CVE-2018-1063 in policycoreutils
Summary
by MITRE
Context relabeling of filesystems is vulnerable to symbolic link attack, allowing a local, unprivileged malicious entity to change the SELinux context of an arbitrary file to a context with few restrictions. This only happens when the relabeling process is done, usually when taking SELinux state from disabled to enable (permissive or enforcing). The issue was found in policycoreutils 2.5-11.
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Analysis
by VulDB Data Team • 01/10/2020
The vulnerability described in CVE-2018-1063 represents a critical security flaw in the SELinux context relabeling process that affects systems using policycoreutils version 2.5-11. This issue occurs during the transition of SELinux from a disabled state to an enabled state, specifically when the system performs filesystem relabeling operations. The vulnerability stems from improper handling of symbolic links during the context assignment process, creating an exploitable race condition that allows local attackers to manipulate file contexts. The flaw is particularly dangerous because it enables unprivileged users to escalate their privileges by changing the security context of arbitrary files to less restrictive ones, effectively bypassing security controls that should prevent such modifications.
The technical implementation of this vulnerability involves a specific weakness in how the relabeling process handles symbolic links within the filesystem. When the system transitions SELinux from disabled to enabled state, it performs a comprehensive relabeling of all filesystem objects to ensure proper security contexts are applied. However, the implementation does not properly validate or resolve symbolic links before applying context changes, creating an opportunity for attackers to manipulate the process. An attacker can create a carefully crafted symbolic link that points to a target file they wish to modify, and during the relabeling process, the system will apply the new context to the target file through the symbolic link rather than the intended file. This behavior directly violates the principle of least privilege and can result in unauthorized access to sensitive system resources.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it fundamentally compromises the integrity of the SELinux security model during system state transitions. When SELinux is enabled or set to permissive mode, the system undergoes a critical phase where all filesystem objects receive appropriate security contexts based on the policy definitions. The vulnerability allows attackers to manipulate this process by creating malicious symbolic links that redirect the relabeling operations to files of their choice, potentially changing the contexts of sensitive system files, configuration files, or even files belonging to other users. This can lead to complete system compromise as attackers can effectively bypass security controls that should prevent unauthorized access to system resources, making the entire SELinux framework ineffective during the relabeling phase.
Mitigation strategies for CVE-2018-1063 focus primarily on patching the affected policycoreutils package to version 2.5-11.2 or later, which contains the necessary fixes for proper symbolic link handling during relabeling operations. Organizations should also implement immediate remediation measures such as ensuring that system administrators perform proper filesystem integrity checks before enabling SELinux, particularly during system initialization or state transitions. The vulnerability aligns with CWE-367, which describes the issue of Time-of-Check to Time-of-Use (TOCTOU) race conditions in file operations, and can be mapped to ATT&CK technique T1068, which covers 'Exploitation for Privilege Escalation' through local system vulnerabilities. Security teams should also consider implementing additional monitoring for unusual filesystem operations during SELinux state changes, as well as conducting regular audits of symbolic link structures in critical system directories to prevent exploitation attempts. The fix implemented in the patched version ensures that symbolic links are properly resolved and validated before any context changes are applied, eliminating the race condition that allowed attackers to manipulate the relabeling process.