Description

The attack aims to explore flaws in the decode mechanism implemented on applications when decoding Unicode data format. An attacker can use this technique to encode certain characters in the URL to bypass application filters, thus accessing restricted resources on the Web server or force browsing to protected pages.

Severity

High

Likelihood of exploitation

High

Examples

Consider a web application that has restricted directories or files (e.g. a file containing application usernames: appusers.txt). An attacker can encode the character sequence “../” (Path Traversal Attack) using Unicode format and attempt to access the protected resource, as follows:

Original Path Traversal attack URL (without Unicode Encoding):

http://vulneapplication/../../appusers.txt

Path Traversal attack URL with Unicode Encoding:

http://vulneapplication/%C0AE/%C0AE%C0AF%C0AE%C0AE%C0AFappusers.txt

The Unicode encoding for the URL above will produce the same result as the first URL (Path Traversal Attack). However, if the application has certain input security filter mechanism, it could refuse any request containing “../” sequence, thus blocking the attack. However, if this mechanism doesn’t consider character encoding, the attacker can bypass and access protected resource.

Other consequences of this type of attack are privilege escalation, arbitrary code execution, data modification and denial of service.

External References

http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2000-0884 - CVE-2000-0884

http://capec.mitre.org/data/definitions/71.html - Using Unicode Encoding to Bypass Validation Logic

http://www.microsoft.com/technet/security/bulletin/MS00-078.mspx - Patch Available for 'Web Server Folder Traversal' Vulnerability

http://www.kb.cert.org/vuls/id/739224 - HTTP content scanning systems full-width/half-width Unicode encoding bypass

http://scissec.scis.ecu.edu.au/conferences2007/documents/cheong_kai_wai_1.pdf - Penetration testing of cross site scripting and SQL injection on web application by Cheong Kai Wee

http://www.cgisecurity.com/lib/URLEmbeddedAttacks.html - URL encoded attacks, by Gunter Ollmann