CVE-2011-3151 in SELinux
Summary
by MITRE
The Ubuntu SELinux initscript before version 1:0.10 used touch to create a lockfile in a world-writable directory. If the OS kernel does not have symlink protections then an attacker can cause a zero byte file to be allocated on any writable filesystem.
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Analysis
by VulDB Data Team • 09/06/2023
The vulnerability identified as CVE-2011-3151 resides within the Ubuntu SELinux initscript implementation prior to version 1:0.10, representing a critical flaw in privilege escalation and system integrity protection mechanisms. This issue stems from improper handling of lockfile creation during system initialization processes, where the script employs the touch command to generate lockfiles in directories that are world-writable. The fundamental security weakness becomes apparent when considering that the operating system kernel lacks symlink protection capabilities, creating an exploitable condition that allows malicious actors to manipulate filesystem objects in ways that compromise system security. The vulnerability specifically targets the initialization sequence of SELinux policies, which are designed to enforce mandatory access controls and maintain system security boundaries.
The technical flaw manifests through a race condition and symlink manipulation attack vector where an unprivileged user can create symbolic links in the world-writable directory path used by the initscript. When the touch command executes, it follows these symbolic links and creates zero-byte files at arbitrary locations within the filesystem where the attacker has write permissions. This behavior directly violates the principle of least privilege and undermines the security model that SELinux aims to enforce. The vulnerability operates under CWE-367, which specifically addresses Time-of-Check to Time-of-Use (TOCTOU) race conditions, and represents a classic example of how improper file handling can lead to privilege escalation. The attack requires minimal privileges but can result in significant system compromise, as the attacker can potentially overwrite critical system files or create malicious files in locations where the system expects legitimate lockfiles to exist.
The operational impact of this vulnerability extends beyond simple privilege escalation, affecting the overall integrity and availability of SELinux policy enforcement mechanisms. When an attacker successfully exploits this condition, they can disrupt the normal initialization process of SELinux, potentially causing the security framework to fail or behave unpredictably. The zero-byte file allocation can lead to various system instability issues, including denial of service conditions where legitimate lockfile operations fail due to corrupted or malicious file creation. This vulnerability particularly affects systems running older versions of Ubuntu where the initscript implementation was not properly secured against symlink attacks. The attack can be executed without requiring elevated privileges, making it particularly dangerous as it allows any local user to potentially compromise the security posture of the entire system. The implications are significant for enterprise environments where SELinux is used for mandatory access controls, as it undermines the fundamental security assumptions that these mechanisms provide.
Mitigation strategies for CVE-2011-3151 should focus on immediate patching of the affected Ubuntu SELinux initscript to version 1:0.10 or later, which addresses the improper lockfile creation behavior through proper file handling and directory permissions enforcement. Organizations should also implement proper symlink protection mechanisms at the kernel level, ensuring that symlink protections are enabled to prevent the exploitation of race conditions. System administrators should audit the directories used by SELinux initscripts for world-writable permissions and restrict access to only necessary parties. The fix typically involves using atomic file creation methods instead of touch commands in world-writable directories, or moving lockfile creation to secure, non-world-writable locations. Additionally, implementing proper monitoring and alerting for unusual lockfile creation patterns can help detect exploitation attempts. From an ATT&CK framework perspective, this vulnerability maps to T1068 (Exploitation for Privilege Escalation) and T1059 (Command and Scripting Interpreter) as attackers can leverage the compromised initscript to execute malicious code. Regular security audits and vulnerability assessments should include checking for similar patterns in other system initialization scripts to prevent analogous issues from existing in the broader system ecosystem.