CVE-2005-2959 in sudo
Summary
by MITRE
incomplete blacklist vulnerability in sudo 1.6.8 and earlier allows local users to gain privileges via the (1) shellopts and (2) ps4 environment variables before executing a bash script on behalf of another user which are not cleared even though other variables are.
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Analysis
by VulDB Data Team • 07/06/2021
The vulnerability identified as CVE-2005-2959 represents a critical privilege escalation flaw in sudo versions 1.6.8 and earlier, specifically targeting the incomplete blacklist mechanism that governs environment variable handling during command execution. This weakness stems from sudo's improper clearing of certain environment variables when executing commands on behalf of other users, creating a persistent security gap that local attackers can exploit to elevate their privileges.
The technical flaw manifests in sudo's handling of environment variables through two specific parameters: shellopts and ps4. While sudo properly clears numerous environment variables to prevent malicious injection during privilege escalation scenarios, it fails to clear these two variables which are particularly dangerous when present in the environment. The shellopts variable controls shell options that can influence command execution behavior, while ps4 provides a prompt string for debugging shell scripts. When these variables remain uncleared, they can be manipulated by local users to inject malicious code that executes with elevated privileges.
This vulnerability operates under the broader context of environment variable manipulation attacks, which fall under the CWE-15 category of "Direct Reference to External Resource" and align with ATT&CK technique T1068, "Local Privilege Escalation." The flaw exploits the fundamental principle that environment variables can contain executable code or influence command behavior, particularly when sudo executes commands through shell processes. The incomplete blacklist approach means that while some variables are properly sanitized, others are overlooked, creating a false sense of security that attackers can leverage.
The operational impact of this vulnerability is significant as it allows local users to bypass sudo's intended security controls and execute commands with the privileges of other users, potentially including root access. Attackers can manipulate the shellopts and ps4 variables to inject malicious shell code that executes during the sudo execution process, effectively circumventing the privilege separation mechanisms that sudo is designed to enforce. This creates a persistent backdoor for privilege escalation that can be exploited repeatedly until the vulnerability is patched.
Mitigation strategies for CVE-2005-2959 require immediate patching of affected sudo versions to implement a complete blacklist of dangerous environment variables, including shellopts and ps4. Organizations should also implement comprehensive environment variable sanitization policies that ensure all potentially dangerous variables are cleared during privilege escalation scenarios. Additional defensive measures include monitoring for suspicious environment variable modifications and implementing principle of least privilege controls that limit local user access to systems where sudo is deployed. The vulnerability highlights the importance of complete sanitization approaches rather than partial blacklist implementations, as demonstrated by the ATT&CK framework's emphasis on proper environment variable handling in privilege escalation scenarios.