5. Advantages and Disadvantages of PAC in Kerberos
Kerberos Lab Experiment
AS-REQ & AS-REP
1. AS-REQ Packet Analysis
2. Analysis of AS-REP Response Packets
(1) TGT Call Warrant
(2) Logon Session Key
TGS-REQ & TGS-REP
1. Analysis of TGS-REQ Request Pack
2. Analysis of TGS-REP Response Packet
(1) ST Service Ticket
(2) Service Session Key
How to perform mutual authentication? (AP-REQ & AP-REP)
1. Analysis of the AP-REQ Request Packet
2. Analysis of AP-REP Response Packet
S4u2Self&S4u2Proxy
1. S4u2Self
2. S4u2Proxy
Security Issues with the Kerberos Protocol
This excerpt is from the “Guide to Offensive and Defensive Domain Penetration.”
The Kerberos protocol, developed by the Massachusetts Institute of Technology (MIT), is a network authentication protocol that provides a method for authenticating user identity information over open and unsecured networks. It aims to deliver strong authentication for client/server applications through the use of key encryption technology. The name “Kerberos” is derived from the three-headed dog that guards the gates of the underworld in Western mythology. This name is chosen because Kerberos authentication requires the participation of three parties to complete an authentication process. The most widely used version of Kerberos today is version 2005 RFC 4120.(https://www.rfc-editor.org/rfc/rfc4120.html)The standard-defined version of Kerberos V5, the Kerberos protocol is supported by Windows, Linux, and Mac OS.
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Introduction to Kerberos
In the Kerberos protocol, there are primarily three roles:
- Access Service Client: A Kerberos client is an application that operates on behalf of a user needing access to resources, such as opening files, querying a database, or printing documents. Each Kerberos client requests authentication before accessing resources.
- Provider of Services: In-domain service provider server, each server has a unique SPN.
- Key Distribution Center (KDC) Providing Authentication Services: The KDC is a network service that issues session tickets and temporary session keys to users and computers within an Active Directory domain. Its service account is krbtgt. The KDC operates as part of the Active Directory Domain Services (AD DS) on each domain controller.
Let’s discuss the krbtgt account. This account is automatically created by the system when the Active Directory is set up, and it serves as the service account for the KDC. Its password is randomly generated by the system, making it impossible to log into the host normally. You will often interact with the krbtgt account during domain studies, and additional information about this account will be explained in detail in future articles.
As shown in the image, you can see information about the krbtgt account.
Kerberos is a ticket-based authentication method. When a client wants to access a specific service on the server, it must first acquire a ticket recognized by the server.ST Service TicketIn other words, before the client can access the service, they need to purchase a ticket in advance. Only after this ticket is verified by the service can they gain access. However, this ticket cannot be purchased directly and requires aTicket Granting Ticket (TGT) Subscription WarrantIn other words, the client must first obtain a TGT subscription warrant before purchasing a ticket.TGT Call WarrantandST Service TicketBoth are issued by the KDC (Key Distribution Center); since the KDC runs on the domain controller, the TGT (Ticket Granting Ticket) and the ST (Service Ticket) are both issued by the domain controller.
Kerberos utilizes TCP/UDP port 88 for authentication and TCP/UDP port 464 for password resets. As shown in the illustration, you can observe the open ports 88 and 464 on the domain controller AD01.
The abbreviations and meanings of certain terms in Kerberos are shown in the table below:
|
Acronym |
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|---|---|
|
DC |
Domain Controller, Domain Control |
|
krbtgt |
KDC Key Distribution Center Service Account |
|
KDC |
Key Distribution Center: This is a service responsible for distributing cryptographic keys and is typically handled by the domain controller. |
|
AD |
Active Directory: An active directory that contains the domain user database. |
|
AS |
Authentication Service: Authentication Service |
|
TGT |
Ticket Granting Ticket: TGT, a warrant issued by the Authentication Service (AS) of the Key Distribution Center (KDC). |
|
TGS |
Ticket Granting Service: Ticket Granting Service |
|
ST |
Service Ticket: An ST (Service Ticket) is issued by the Key Distribution Center’s Ticket Granting Service for service provisioning. |
The Kerberos protocol features two fundamental authentication modules:AS_REQ & AS_REP andTGS_REQ & TGS_REP, as well as the two certification modules Microsoft extended: S4U and PAC. S4U is a module Microsoft extended to implement delegation, divided into S4U2Self and S4U2Proxy. In the original design of the Kerberos process, it only explained how to authenticate the client’s true identity but did not address whether the client had permission to access the service. This is because different users with varying permissions can access different resources within a domain. To resolve this permissions issue, Microsoft introduced the concept of PAC (Privilege Attribute Certificate).
Before analyzing AS_REQ & AS_REP and TGS_REQ & TGS_REP, let’s first take a look at what PAC is.
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PAC Privilege Attribute Certificate
PAC (Privilege Attribute Certificate) encompasses various authorization information, additional credential details, configuration files, and policy information, such as user groups to which a user belongs and the permissions the user holds. In the original RFC 1510-defined standard Kerberos authentication process, PAC was not included. Microsoft integrated the concept of PAC into its implementation of the Kerberos process to distinguish users’ identities and permissions because users with different permissions in a domain can access different resources.
In a typical Kerberos authentication process, the Ticket Granting Ticket (TGT) and Service Ticket (ST) issued by the Key Distribution Center (KDC) include a Privilege Attribute Certificate (PAC). The advantage of this approach is that during subsequent resource access, the server, upon receiving a client request, does not need additional assistance from the KDC to obtain comprehensive authorization information to assess user permissions. Instead, it can make decisions based directly on the PAC information included in the request, comparing it with the local resource’s Access Control List (ACL).
1
PAC Structure
The top structure of a PAC is as follows:
typedef unsigned long ULONG;
typedef unsigned short USHORT;
typedef unsigned long64 ULONG64;
typedef unsigned char UCHAR;
typedef struct _PACTYPE {
ULONG cBuffers;
ULONG Version;
PAC_INFO_BUFFER Buffers[1];
} PACTYPE;The definitions of these top fields are as follows:
· cBuffers:Contains the number of entries in the array buffer.
