Abstract

The program can potentially fail to release a system resource.

Description

Most unreleased resource issues result in general software reliability problems, but if an attacker can intentionally trigger a resource leak, the attacker might be able to launch a denial of service attack by depleting the resource pool.

Resource leaks have at least two common causes:

• Error conditions and other exceptional circumstances.
• Confusion over which part of the program is responsible for releasing the resource.

Examples

Example 1: The following Java method never closes the file handle it opens. The finalize() method for FileInputStream eventually calls close(), but there is no guarantee as to how long it will take before the finalize() method will be invoked. In a busy environment, this can result in the JVM using up all of its file handles.

	private void processFile(String fName) throws FileNotFoundException, IOException
	{
	  FileInputStream fis = new FileInputStream(fName);
	  int sz;
	  byte[] byteArray = new byte[BLOCK_SIZE];
	  while ((sz = fis.read(byteArray)) != -1) {
		processBytes(byteArray, sz);
	  }
	}

Example 2: Under normal conditions the following C# code executes a database query, processes the results returned by the database, and closes the allocated SqlConnection object. But if an exception occurs while executing the SQL or processing the results, the SqlConnection object is not closed. If this happens often enough, the database will run out of available cursors and not be able to execute any more SQL queries.

	...
	SqlConnection conn = new SqlConnection(connString);
	SqlCommand cmd = new SqlCommand(queryString);
	cmd.Connection = conn;
	conn.Open();
	SqlDataReader rdr = cmd.ExecuteReader();
	HarvestResults(rdr); 
	conn.Connection.Close();
	...

Example 3: The following C function does not close the file handle it opens if an error occurs. If the process is long-lived, the process can run out of file handles.

	int decodeFile(char* fName)
	{
		char buf[BUF_SZ];
		FILE* f = fopen(fName, "r");
		
		if (!f) {
			printf("cannot open %s\n", fName);
			return DECODE_FAIL;
		} else {
			while (fgets(buf, BUF_SZ, f)) {
				if (!checkChecksum(buf)) {
				  return DECODE_FAIL;
				} else {
				  decodeBlock(buf);
				}
			}
		}
		fclose(f);
		return DECODE_SUCCESS;
	}