CVE-2008-0777 in FreeBSD
Summary
by MITRE
The sendfile system call in FreeBSD 5.5 through 7.0 does not check the access flags of the file descriptor used for sending a file, which allows local users to read the contents of write-only files.
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Analysis
by VulDB Data Team • 05/04/2017
The vulnerability described in CVE-2008-0777 represents a critical access control flaw within the FreeBSD operating system's implementation of the sendfile system call. This issue affects FreeBSD versions 5.5 through 7.0 and stems from insufficient validation of file descriptor access flags during file transmission operations. The sendfile system call is designed to efficiently transfer data from file descriptors to socket descriptors, commonly used in web servers and network applications to serve files. The flaw occurs because the system call fails to verify that the file descriptor possesses appropriate read permissions before attempting to read file contents, creating a scenario where local attackers can bypass normal file access controls.
The technical nature of this vulnerability aligns with CWE-284, which addresses improper access control mechanisms, and specifically demonstrates weaknesses in file descriptor validation and access flag checking. When a process invokes sendfile with a file descriptor that has been opened in write-only mode, the system should reject the operation due to insufficient read permissions. However, the FreeBSD implementation omits this crucial check, allowing the system call to proceed and read the file contents despite the descriptor's write-only nature. This behavior violates fundamental security principles of least privilege and mandatory access controls, as it enables unauthorized data extraction from files that should be protected from reading access.
From an operational perspective, this vulnerability presents significant risks for local attackers who can leverage it to access sensitive information stored in write-only files. The impact extends beyond simple information disclosure, as it can enable privilege escalation attacks when combined with other vulnerabilities or when attackers can manipulate file descriptors in specific ways. The vulnerability is particularly concerning in server environments where the sendfile system call is frequently used for serving web content, as it could allow attackers to read configuration files, database files, or other sensitive data that should remain protected. The local nature of this exploit means that attackers do not require network access or remote code execution capabilities, making it a particularly dangerous flaw for systems where local access is possible.
Mitigation strategies for this vulnerability involve both immediate system updates and defensive programming practices. The primary solution is to upgrade to FreeBSD versions that contain the patched implementation of the sendfile system call, which properly validates file descriptor access flags before proceeding with file operations. System administrators should also implement proper access control policies and file permissions to minimize the impact of potential exploitation. Additionally, monitoring for unusual sendfile system call usage patterns can help detect potential exploitation attempts. Organizations should consider implementing application-level controls that validate file access permissions before allowing operations that might trigger this vulnerability, particularly in applications that handle user-provided file descriptors or that rely heavily on the sendfile system call for performance optimization. The vulnerability highlights the importance of thorough input validation and access control checking in system call implementations, particularly for operations that involve multiple file descriptors and complex permission scenarios.