· Version:Version
– Buffers:An array containing a PAC_INFO_BUFFER structure.
As illustrated in the image, it is the PAC structure segment captured by WireShark. You can observe the cBuffers, Version, and Buffers sections:
The PAC_INFO_BUFFER structure contains information about each part of the PAC, and this section is the most important. The structure is as follows:
typedef struct _PAC_INFO_BUFFER { ULONG ulType; ULONG cbBufferSize; ULONG64 Offset;} PAC_INFO_BUFFER;The definition of the type field is as follows:
– ulType:Type of data contained within this buffer. It may be one of the following:
· Logon Info (1)
– Client Information Type (10)
· UPN DNS Info (12)
· Sserver Cechksum (6)
· Privsvr Cechksum (7)
– `cbBufferSize`:Buffer Size
· Offset:Buffer Offset
As shown in the diagram, this is the PAC_INFO_BUFFER structure part captured by WireShark.
2
PAC Credential Information
The PAC_LOGON_INFO of type LOGON INFO includes the credential data for the Kerberos ticket client. The data itself is contained within a KERB_VALIDATION_INFO structure, which is encoded using NDR. The output of the NDR encoding is placed into a PAC_INFO_BUFFER structure of type LOGON INFO. As follows:
typedef struct _KERB_VALIDATION_INFO {
FILETIME Reserved0;
FILETIME Reserved1;
FILETIME KickOffTime;
FILETIME Reserved2;
FILETIME Reserved3;
FILETIME Reserved4;
UNICODE_STRING Reserved5;
UNICODE_STRING Reserved6;
UNICODE_STRING Reserved7;
UNICODE_STRING Reserved8;
UNICODE_STRING Reserved9;
UNICODE_STRING Reserved10;
USHORT Reserved11;
USHORT Reserved12;
ULONG UserId;
ULONG PrimaryGroupId;
ULONG GroupCount;
[size_is(GroupCount)] PGROUP_MEMBERSHIP GroupIds;
ULONG UserFlags;
ULONG Reserved13[4];
UNICODE_STRING Reserved14;
UNICODE_STRING Reserved15;
PSID LogonDomainId;
ULONG Reserved16[2];
ULONG Reserved17;
ULONG Reserved18[7];
ULONG SidCount;
[size_is(SidCount)] PKERB_SID_AND_ATTRIBUTES ExtraSids;
PSID ResourceGroupDomainSid;
ULONG ResourceGroupCount;
[size_is(ResourceGroupCount)] PGROUP_MEMBERSHIP ResourceGroupIds;
} KERB_VALIDATION_INFO;Primarily focus on the following fields:
· Account Name:The value for this field corresponds to the user’s sAMAccountName attribute.
– Full Name:The value for this field corresponds to the user’s displayName attribute.
I’m sorry, but it seems there was no text provided for translation. Please provide the text content from your WordPress post, and I’ll be glad to assist you with the translation while maintaining the original formatting.The value in this field corresponds to the user’s RID, which is the last part of the user’s SID.
I’m here to help with translating content while keeping the structure intact, but it looks like there’s no plain text content provided for translation. Please include the text you need translated, and I’ll ensure the HTML and formatting remain unaltered.For this field, the Group RID for domain users is consistently 513 (which corresponds to the Domain Users RID), the Group RID for machine users is consistently 515 (which corresponds to the Domain Computers RID), and the Group RID for domain controllers is consistently 516 (which corresponds to the Domain Controllers RID).
· Number of RIDS:Number of groups the user belongs to
· GroupIDS:RID of all groups to which the user belongs.
As shown in the image, this is the PAC_LOGON_INFO section captured using WireShark.
3
PAC Signature
The PAC contains two digital signatures:PAC_SERVER_CHECKSUM and PAC_PRIVSVR_CHECKSUM. PAC_SERVER_CHECKSUM is signed using the service key, while PAC_PRIVSVR_CHECKSUM is signed using the KDC key. The signatures serve two purposes. First, there is a signature with the service key to verify that this PAC was signed by the service. Second, the signature with the KDC key prevents untrusted services from forging tickets for themselves with an invalid PAC.
These two signatures are sent as PAC_INFO_BUFFER types, namely PAC_SERVER_CHECKSUM and PAC_PRIVSVR_CHECKSUM. Before the PAC data is used for access control, the PAC_SERVER_CHECKSUM signature must be checked. This will verify whether the client knows the service’s key. The verification of the PAC_PRIVSVR_CHECKSUM signature is optional and disabled by default. It is used to verify whether the PAC was issued by the KDC and not inserted into the ticket by someone with access to the service key outside of the KDC.
The signature is included in the structure below:
typedef struct _PAC_SIGNATURE_DATA {
ULONG SignatureType;
UCHAR Signature[1];
} PAC_SIGNATURE_DATA, *PPAC_SIGNATURE_DATA;These fields are defined as follows:
· SignatureType:This field contains the type of checksum used to create the signature, which must be a keyed checksum.
· Signature:This field consists of a byte array containing checksum data. The length of the byte can be determined by wrapping the PAC_INFO_BUFFER structure.
As shown in the image, this is the signature section in the PAC, where you can see PAC_SERVER_CHECKSUM and PAC_PRIVSVR_CHECKSUM.
4
Kerberos Distribution Center (KDC) Verifies Privilege Attribute Certificate (PAC)
We have noticed there are two signatures within the PAC: PAC_SERVER_CHECKSUM and PAC_PRIVSVR_CHECKSUM. One is signed using the service key (PAC_SERVER_CHECKSUM), while the other is signed with the KDC key (PAC_PRIVSVR_CHECKSUM). When the server receives an AP-REQ message from the client, it can only verify the PAC_SERVER_CHECKSUM signature and is unable to verify the PAC_PRIVSVR_CHECKSUM signature. Therefore, normally, if the PAC_PRIVSVR_CHECKSUM signature needs to be verified, the server would need to send the PAC signature from the Service Ticket (ST) received from the client to the KDC for verification.
