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CVE-2026-16922 is a vulnerability affecting IBM AIX versions 7.2 and 7.3, as well as IBM PowerVM VIOS version 4.1. The flaw is a Time-Of-Check to Time-Of-Use (TOCTOU) race condition, categorized under CWE-367. This class of vulnerability arises when a system checks the state of a resource (like a file) and then uses that resource, but an attacker can change the resource between the check and the use. In the context of AIX and VIOS, this TOCTOU vulnerability can be exploited by a local attacker to execute arbitrary code.
The core of the issue lies in how the operating system handles certain file operations. When a privileged process performs an action, it typically verifies that the calling user has the necessary permissions. This verification is the “check” part of the TOCTOU. However, there is a small window of time between this permission check and the actual execution of the operation (the “use”). An attacker with local access can exploit this window. By using techniques like symbolic links (symlinks), they can trick the system into checking permissions on a harmless file they own, and then, in the split second before the operation is performed, change the symlink to point to a critical system file. The system, having already verified the permissions, will then perform the operation on the target file, leading to unauthorized actions. This could allow a low-privileged user to write to or execute files they should not have access to. The vulnerability is considered local because the attacker needs an account on the target system.
DailyCVE Form:
Platform: IBM AIX / PowerVM VIOS
Version: 7.2, 7.3, 4.1
Vulnerability: TOCTOU Race Condition
Severity: HIGH (CVSS 3.1: 7.0)
Date: 2026-08-20
Prediction: 2026-09-15 (Based on standard patch cycles)
What Undercode Say:
Analytics indicate that local privilege escalation is the primary concern. The vulnerability requires local access, limiting the attack surface but making it critical for multi-user environments. The CVSS v3.1 score of 7.0 (AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H) reflects the high impact on confidentiality, integrity, and availability, though the attack complexity is high. An attacker needs to win a race condition, which can be difficult but not impossible. The vulnerability is in a core component of AIX and VIOS, meaning a successful exploit could lead to full system compromise.
Exploit: (Educational Purposes!)
A typical exploit for this class of vulnerability follows a pattern:
1. Identify Target: Find a setuid binary or a system service that performs a file operation with a TOCTOU vulnerability.
2. Create a Decoy: The attacker creates a file they own, e.g., /tmp/decoy.
3. Set Up a Race: The attacker writes a script that continuously changes a symbolic link.
!/bin/bash while true; do ln -sf /tmp/decoy /tmp/victim_file ln -sf /etc/passwd /tmp/victim_file done
4. Trigger the Vulnerable Process: Simultaneously, the attacker triggers the vulnerable process many times, hoping it will perform the “check” when the link points to `/tmp/decoy` (which the attacker owns) and the “use” when it points to /etc/passwd.
!/bin/bash
for i in {1..1000}; do
/usr/bin/vulnerable_binary /tmp/victim_file
done
5. Achieve Code Execution: If successful, the vulnerable binary will write to or execute `/etc/passwd` with its elevated privileges. This could allow the attacker to add a new user with root privileges.
Protection:
- Apply Patches: The primary and most effective mitigation is to apply the official IBM fix for APAR PH71756 as soon as it is available.
- Restrict Local Access: Limit local user accounts and access to the system to trusted personnel only.
- Monitor Logs: Monitor system logs for unusual activity, such as repeated execution of sensitive binaries or attempts to create many symbolic links in a short period.
- Principle of Least Privilege: Run services and applications with the minimum privileges necessary to reduce the potential impact of a successful exploit.
Impact:
Successful exploitation of this vulnerability allows a local, unprivileged attacker to execute arbitrary code with elevated privileges. This could lead to a complete system compromise, including the ability to install malicious software, steal sensitive data, modify system configurations, or cause a denial of service. The impact is considered severe due to the potential for full control over the affected AIX or VIOS system.
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Sources:
Reported By: nvd.nist.gov
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