CVE-2025-71239: Linux Kernel Auth Bypass Vulnerability

CVE-2025-71239 Overview

A vulnerability has been resolved in the Linux kernel where the fchmodat2() system call was not included in the audit change attributes class. This syscall, introduced in Linux kernel version 6.6, allows file attribute changes that bypass audit rules, enabling potential security monitoring evasion.

When fchmodat2() is used to modify file attributes in the same manner as chmod() or fchmodat(), the operation will not trigger audit rules that are configured to monitor file attribute changes. This creates a blind spot in security monitoring and compliance logging for systems relying on Linux audit infrastructure.

Critical Impact

Attackers can use fchmodat2() to change file permissions without triggering audit rules, effectively bypassing security monitoring and compliance controls on Linux kernel 6.6 and later systems.

Affected Products

• Linux kernel version 6.6 and later (where fchmodat2() is available)
• Systems using Linux audit infrastructure for security monitoring
• Systems with audit rules targeting file attribute changes (e.g., -p a or -p wa permissions)

Discovery Timeline

• 2026-03-17 - CVE CVE-2025-71239 published to NVD
• 2026-03-18 - Last updated in NVD database

Technical Details for CVE-2025-71239

Vulnerability Analysis

The fchmodat2() syscall was introduced in Linux kernel 6.6 as an extended version of fchmodat(), providing additional flags for file permission modification operations. However, when the kernel's audit subsystem was updated, this new syscall was not added to the change attributes class, which is responsible for triggering audit events when file attributes are modified.

The audit subsystem in Linux maintains syscall classifications that determine which operations generate audit records. The change attributes class includes syscalls like chmod(), fchmod(), and fchmodat() that modify file permissions. The omission of fchmodat2() from this class means that an attacker or malicious process could use this syscall to modify file permissions without generating the expected audit trail.

This is particularly concerning for security-conscious environments that rely on audit rules to detect unauthorized permission changes on sensitive files and directories.

Root Cause

The root cause of this vulnerability is the incomplete integration of the fchmodat2() syscall into the kernel audit framework. When fchmodat2() was added to the kernel in version 6.6, it was not registered with the audit subsystem's change attributes class alongside similar syscalls like chmod(), fchmodat(), and fchmod(). This oversight resulted in the syscall being able to modify file attributes without generating corresponding audit events.

Attack Vector

An attacker with local access to a Linux system running kernel 6.6 or later can exploit this vulnerability to modify file permissions while evading detection by audit monitoring systems.

The attack scenario involves using fchmodat2() instead of traditional permission-changing syscalls. When audit rules are configured to monitor file attribute changes, the fchmodat2() call will not trigger the expected audit events, allowing the permission change to go unnoticed by security monitoring tools.

For example, an audit rule such as -w /tmp/test -p rwa -k test_rwa that monitors read, write, and attribute changes on /tmp/test would not detect permission modifications made via fchmodat2().

The patch resolves this issue by adding fchmodat2() to the change attributes audit class, ensuring that all permission-modifying syscalls are properly monitored. Technical details about this syscall audit gap are documented in the Bencteux Blog Post on Syscalls.

Detection Methods for CVE-2025-71239

Indicators of Compromise

• Unexpected file permission changes on sensitive files without corresponding audit logs
• Discrepancies between actual file permissions and audit trail records
• Processes using fchmodat2() syscall when fchmodat() would typically be used

Detection Strategies

• Monitor for fchmodat2() syscall usage via alternative tracing mechanisms (eBPF, ftrace, or strace)
• Compare expected permission states against actual file permissions to detect unauthorized changes
• Implement file integrity monitoring tools that track permissions independently of audit subsystem
• Review syscall usage patterns for processes that interact with sensitive files

Monitoring Recommendations

• Deploy endpoint detection solutions that can monitor syscall activity beyond the traditional audit framework
• Implement behavioral analysis to detect anomalous permission change patterns
• Configure alerts for file permission modifications on critical system files and directories
• Consider using SentinelOne Singularity platform for comprehensive kernel-level visibility and threat detection

How to Mitigate CVE-2025-71239

Immediate Actions Required

• Update the Linux kernel to a patched version that includes fchmodat2() in the audit change attributes class
• Review existing audit rules and ensure comprehensive coverage of file monitoring requirements
• Implement additional file integrity monitoring as a compensating control until patches are applied
• Audit recent file permission changes on critical systems to identify potential exploitation

Patch Information

The Linux kernel team has released patches to address this vulnerability by adding fchmodat2() to the change attributes class of the audit subsystem. Multiple kernel commits have been made available:

• Kernel Git Commit 3e762a0
• Kernel Git Commit 4f493a6
• Kernel Git Commit 4fed776
• Kernel Git Commit 91e27bc
• Kernel Git Commit c4334c0

Apply the appropriate patch for your kernel version from the stable kernel repositories.

Workarounds

• Deploy endpoint detection and response (EDR) solutions that provide syscall-level monitoring independent of the audit subsystem
• Use eBPF-based monitoring tools to capture fchmodat2() syscall activity
• Implement file integrity monitoring (FIM) solutions that detect permission changes regardless of audit coverage
• Consider restricting access to sensitive files using additional access control mechanisms (SELinux, AppArmor)

# Example: Using auditctl to verify current rules and manually trace fchmodat2
# Check existing audit rules
auditctl -l

# Use strace to monitor fchmodat2 syscalls for a specific process
strace -f -e trace=fchmodat2 -p <PID>

# Update kernel to patched version (example for Debian-based systems)
apt update && apt upgrade linux-image-$(uname -r)