However, because most services by default do not perform KDC validation of the PAC (they require target service hosts to be configured to verify the KDC PAC signature, which is disabled by default), the server does not need to send the PAC signature in the ST service ticket for KDC verification. This is a precondition for the success of the Silver Ticket attack. If there is a configuration requiring verification of the PAC_PRIVSVR_CHECKSUM signature, the server will send the digital signature of this PAC to the KDC via an RPC protocol message, KRB_VERIFY_PAC. The KDC will then return the result of verifying this digital signature of the PAC as an RPC return code. The server can thereby determine the authenticity and validity of the PAC based on this return result. Therefore, if the target service host is configured to verify the PAC_PRIVSVR_CHECKSUM signature, even if the attacker possesses the service key and can create an ST service ticket, they cannot forge the KDC’s PAC_PRIVSVR_CHECKSUM signature, and naturally, they cannot pass the KDC’s signature verification.
So how do you configure the service host to enable KDC signature verification? According to the Microsoft official documentation, the following conditions are required to enable KDC PAC verification:
· The application has SeTcbPrivilege rights.The SeTcbPrivilege permission allows a user account to be assigned the “Act as Part of the Operating System” privilege. Local System, Network Service, and Local Service accounts are service user accounts defined by Windows. Each account has a specific set of privileges.
· The application is a service with the registry key for validating KDC PAC signature set to 1, default is 0.Modifications are as follows:
1. Launch the Registry Editor regedit.exe
2. Locate the following subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\Kerberos\Parameters
3. Add a key value named ValidateKdcPacSignature (of DWORD type). When the value is 0, KDC PAC validation will not be performed. When the value is 1, KDC PAC validation will be performed. Therefore, you can set this value to 1 to enable KDC PAC validation.
Regarding the registry key for verifying KDC PAC signatures, consider the following points:
· If the server is not a service program but a regular application, it will not be affected by the above registry and will always perform KDC PAC validation.
· If the server-side is not a program but a driver, with its authentication process completed within the system kernel, it will not be affected by the aforementioned registry and will never perform PAC validation.
· To use the above registry key, you need Windows Server 2003 SP2 or a newer operating system.
· On servers running Windows Server 2008 or later operating systems, the value of this registry key defaults to 0 (with the ValidateKdcPacSignature key value not existing by default), meaning that KDC PAC validation is not performed.
Note:It is important to note that services registered under the local system account will not trigger the KDC to verify the PAC signature, regardless of the configuration. This means that services like SMB, CIFS, and HOST, among others, will not prompt the KDC to verify the PAC signature under any circumstances.
So why, by default, does the KDC not verify the PAC’s signature?
Executing KDC verification of PAC entails costs in terms of response time and bandwidth usage. It requires bandwidth to transmit requests and responses between the application server and the KDC. This could lead to some performance issues in high-volume application servers. In such environments, user authentication might result in additional network latency and significant traffic. Therefore, by default, the KDC does not verify PAC signatures.
5
PAC in Kerberos: Advantages and Disadvantages
Advantages:
1. Enhanced Security: The Privilege Attribute Certificate (PAC) contains detailed information about user permissions and group memberships, enhancing security by ensuring proper access control within Kerberos authentication.
2. Efficient Authorization: By carrying extensive authorization data, the PAC enables efficient access decisions tailored to the specific user context, reducing reliance on separate directory lookups.
3. Interoperability: It facilitates seamless integration with Microsoft environments, where PACs are a critical component of user authorization processes.
Disadvantages:
1. Increased Ticket Size: The inclusion of PACs increases the size of Kerberos tickets, potentially leading to performance issues if network infrastructure or systems are not designed to handle larger authentication tokens.
2. Complexity: Managing the data within PACs can add an additional layer of complexity to the Kerberos authentication process and require robust oversight and maintenance.
3. Potential Compatibility Issues: Systems that do not natively support PACs may face compatibility challenges, requiring additional configuration or middleware to ensure proper function.
What advantages or disadvantages does the presence of PAC bring to our verification process?
As mentioned above, the introduction of PAC actually brings numerous advantages. When the client accesses network resources, the server no longer needs to query the KDC for authorization information. Instead, it directly compares PAC information with ACL locally, thereby conserving network resources.
As illustrated in the image, without a PAC, the Server must engage in querying and returning user authorization information with the KDC:
After introducing the PAC, it transforms into what is shown in the image:
However, the introduction of PAC is not without disadvantages. The introduction of PAC during the user authentication phase will cause the authentication to take too long. The Windows Kerberos client will verify the PAC information by calling the function on the KDC through RPC. At this time, the user will observe an increase in the RPC packet traffic between the server and the KDC. On the other hand, since PAC is a Microsoft-specific feature, servers with other operating systems will not be supported in a domain with PAC enabled, which restricts the flexibility of domain configuration. In addition, in 2014, due to the security of PAC, an extremely serious privilege escalation vulnerability MS14-068 was generated in the domain (we will introduce this vulnerability in the following article).
Kerberos Experiment
In order to analyze the Kerberos protocol more intuitively, we will use the standard domain account xie/hack and Impacket tool to request the CIFS service ticket from a Windows 10 machine. Then, we will perform a remote SMB connection, capturing the process with WireShark.
Experimental environment as follows:
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Domain Host (Win10): 10.211.55.16
Domain Controller (AD01): 10.211.55.4
The command for using impacket is as follows:
# Requesting an ST service ticket for the CIFS service on Windows 10 using the hack account credentials
python3 getST.py -dc-ip 10.211.55.4 -spn cifs/win10.xie.com xie.com/hack:P@ss1234
# Import the ST service ticket
export KRB5CCNAME=hack.ccache
# Remotely connect to Windows 10 using SMB
python3 smbexec.py -no-pass -k win10.xie.comAs shown in the image, after requesting the service ticket, a successful remote SMB connection was established with the Windows 10 machine.
In this process, we use Wireshark to capture packets, enabling further analysis of the Kerberos protocol. As shown in the image, this is the packet capture diagram for this process:
The entire Kerberos authentication process is illustrated as follows:
Let’s specifically analyze each step of the Kerberos authentication process:
Let’s analyze each step of the Kerberos authentication process in detail:
AS-REQ & AS-REP
Let’s first take a look at the AS-REQ & AS-REP request section, which corresponds to the first and second packets captured by Wireshark, as shown in the figure:
As shown in the diagram, this is a simplified process chart for AS-REQ and AS-REP requests, allowing us to intuitively understand the AS-REQ and AS-REP request process.
