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CVE-2026-16896 is a vulnerability in IBM i that allows a local authenticated attacker to gain unauthorized access to files. The root cause is a Time-of-Check Time-of-Use (TOCTOU) race condition, a class of bug where a system checks the state of a resource (like a file’s permissions) before using it, but an attacker can change that state in the brief window between the check and the use. In the context of IBM i, this flaw exists within the file system access controls. When a process attempts to access a file, the operating system performs a permission check. However, due to the TOCTOU vulnerability, a local attacker can exploit this race window. By rapidly swapping a legitimate file with a malicious one or altering its permissions after the check but before the file is opened, the attacker can trick the system into operating on a file they should not have access to. This allows them to read sensitive data, potentially including system files, configuration files, or other users’ data, thereby bypassing the operating system’s security model. The vulnerability is triggered locally, meaning the attacker must already have authenticated access to the system, making it a privilege escalation and information disclosure issue.
DailyCVE Form:
Platform: IBM i
Version: 7.3, 7.4, 7.5, 7.6
Vulnerability: TOCTOU Race Condition
Severity: High (CVSS:7.1)
date: 2026-08-13
Prediction: 2026-09-15
What Undercode Say:
Check for the vulnerability by inspecting file operation logs Look for patterns of rapid file access and modification cat /var/log/ibmi/security/ | grep -i "toctou" Monitor for unusual file access attempts by non-privileged users auditctl -a always,exit -F arch=b64 -S openat -S unlink -k toctou_monitor List all users with local access to assess the attack surface lsuser -a ALL
Exploit: (Educational Purposes!)
A local attacker could write a script that continuously attempts to access a protected file, while simultaneously using another process to change the file’s symbolic link or permissions right after the access check passes but before the file is opened. This technique, known as a “race condition attack,” can be used to read files like `/etc/passwd` or other sensitive configuration data. The following pseudocode illustrates the concept:
// Attacker's process 1: Attempt to open the file
if (access("/etc/protected_file", R_OK) == 0) {
// In the race window, process 2 swaps the file
fd = open("/etc/protected_file", O_RDONLY);
read(fd, buffer, sizeof(buffer));
}
// Attacker's process 2: Swaps the file during the race window
rename("/tmp/malicious_file", "/etc/protected_file");
Protection:
Apply the security patches provided by IBM as soon as they are available. The official IBM Security Bulletin is the primary source for fixes.
Restrict local access to the IBM i system to only trusted users, minimizing the potential attacker pool.
Implement strict file system auditing and monitoring to detect suspicious patterns of file access, which could indicate an attempted TOCTOU attack.
Impact:
Successful exploitation of this vulnerability allows a local authenticated attacker to read files they are not authorized to access. This can lead to the disclosure of sensitive information, including system configuration files, cryptographic keys, user data, and other confidential information stored on the system.
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Sources:
Reported By: nvd.nist.gov
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