CVE-2026-23403: Linux Kernel Memory Leak Vulnerability

CVE-2026-23403 Overview

A memory leak vulnerability has been identified in the Linux kernel's AppArmor security module, specifically within the verify_header function. The flaw occurs when the function incorrectly sets *ns = NULL on every call, causing memory leakage of namespace strings that were allocated during previous iterations when unpacking multiple profiles. This improper memory management not only leads to resource exhaustion over time but also breaks namespace consistency checking since *ns is always NULL when comparisons are made.

Critical Impact

This vulnerability can lead to memory exhaustion on affected Linux systems running AppArmor, potentially causing denial of service conditions and undermining the security consistency of AppArmor namespace enforcement.

Affected Products

• Linux Kernel (multiple stable versions)
• Systems with AppArmor security module enabled
• Linux distributions using AppArmor for mandatory access control

Discovery Timeline

• 2026-04-01 - CVE CVE-2026-23403 published to NVD
• 2026-04-01 - Last updated in NVD database

Technical Details for CVE-2026-23403

Vulnerability Analysis

The vulnerability resides in the AppArmor profile unpacking mechanism within the Linux kernel. When multiple AppArmor profiles are being unpacked, the verify_header function is called iteratively. The root issue is that on each invocation, the function explicitly sets the namespace pointer (*ns) to NULL, discarding any previously allocated namespace string without properly freeing it first.

This behavior has two significant consequences. First, it creates a classic memory leak where namespace strings allocated in prior iterations are orphaned and never deallocated, leading to gradual memory consumption. Second, it introduces a logic flaw in namespace consistency checking—since *ns is always NULL when the comparison occurs, the validation mechanism cannot properly verify that profiles belong to consistent namespaces.

The fix is straightforward: remove the incorrect *ns = NULL assignment from verify_header. The calling function aa_unpack already initializes *ns to NULL once before entering the loop, which is the correct and sufficient initialization point.

Root Cause

The root cause is an incorrect variable initialization within the verify_header function. By resetting the namespace pointer to NULL on every call rather than preserving the allocated value across iterations, the function both leaks memory and defeats the namespace consistency checking logic that depends on comparing namespace values across multiple profile headers.

Attack Vector

While this vulnerability does not appear to be directly exploitable for arbitrary code execution, it presents a denial of service vector. An attacker with the ability to load AppArmor profiles (typically requiring elevated privileges) could repeatedly trigger the memory leak by loading policies with multiple profiles, gradually consuming kernel memory until system stability is affected.

The vulnerability mechanism involves the verify_header function in the AppArmor policy unpacking code path. When profiles are loaded, namespace strings are allocated but never freed due to the premature NULL assignment, causing unbounded memory growth with repeated policy operations. For full technical details, see the kernel git commits referenced below.

Detection Methods for CVE-2026-23403

Indicators of Compromise

• Gradual increase in kernel memory usage on systems with active AppArmor profile loading
• Memory allocation warnings in kernel logs related to AppArmor subsystem
• Unexpected system slowdowns or OOM (Out of Memory) conditions on systems with AppArmor enabled

Detection Strategies

• Monitor kernel memory usage trends, particularly slab allocations associated with AppArmor
• Review kernel logs for memory allocation failures or warnings in the apparmor subsystem
• Implement memory profiling to detect sustained memory growth during AppArmor policy operations
• Check running kernel version against patched versions listed in the kernel git commits

Monitoring Recommendations

• Configure alerting for abnormal kernel memory consumption patterns
• Enable AppArmor audit logging to track profile loading events
• Deploy kernel memory leak detection tools such as kmemleak in development and testing environments
• Regularly audit AppArmor profile loading frequency on production systems

How to Mitigate CVE-2026-23403

Immediate Actions Required

• Update the Linux kernel to a patched version that includes the fix
• Limit AppArmor profile loading operations to reduce memory leak impact until patched
• Monitor system memory usage on affected systems
• Review which users and processes have permissions to load AppArmor profiles

Patch Information

The vulnerability has been addressed in multiple kernel stable branches. Patches are available through the following kernel git commits:

• Kernel Git Commit 42fd831
• Kernel Git Commit 4f0889f
• Kernel Git Commit 663ce34
• Kernel Git Commit 786e2c
• Kernel Git Commit e38c55d

Apply the appropriate patch for your kernel version by updating through your distribution's package manager or building from the patched kernel source.

Workarounds

• Restrict AppArmor profile loading privileges to minimize exposure until patching is complete
• Schedule periodic system reboots to reclaim leaked memory on systems that cannot be immediately patched
• Reduce the frequency of AppArmor profile reloading operations where possible
• Consider temporarily disabling AppArmor if the security trade-off is acceptable for your environment

# Check current kernel version
uname -r

# Update kernel on Debian/Ubuntu systems
sudo apt update && sudo apt upgrade linux-image-generic

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

# Verify AppArmor status
sudo aa-status