Let’s specifically analyze the details of the packets during the AS-REQ and AS-REP process:
First, let’s examine how the client obtains a TGT subscription warrant. The TGT subscription warrant is issued by the AS (Authentication Service) of the KDC.
1
AS-REQ Packet Analysis
AS-REQ:When a domain user wants to access a service within the domain, they input their username and password. The local machine sends an AS-REQ authentication request to the KDC’s Authentication Service (AS). This request packet contains the following information:
– Requested Username (cname).
· Domain (realm).
· Authenticator:An abstract concept representing an authentication. Here is the timestamp for user key encryption.
· Requested Service Name (sname):During the AS-REQ phase, the requested service is always krbtgt.
· Encryption type (etype).
· And some additional information:Version number, message type, ticket validity period, whether it includes PAC, negotiation options, etc.
As shown in the image, it is the detailed AS-REQ request packet:
Below, let’s take a look at the meaning of each field in the AS-REQ request packet, as follows:
pvno: Kerberos version number, here it is 5
msg-type: Message type, AS_REQ corresponds to krb-as-req (10)
padata: Primarily authentication information, each auth message has a type and a value.
PA-DATA PA-ENC-TIMESTAMP: This is pre-authentication, where a timestamp is encrypted with the user's hash and sent as a value to the KDC's AS service. Then, the KDC queries the directory for the user's hash, decrypts the timestamp, and if it can decrypt and the timestamp is within a certain range, it authenticates successfully. Because the timestamp is encrypted with the user's password hash, it leads to pass-the-hash attacks.
padata-type: padata type, here it is pA-ENC-TIMESTAMP (2)
padata-value: Encrypted value
etype: Encryption type, here it is eTYPE-AES256-CTS-HMAC-SHA1-96 (18)
cipher: Key
PA-DATA PA-PAC-REQUEST: Extension for enabling PAC support. The value is True or False, and the KDC determines whether to include the PAC in the returned ticket based on this.
padata-type: padata type, here it is pA-PAC-REQUEST (128)
padata-value: padata value
include-pac: Whether PAC is included, here it is True, indicating PAC is needed
req-body: Request body
padding: padding, here it is 0
kdc-options: Options set for negotiation with the KDC
reserved
forwardable
forwarded
proxiable
proxy
allow-postdate
postdated
unused7
renewable
unused9
unused10
opt-hardware-auth
unused12
unused13
constrained-delegation
canonicalize
request-anonymous
unused17
unused18
unused19
unused20
unused21
unused22
unused23
unused24
unused25
disable-transited-check
renewable-ok
enc-tkt-in-skey
unused29
renew
validate
cname: The requested username; its presence or absence results in differences in the response packet, which can be used to enumerate domain usernames. Also, when the username is correct or incorrect, the response packet differs, allowing for password spraying.
name-type: Name type, here it is KRB5-NT-PRINCIPAL (1)
cname-string: Name, which is the requested username
CNameString: The requested username, here it is hack
realm: Domain name, here it is XIE.COM
sname: Requested service, includes type and value. In AS-REQ, sname is always krbtgt
name-type: Name type, here it is KRB5-NT-PRINCIPAL (1)
sname-string: krbtgt user information, consists of 2 items
SNameString: This is the krbtgt username
SNameString: This is the domain name XIE.COM
till: Expiry time
rtime: Also the expiry time
nonce: A randomly generated number
etype: Encryption type
ENCTYPE: eTYPE-AES256-CTS-HMAC-SHA1-96 (18) Let’s focus on the PA-DATA PA-ENC-TIMESTAMP field, which is used for pre-authentication.
In the AS-REQ request packet, only the PA-DATA PA-ENC-TIMESTAMP section is encrypted, which constitutes pre-authentication. We refer to this section as the Authenticator.
As shown in the figure, in impacket/krb5/kerberosv5.py, you can see the use of the user’s password hash or the user’s password AES key to encrypt the timestamp.
To decrypt the traffic captured by WireShark, as shown in the image, you can see that the key of the hack user is used to decrypt this value. The following patimestamp and pauses are the decrypted values.
2
AS-REP Response Packet Analysis
AS-REP:When the KDC’s AS authentication service receives the AS-REQ request from the client, it retrieves the user’s key from the Active Directory database. Then, it decrypts the Authenticator pre-authentication part of the request packet using this key. If the decryption is successful and the timestamp is within the valid range, it proves that the requester has provided the correct user key.
After successfully authenticating the client’s identity, the KDC’s AS authentication service sends an AS-REP response packet to the client. The AS-REP response packet mainly includes the following information:
– Requested username (cname).
· Domain (crealm).
– TGT Call Warrants:Contains plain text version numbers, domain names, service names being requested, and the encrypted portion (enc-part). The encrypted part is encoded with the krbtgt key. It includes the Logon Session Key, username, domain name, authentication time, expiration time, and authorization-data. Within the authorization-data, the most crucial PAC (Privilege Attribute Certificate) is included, which contains the user’s RID and the RIDs of the groups the user belongs to.
· enc_Logon Session Key:The value obtained by encrypting the Logon Session Key with the user key is used to ensure secure communication between the client and the KDC in subsequent phases. This refers to the outermost enc-part in the AS-REP.
· And some additional information:Such as version numbers, message types, etc.
