CVE-2014-8989 in Linux
Summary
by MITRE
The Linux kernel through 3.17.4 does not properly restrict dropping of supplemental group memberships in certain namespace scenarios, which allows local users to bypass intended file permissions by leveraging a POSIX ACL containing an entry for the group category that is more restrictive than the entry for the other category, aka a "negative groups" issue, related to kernel/groups.c, kernel/uid16.c, and kernel/user_namespace.c.
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Analysis
by VulDB Data Team • 03/02/2022
The vulnerability described in CVE-2014-8989 represents a critical access control flaw within the Linux kernel that affects versions through 3.17.4. This issue stems from improper handling of supplemental group memberships within namespace scenarios, creating a pathway for local attackers to circumvent security controls that should otherwise restrict file access. The vulnerability specifically manifests when POSIX Access Control Lists contain entries where group permissions are more restrictive than other permissions, creating what is termed a "negative groups" scenario. This flaw enables attackers to escalate privileges and gain unauthorized access to files that should be protected by their restrictive group permissions.
The technical root cause of this vulnerability lies in the kernel's group management functions located in kernel/groups.c, kernel/uid16.c, and kernel/user_namespace.c. These components fail to properly validate and restrict the dropping of supplemental group memberships when processes operate within user namespaces. The flaw occurs during the evaluation of POSIX ACLs where the kernel does not adequately enforce the principle of least privilege by allowing certain group memberships to persist even when they should be dropped. This improper restriction creates a scenario where a process running with specific group memberships can retain access to resources that should be restricted based on the more restrictive group category entries in the ACL.
From an operational perspective, this vulnerability allows local users to bypass intended file permissions and access restricted resources without proper authorization. Attackers can leverage this weakness to gain elevated privileges and access files that should be protected by their group permissions. The impact extends beyond simple file access violations as it can potentially enable privilege escalation attacks, where malicious actors can manipulate their group memberships to access system resources they should not normally be able to reach. This type of vulnerability is particularly dangerous in multi-user environments where proper access controls are essential for system security and data protection.
The vulnerability aligns with CWE-284, which addresses improper access control, and can be mapped to ATT&CK technique T1068, which covers "Exploitation for Privilege Escalation." Organizations should implement immediate mitigations including kernel updates to versions that contain the necessary patches addressing the group membership handling logic. System administrators should also consider implementing additional monitoring for suspicious group membership changes and access pattern anomalies. The fix typically involves modifying the kernel's group management routines to properly enforce namespace boundaries and ensure that supplemental group memberships are correctly dropped when transitioning between different namespace contexts. Regular security auditing and access control reviews should be conducted to identify potential exploitation vectors and maintain overall system integrity.
This vulnerability demonstrates the complexity of access control mechanisms in modern operating systems and highlights the importance of proper namespace handling in kernel security implementations. The issue underscores the need for comprehensive testing of privilege escalation scenarios and proper validation of access control lists under various namespace configurations. Organizations should prioritize patch management and maintain up-to-date kernel versions to protect against similar vulnerabilities that could compromise system security and data integrity.