CVE-2015-8239 in sudo
Summary
by MITRE
The SHA-2 digest support in the sudoers plugin in sudo after 1.8.7 allows local users with write permissions to parts of the called command to replace them before it is executed.
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Analysis
by VulDB Data Team • 01/03/2023
The vulnerability identified as CVE-2015-8239 represents a critical security flaw in the sudoers plugin mechanism within the sudo utility version 1.8.7 and later. This issue specifically affects the implementation of SHA-2 digest verification functionality that was introduced to enhance command integrity checking. The vulnerability stems from a race condition that occurs during the execution of commands where sudo verifies the integrity of commands through SHA-2 hash calculations. When local users possess write permissions to portions of the command being executed, they can exploit this timing window to substitute the intended command with a malicious alternative before sudo completes its integrity verification process.
The technical exploitation of this vulnerability relies on the fundamental flaw in how sudo handles command verification during execution. The sudoers plugin maintains SHA-2 digests of commands to ensure they have not been tampered with between the time they are specified and when they are executed. However, the implementation contains a critical timing gap where the verification occurs after the command has already been processed by the shell, but before it is executed. This creates a window where an attacker with write access to command components can replace the intended command with a malicious version that will be executed with elevated privileges. The flaw is particularly dangerous because it allows attackers to bypass intended security controls and execute arbitrary code with root privileges.
The operational impact of CVE-2015-8239 extends beyond simple privilege escalation to encompass broader system compromise and persistence mechanisms. Attackers can leverage this vulnerability to maintain long-term access to compromised systems by replacing legitimate commands with backdoor implementations that execute in the context of elevated privileges. The vulnerability affects systems where sudo is configured with command integrity checking enabled, particularly those using the sudoers plugin for enhanced security controls. This creates a significant risk for enterprise environments where sudo is extensively used for privilege management, as it allows attackers to subvert security policies designed to prevent unauthorized command execution.
Mitigation strategies for this vulnerability require immediate patching of affected sudo versions to address the race condition in digest verification. System administrators should implement comprehensive monitoring for unauthorized modifications to command files and directories that sudo processes. The recommended approach includes upgrading to sudo version 1.8.12 or later, which contains the necessary fixes for this race condition. Additionally, organizations should review sudoers configurations to minimize write permissions for command components and implement proper file integrity monitoring solutions. The vulnerability aligns with CWE-367, which addresses time-of-check to time-of-use (TOCTOU) race conditions, and represents a specific instance where ATT&CK technique T1068 is applicable for privilege escalation through command substitution. Organizations should also consider implementing additional security controls such as mandatory access controls and privilege separation to reduce the attack surface and limit the impact of potential exploitation.