As shown in the figure, this is the AS-REP response packet details:
Let’s take a look at the significance of each field in the AS-REP response packet, as follows:
pvno: Kerberos version number, here it's 5
msg-type: Message type, AS_REP corresponds to krb-as-rep(11)
padata: Primarily authentication information. It's a list containing several authentication messages used for verification
PA-DATA PA-ENCTYPE-INFO2
padata-type: Type of padata, here it's pA-ETYPE-INFO2(19)
padata-value: Encrypted value
ETYPE-INFO2-ENTRY
etype: eTYPE-AES256-CTS-HMAC-SHA1-96(18)
salt: Salt value, here it's XIE.COMhack
crealm: Realm, here it's XIE.COM
cname: Requested username
name-type: Username type, here it’s kRB5-NT-PRINCIPAL(1)
cname-string: 1 item
CNameString: hack
ticket: TGT subscription voucher
tkt-vno: TGT version number, here it's 5
realm: Realm, here it's XIE.COM
sname: Service username, here it's the krbtgt password distribution center service account
name-type: KRB5-NT-SRV-INST(2)
sname-string: 2 items
SNameString: krbtgt
SNameString: XIE.COM
enc-part: Encrypted part of the TGT ticket, which is encrypted with the password hash of krbtgt. Therefore, if we have the hash of krbtgt, we can forge a ticket ourselves, leading to a golden ticket attack
etype: Encryption type, here it's eTYPE-AES256-CTS-HMAC-SHA1-96(18)
kvno: Version number, here it's 2
cipher: Encrypted value
enc-part: Login session key, this part is encrypted with the hash of the requested user's password, serving as the authentication key for the next stage.
etype: eTYPE-AES256-CTS-HMAC-SHA1-96(18)
kvno: Version number, here it's 3
cipher: Encrypted value In the AS-REP response packet, the most important elements are the TGT (Ticket Granting Ticket) and the encrypted Logon Session Key. The encrypted part of the TGT is secured using the krbtgt key, while the Logon Session Key is encrypted using the requesting user’s key.
Now, let’s decrypt WireShark to see exactly what is contained within the TGT warrants and the Logon Session Key.
1
TGT Call Warrant
The ticket in the AS-REP response packet is essentially the TGT (Ticket Granting Ticket). The TGT contains some information displayed in plaintext, such as the version number (tkt-vno), the realm, and the requested service name (sname). However, the most crucial part of the TGT is the encrypted section, which is encrypted using the krbtgt account’s key. This encrypted section primarily includes details like the Logon Session Key, the requesting user’s name (cname), the realm (crealm), the authentication time (authtime), the expiration time (endtime), and authorization data, among other information.
The most important part is the authorization-data section, which includes client identity and permissions information. This information is contained within the PAC.
As shown in the image, this is the TGT call warrant:
Let’s take a look at what the PAC, which represents user identity permissions under the authorization-data field in TGT subscription warrants, looks like. We will decrypt the PAC and examine only the credential information portion, PAC_LOGON_INFO.
As shown in the image, the primary method for distinguishing user permissions is through the User RID and Group RID.
The process by which the KDC generates a PAC is as follows:After the Key Distribution Center (KDC) receives the AS-REQ request from the client, it extracts the `cname` field from the request. The KDC then queries the Active Directory database to locate the user whose `sAMAccountName` attribute matches the value of the `cname` field. Using the identity of this user, a corresponding Privilege Attribute Certificate (PAC) is generated.
2
Logon Session Key
The outermost part of the AS-REP response packet is the encrypted Login session Key, which serves to ensure secure communication between the client and the KDC in the next phase. It is encrypted with the requested user’s key.
We decrypt the outermost enc-part section as shown in the diagram, and you can see that it is decrypted using the hack user’s key.
Decryption results are as follows:The primary contents include the authentication time `authtime`, the authentication expiration time `endtime`, the domain name `srealm`, the requested service name `sname`, negotiation flags `flags`, and other information. It is important to note that the TGT (Ticket Granting Ticket) also contains the Logon Session Key.
Five
TGS-REQ&TGS-REP
Let’s take another look at the TGS-REQ & TGS-REP request sections, which are the third and fourth packets captured by WireShark, as shown in the image:
As shown in the figure, it is a simple TGS-REQ & TGS-REP request process diagram, which allows us to intuitively understand the TGS-REQ & TGS-REP request process.
Let’s delve into a detailed analysis of the packet specifics in the TGS-REQ & TGS-REP process:
After the client receives the AS-REP response from the KDC, it uses the user’s key to decrypt the enc_Logon Session Key (which is the outermost enc-part) to obtain the Logon Session Key, and it also acquires the TGT credential. Subsequently, the client caches this TGT credential and Logon Session Key locally. Now, the client needs to use the TGT credential to request the corresponding Service Ticket (ST) from the KDC. The Service Ticket is issued by another KDC service called the TGS (Ticket Granting Service). In this phase, Microsoft introduces two extended sub-protocols, S4u2self and S4u2Proxy (which are utilized during delegation; further details will be provided in Section 4.5).
1
Analysis of TGS-REQ Request Packet
TGS-REQ:The client, holding the TGT (Ticket Granting Ticket) acquired in the previous step, sends a TGS-REQ request to the KDC (Key Distribution Center) to obtain a Service Ticket (ST) for the specified service. This request primarily includes the following information:
· Domain (realm).
· Requested service name (sname).
– TGT Call Warrant.
· Authenticator:An abstract concept that represents an authentication. Here, a timestamp encrypted with the Logon Session Key is used.
· Encryption Type (etype).
– And some other information:Such as version number, message type, negotiation options, token expiration time, etc.
As shown in the image, here are the details of the TGS-REQ request packet:
Let’s take a look at the meaning of each field in the TGS-REQ request packet, as shown below:
Your text primarily discusses the structure and components of a Kerberos Ticket Granting Service Request (TGS-REQ) message. Here's the translation for the relevant parts:
```plaintext
pvno: Kerberos version number, it is 5 here
msg-type: Message type, TGS_REQ corresponds to krb-tgs-req(12)
padata: padata contains ap_req, which is mandatory for TGS_REQ. This section carries the Ticket Granting Ticket (TGT) obtained in the AS_REP and a timestamp encrypted with the original Logon Session Key. There might also be PA_FOR_USER, type S4U2SELF, which is a unique identifier indicating the user's identity, made up of a username and domain name. S4U2Proxy must extend the PA_FOR_USER structure, specifying that a service is requesting a Kerberos service ticket on behalf of a user for the service itself. There might also be PA_PAC_OPTIONS, type PA_PAC_OPTIONS. S4U2Proxy must extend the PA-PAC-OPTIONS structure. If it's Resource-based Constrained Delegation, the Resource-based Constrained Delegation bit needs to be specified.
