CVE-2021-23239: Sudo Race Condition Vulnerability

CVE-2021-23239 Overview

The sudoedit personality of Sudo before version 1.9.5 contains a race condition vulnerability that may allow a local unprivileged user to perform arbitrary directory-existence tests. The flaw exists in the sudo_edit.c file where a race condition can be exploited by replacing a user-controlled directory with a symlink to an arbitrary path, enabling information disclosure about the file system structure.

Critical Impact

Local attackers can exploit this symlink race condition to determine the existence of arbitrary directories on the system, potentially aiding in reconnaissance for further attacks.

Affected Products

• Sudo Project Sudo (versions before 1.9.5)
• NetApp Cloud Backup
• NetApp HCI Management Node
• NetApp SolidFire
• Fedora 32 and 33
• Debian Linux 10.0

Discovery Timeline

• 2021-01-12 - CVE-2021-23239 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-23239

Vulnerability Analysis

This vulnerability is classified as CWE-59 (Improper Link Resolution Before File Access), commonly known as a symlink attack. The flaw resides in the sudoedit functionality of Sudo, which is designed to allow users to edit files with elevated privileges in a controlled manner.

The vulnerable code path in sudo_edit.c fails to properly handle the time-of-check to time-of-use (TOCTOU) window when processing user-controlled directories. An attacker with local access can exploit this race condition by rapidly replacing a directory they control with a symbolic link pointing to an arbitrary location on the file system.

When the sudoedit process attempts to access the directory, the attacker can determine whether the target path exists based on the behavior or error messages returned. While this vulnerability does not directly enable code execution or privilege escalation, it facilitates information disclosure that could be leveraged as part of a more sophisticated attack chain.

Root Cause

The root cause is a race condition in the sudo_edit.c file where the application checks directory permissions and then subsequently accesses the directory without atomic verification. This TOCTOU gap allows an attacker to substitute a legitimate directory with a malicious symlink between the check and use operations, bypassing the intended security constraints.

Attack Vector

The attack requires local access to the system with low privileges. An attacker must be able to:

1. Create directories in a location accessible to their user account
2. Execute sudoedit with a path they control
3. Win the race condition by replacing the directory with a symlink at the precise moment between Sudo's check and use operations

The exploitation is performed locally and requires the attacker to repeatedly attempt the race condition until successful. The attacker can then probe for the existence of arbitrary directories on the file system by observing the behavior differences when the symlink target exists versus when it does not.

Detection Methods for CVE-2021-23239

Indicators of Compromise

• Unusual sudoedit invocations with rapidly changing directory paths
• High volume of directory creation and deletion operations in user-controlled paths
• Symlink creation activity targeting sensitive system directories
• Repeated sudoedit failures or unusual error patterns in authentication logs

Detection Strategies

• Monitor /var/log/auth.log or /var/log/secure for anomalous sudoedit command patterns
• Implement file integrity monitoring on directories commonly targeted for symlink attacks
• Configure auditd rules to track symlink creation and directory manipulation in user home directories
• Deploy behavioral analysis to detect rapid file system operations indicative of race condition exploitation

Monitoring Recommendations

• Enable verbose Sudo logging by configuring log_input and log_output in sudoers
• Set up alerting for sudoedit commands executed with unusual path arguments
• Monitor process creation events for sudoedit invocations paired with rapid filesystem modifications
• Correlate sudo activity with file system events to identify potential race condition attempts

How to Mitigate CVE-2021-23239

Immediate Actions Required

• Upgrade Sudo to version 1.9.5 or later immediately
• Review sudo logs for evidence of exploitation attempts
• Audit systems for unauthorized symlinks in user-writable directories
• Consider temporarily restricting sudoedit access if immediate patching is not feasible

Patch Information

The Sudo Project has addressed this vulnerability in version 1.9.5. System administrators should update to this version or later to remediate the race condition in sudoedit. Patches are available from the Sudo Project Stable Release page.

Distribution-specific patches have been released:

• Fedora users should apply updates via the Fedora Package Announcement
• Debian users can reference the Debian LTS Announcement
• Gentoo users should follow the Gentoo GLSA 202101-33 advisory
• NetApp customers should review the NetApp Security Advisory

Workarounds

• Restrict sudoedit access to trusted users only by modifying sudoers configuration
• Implement filesystem quotas and inode limits to make race condition exploitation more difficult
• Use mandatory access control systems like SELinux or AppArmor to restrict symlink creation in sensitive paths
• Monitor and alert on sudoedit usage until patches can be applied

# Configuration example
# Verify current sudo version
sudo --version

# Update sudo on Debian/Ubuntu systems
sudo apt-get update && sudo apt-get install sudo

# Update sudo on RHEL/CentOS/Fedora systems
sudo dnf update sudo

# Restrict sudoedit in /etc/sudoers (temporary workaround)
# Comment out or remove sudoedit permissions for non-essential users
# Example: Remove NOPASSWD entries for sudoedit