padata-type: The type of padata, here it is pA-TGS-REQ(1)
padata-value: The value of padata
ap-req: This is a mandatory part of TGS_REQ
pvn0: 5
msg-type: krb-ap-req(14)
padding: 0
ap-options: 00000000
reserved: False
use-session-key: False
mutual-required: False
ticket: TGT obtained in the AS-REP response packet
tkt-vno: 5
realm: XIE.COM
sname
name-type: kRB5-NT-SRV-PRINCIPAL(1)
sname-string: 2 items
SNameString: krbtgt
SNameString: XIE.COM
enc-part
etype: eTYPE-AES256-CTS-HMAC-SHA1-96 (18)
kvno: 2
cipher: Encrypted value
authenticator: Timestamp encrypted with the original Logon Session Key to ensure session security
etype: eTYPE-AES256-CTS-HMAC-SHA1-96 (18)
cipher: Encrypted value
req-body: Request body
padding: Here it is 0
kdc-options: Options agreed upon with the KDC
reserved
forwardable
forwarded
proxiable
proxy
allow-postdate
postdated
unused7
renewable
unused9
unused10
opt-hardware-auth
unused12
unused13
constrained-delegation
canonicalize
request-anonymous
unused17
unused18
unused19
unused20
unused21
unused22
unused23
unused24
unused25
disable-transited-check
renewable-ok
enc-tkt-in-skey
unused29
renew
validate
realm: Domain name, here it is XIE.COM
sname: Service name to be requested
name-type: KRB5-NT-SRV-INST(2)
sname-string: 2 items
SNameString: cifs
SNameString: win10.xie.com
till: Expiration time, rubeus and kekeo use 20370913024805Z, this can be used as a signature to detect the tool.
nonce: A randomly generated number.
etype: Encryption type
ENCTYPE: eTYPE-ARCFOUR-HMAC-MD5(23)
ENCTYPE: eTYPE-DES3-CBC-SHA1(16)
ENCTYPE: eTYPE-DES-CBC-MD5 (3)
ENCTYPE: eTYPE-AES256-CTS-HMAC-SHA1-96(18)
```
This gives an overview of what elements are typically included in a TGS_REQ message, using Kerberos protocol-specific terminology and structures.Let’s focus on the authenticator field in the ap-req, which is primarily used for session security authentication in the later stages.
To ensure the subsequent session security, the value of the authenticator field in the TGS-REQ’s ap-req is encrypted using the timestamp with the Logon Session Key returned in the previous AS-REP, as shown in the diagram:
As shown in the image, in `impacket/krb5/kerberosv5.py`, you can see that the timestamp is encrypted using the Logon Session Key with the following encryption method.
2
Analysis of TGS-REP Response Packet
TGS-REP:When the TGS service of the KDC receives a TGS-REQ request, it first decrypts the encrypted part of the TGT using the krbtgt key to obtain information such as the Logon Session key and PAC. If decryption is successful, it indicates that the TGT was issued by the KDC. It then verifies the signature of the PAC; if the signature is correct, it proves the PAC has not been tampered with. Next, the Logon Session Key is used to decrypt the Authenticator to obtain timestamp information. If successful decryption occurs and the ticket time is within range, it verifies the security of the session. After completing these checks, the TGS service of the KDC has authenticated the client and sends a response packet back to the client. This response packet primarily includes the following details:
· Requested Username (cname)
· Domain Name (crealm)
– ST Service Ticket:Includes plaintext version numbers, domain names, requested service name, and an encrypted section called `enc-part`, which is encrypted using the service key. The encrypted section contains the username, domain name, authentication time, expiration time, Service Session key, and authorization data. The authorization data includes the crucial Privilege Attribute Certificate (PAC) that contains the user’s RID and the RIDs of groups to which the user belongs, among other details.
· enc_Service Session key:The Service Session key encrypted with the Logon Session key serves to ensure secure communication between the client and the KDC in the next stage.
· And some additional information:Version number, message type, etc.
Note:It should be noted here that in the TGS-REP step, the KDC does not verify whether the client has permission to access the server. Therefore, regardless of whether the user has access permissions, as long as the TGT is correct, the ST service ticket will be returned. This is the reason why kerberoasting can be exploited; any domain user can request the ST service ticket for any service within the domain.
As shown in the image, here is the detailed information of the TGS-REP reply packet:
Now let’s take a look at the meaning of each field in the TGS-REP response packet, as follows:
pvno: The Kerberos version number, here it is 5
msg-type: Message type, TGS_REP corresponds to krb-tgs-rep (13)
crealm: Realm, here it is XIE.COM
cname: The requested username
name-type: Name type, here it is KRB5-NT-PRINCIPAL (1)
cname-string: 1 item
CNameString: hack
ticket: This is the Service Ticket (ST)
tkt-vno: Service Ticket version number, here it is 5
realm: Realm, here it is XIE.COM
sname:
name-type: KRB5-NT-SRV-HST (3)
sname-string: 2 items
SNameString: cifs
SNameString: win10.xie.com
enc-part: This part is encrypted with the service's key
etype: Encryption type, eTYPE-AES256-CTS-HMAC-SHA1-96 (18)
kvno: Version number, here it is 3
cipher: Encrypted value
enc-part: This part is encrypted with the original Logon Session Key. The most important field here is the Service session key, which serves as the authentication key for the next phase.
etype: Encryption type eTYPE-AES256-CTS-HMAC-SHA1-96 (18)
cipher: Encrypted valueThe most important elements in the TGS-REP return package are the ST service ticket and the Service Session key. The encrypted portion of the ST service ticket is encrypted using the service key, while the Service Session key is encrypted using the Logon Session Key.
Below, we will use Wireshark decryption to examine what exactly is contained in the ST service ticket and the Service Session key.
1
ST service tickets
The ticket within the TGS-REP response package is the Service Ticket (ST). The ST contains information displayed in plaintext, such as the version number (tkt-vno), realm name, and service name requested (sname). However, the most critical part of the ST is the encrypted segment, which is encrypted using the service key. The encrypted segment primarily includes the Server Session Key, the client’s username (cname), the client realm (crealm), the authentication time (authtime), the ticket expiration time (endtime), and authorization data information. The authorization-data is particularly important because it contains the client’s identity and access rights, encapsulated within the Privilege Attribute Certificate (PAC).
As shown in the image, this is an ST service ticket:
Let’s examine what the PAC, which represents user identity permissions in the authorization-data field of the ST service ticket, looks like. We will decrypt the PAC and only view the credential information part, PAC_LOGON_INFO.
As illustrated, the primary method of identifying user privileges is through the User RID and Group RID. It can be observed that the PAC in the ST service ticket is consistent with the PAC in the TGT subscription token. In a typical TGS process that does not involve an S4u2Self request, the KDC directly copies the PAC from the TGT ticket to the ST service ticket.
2
Service Session Key
The outermost part of the TGS-REP response packet is the Service Session Key. Its purpose is to ensure secure communication between the client and the KDC in the next phase, and it is encrypted using the Logon Session Key.
As shown in the illustration, decrypt it to reveal the main contents, which include the authentication time (authtime), authentication expiration time (endtime), domain name (srealm), requested service name (sname), negotiation flags (flags), among other information. It’s important to note that the ST service ticket also contains the Service Session Key.
Six
How to Implement Two-Factor Authentication?
(AP-REQ&AP-REP)
After the client receives the TGS-REP message from the KDC and extracts the ST service ticket, it is ready to begin requesting access to the service.
Since we are connecting remotely via the SMB protocol, the AP-REQ & AP-REP messages are embedded within the SMB protocol. As shown in the figure:
Note:The default remote connection through Impacket does not require authenticating the service’s server, so there’s no AP-REP reply here.
As illustrated in the figure, this is a simplified diagram of the AP-REQ & AP-REP request process, which helps us intuitively understand the AP-REQ & AP-REP request sequence.
1
AP-REQ Request Packet Analysis
AP-REQ:After the client receives the KDC’s TGS reply, it decrypts the enc_Service Session key with the cached Logon Session Key to obtain the Service Session Key. At the same time, it also receives the ST (Service Ticket). The Service Session Key and the ST service ticket will be cached by the client.
When a client accesses a specified service, an AP-REQ request is initiated, which primarily includes the following content:
– ST Service Ticket
· Authenticator:An abstract concept representing an authentication. Here it refers to the timestamp encrypted with the Service Session Key.
· And some other information:Such as version number, message type, negotiation options, etc.
As shown in the diagram, here are the details of the AP-REQ request packet:
Below, let’s examine the meaning of each field in the AP-REQ message packet, as shown:
pvno: Kerberos Version Number, here it is 5
msg-type: Message Type, AP_REQ corresponds to krb-ap-req (14)
Padding: Padding, here it is 0
ap-options: Some negotiation options
reserved
use-session-key
mutual-required: This option indicates whether the client wishes to authenticate the service-providing server
ticket: ST Service Ticket
tkt-vno: Version Number, here it is 5
realm: XIE.COM
sname: Service Name
name-type: KRB5-NT-SRV-INST(2)
sname-string: 2 items
SNameString: cifs
SNameString: win10.xie.com
enc-part: Encrypted part of the ST Service Ticket
etype: Encryption Type, eTYPE-AES256-CTS-HMAC-SHA1-96(18)
kvno: Version Number, here it is 2
cipher: Encrypted Value
authenticator: Encrypted timestamp using the Service Session Key
etype: Encryption Type
cipher: Encrypted Value 2
AP-REP response packet analysis
AP-REP:This step is optional. When the client desires to verify the service provided by the server (i.e., the mutual-required negotiation option in the AP-REQ request is True), the server returns an AP-REP message. Upon receiving the AP-REQ message from the client, the server decrypts the ST service ticket using the service key to obtain the Service Session Key and PAC information, among other details. Then, it uses the Service Session Key to decrypt the Authenticator to get the timestamp. If successful decryption is achieved and the timestamp is within the valid range, the client’s identity is confirmed. After confirming the client’s identity, the server extracts data from the PAC in the ST service ticket, representing the user’s identity and permission information, and compares it to the requested service ACL to generate the appropriate access token. Additionally, the server will check if the mutual-required negotiation option in the AP-REQ request is True, indicating the client wants to verify the server’s identity. At this point, the server encrypts the timestamp using the Service Session Key as the Authenticator and sends it in the AP-REP response packet for client verification.
If the mutual-required option is set to False, the server will decide whether to return the corresponding services to the client based on the permissions of the access token.
Note:Due to Impacket’s default setting not requiring server identity verification, as shown in the image, here is a screenshot of the AP-REP response packet from other request methods.
The AP-REP response packet primarily includes the following information:
· Version Number
· Message Type
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Below are the details of the AP-REP reply packet:
5
Message type, AP_REP corresponds to krb-ap-rep(15)
Service Session Key encrypted timestamp
Encryption type
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S4u2Self&S4u2Proxy
To support constrained delegation at the Kerberos protocol layer, Microsoft extended the Kerberos protocol with two sub-protocols: S4u2self (Service for User to Self) and S4u2Proxy (Service for User to Proxy). S4u2self allows a service to request a service ticket for itself on behalf of any user; S4u2Proxy uses the ST service ticket obtained in the previous step to request ST service tickets for other specified services on behalf of the user.
Execute the following command, machine$ to simulate the machine user accessing its own services as an administrator.
python3 getST.py -dc-ip AD01.xie.com xie.com/machine\$:root -spn cifs/ad01.xie.com -impersonate administrator As shown in the image, the machine account `machine$` uses the S4u2Self protocol to impersonate the `administrator` identity to access its own services.。
1
S4u2Self
Compared to a standard TGS-REQ request packet, the TGS-REQ request packet of the S4u2Self protocol includes an additional PA-DATA, specifically a pA-TGS-USER, where the name is set to the user to be impersonated.
And the sname is also the service itself that is being requested. As shown in the figure:
2
S4u2Proxy
Compared to a normal TGS-REQ request packet, the TGS-REQ request packet of the S4u2Proxy protocol includes an additional-tickets field. The content of this field is the ST service ticket acquired in the previous step using the S4u2Self request. As shown in the figure:
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Security Issues with the Kerberos Protocol
The Kerberos protocol, although widely used for network authentication, is not immune to certain security issues. Understanding these weaknesses is crucial for implementing robust security measures. Here are some of the main security concerns:
1. Password Guessing Attacks: Since Kerberos relies heavily on shared secrets, it is susceptible to password guessing attacks. An attacker who obtains encrypted credentials can attempt to crack them by guessing the password.
2. Replay Attacks: Kerberos is vulnerable to replay attacks if timestamps are not properly synchronized between the client and the server. An attacker could capture a valid ticket and reuse it.
3. Single Point of Failure: The Key Distribution Center (KDC) in Kerberos is a single point of failure. If it is compromised, the entire authentication framework can be undermined.
4. Credential Forwarding Issues: While credential forwarding in Kerberos can provide flexibility, it also increases the risk if an attacker gains access to the forwarded credentials.
5. Clock Synchronization: Accurate clock synchronization between clients and servers is critical. Without it, legitimate authentication requests might be denied, or security might be compromised.
6. Man-in-the-Middle Attacks: While Kerberos isn’t vulnerable to traditional man-in-the-middle attacks due to mutual authentication, if encryption keys are weak or improperly managed, the risk increases.
Mitigating these risks involves enforcing strong passwords, ensuring proper synchronization of system clocks, and setting up redundant KDCs to prevent single points of failure. Additionally, regular security audits and the implementation of multifactor authentication can enhance the security of the Kerberos protocol.
As shown in the diagram, these are the security issues that can arise in various stages of the Kerberos protocol:
In the AS-REQ request phase, the timestamp is encrypted with the user password Hash or AES Key. Therefore, when only the user password Hash is obtained, the AS-REQ request can also be initiated, which results in a PTH hash transfer attack; when only the user password AES Key is obtained, the AS-REQ request can also be initiated, which results in a PTK key transfer attack.
The value of the cname field in the AS-REQ request packet represents the user name. The returned packet is different when this value exists or does not exist, so it can be used to enumerate domain user names. This attack method is called domain user enumeration attack (when valid domain user permissions are not obtained, this method can be used to enumerate domain users). And when the user name exists, the password is correct and the password is wrong, the returned packet is also different, so the username and password can be blasted. However, in actual combat, penetration testers usually use an attack method called Password Spraying to test and attack. The term “spraying” is very vivid because it is a type of automated password guessing. This automatic password spraying for all users is usually done to avoid account lockouts, because continuous password guessing for the same user will result in account lockouts. Therefore, only by performing specific password login attempts on all users at the same time can the probability of cracking be increased and the probability of account lockout be eliminated. Ordinary brute force is to fix the username and brute force the password, but password spraying uses a fixed password to run all usernames.
In the realm of web security, a domain user enumeration attack is a tactic employed by malicious actors to gather information about the users within a specific domain. This type of attack is primarily aimed at identifying valid usernames or account information that can be subsequently used in more targeted intrusion attempts such as brute force attacks.
This threat underscores the importance of implementing robust security measures, such as disabling unnecessary directory listing features, deploying intrusion detection systems, and ensuring proper access controls to mitigate unauthorized probing of user information.
Organizations should also monitor network logs for unusual activities that could indicate a user enumeration attempt, thereby allowing for prompt response and safeguarding against potential breaches.(When valid domain user permissions cannot be obtained, this method can be used to enumerate users within the domain). Additionally, when the username exists, the password is correct, or the password is incorrect, the returned packets differ, allowing for username and password brute force attacks. However, in practical scenarios, penetration testers typically use a technique calledPassword SprayingTesting and attacking using an attack method known as password spraying. This term is quite descriptive as it falls under automated password guessing. Such automated password spraying across all users is typically intended to evade account lockouts because repeatedly guessing passwords for a single user results in account lockouts. Hence, only by conducting specific password login attempts across all users simultaneously can the chance of successful compromise be increased while reducing the probability of account lockouts. Traditional brute force involves a fixed username while brute-forcing passwords; however, password spraying employs a fixed password across all usernames.
During the AS-REP phaseDue to the fact that the returned TGT (Ticket Granting Ticket) is encrypted using the krbtgt user’s password hash, if we possess the krbtgt password hash, we can create our own TGT. This type of attack is known asGolden Ticket AttackSimilarly, in the TGS-REP phase, the ST service ticket within the TGS_REP is encrypted using the service’s hash. If we possess the service’s hash, we can issue ST service tickets for any user, which is referred to as a Silver Ticket. This attack method is known asSilver Ticket AttackCompared to Golden Tickets, Silver Tickets use the hash of the service to be accessed rather than the krbtgt hash.
In the AS-REP stage, the Login session key is encrypted with the user password Hash. For domain users, if the “Do not require Kerberos preauthentication” option is set, the attacker sends an AS_REQ request to the domain controller’s port 88. The domain controller will return the TGT subscription certificate and the user’s hash-encrypted Login Session Key without any verification. Therefore, the attacker can crack the obtained user’s hash-encrypted Login Session Key offline. If the cracking is successful, the user’s password in plain text can be obtained. This attack method is called AS-REP Roasting attack
During the TGS-REP phaseDue to the fact that ST service tickets are encrypted using a service hash, if we can obtain the ST service ticket, we can conduct a brute force attack on the ST service ticket to derive the service’s hash. This type of attack is known asKerberoasting AttackAn additional factor contributing to this issue is that when a user sends a TGS_REQ request to the KDC, the KDC will return an ST service ticket as long as the TGT credential is correct, regardless of whether the user has access to the service. In fact, the service in the AS_REQ is krbtgt, which means this attack method can also be used to brute force the TGT credentials in the AS_REP. However, this type of attack is generally not observed because the password for krbtgt is randomly generated, making it resistant to brute-forcing.
Reference:
https://www.rfc-editor.org/rfc/rfc4120.html
https://www.cnblogs.com/artech/archive/2011/01/24/kerberos.html
https://docs.microsoft.com/zh-cn/archive/blogs/apgceps/packerberos-2
https://docs.microsoft.com/en-us/previous-versions/aa302203(v=msdn.10)#top-level-pac-structure