CVE-2026-23405: Linux Kernel AppArmor DOS Vulnerability

CVE-2026-23405 Overview

A resource exhaustion vulnerability has been identified in the Linux kernel's AppArmor security module. The vulnerability exists in the handling of policy namespaces, where the number of namespace levels is not properly bounded. While the implementation relies on user namespace limits, policy namespaces are not strictly tied to user namespaces, allowing attackers to create and nest them arbitrarily deep. This can be exploited to exhaust system resources, leading to denial of service conditions.

Critical Impact

Local attackers can exhaust system resources by creating deeply nested AppArmor policy namespaces, potentially causing system instability or denial of service.

Affected Products

• Linux Kernel (multiple versions with AppArmor enabled)
• Systems utilizing AppArmor mandatory access control
• Linux distributions with AppArmor security profiles enabled

Discovery Timeline

• April 1, 2026 - CVE-2026-23405 published to NVD
• April 1, 2026 - Last updated in NVD database

Technical Details for CVE-2026-23405

Vulnerability Analysis

The vulnerability stems from an architectural oversight in the AppArmor security module's namespace management. AppArmor uses policy namespaces to isolate and organize security policies, providing hierarchical containment for different security contexts. However, the implementation failed to enforce a hard limit on the depth of namespace nesting.

The core issue is that while AppArmor policy namespaces conceptually relate to user namespaces, they operate independently without inheriting the depth restrictions applied to user namespaces. An attacker with the ability to create AppArmor policy namespaces can recursively nest them to arbitrary depths, consuming kernel memory and other system resources with each additional level.

Root Cause

The root cause is the absence of a hard cap on policy namespace depth in the AppArmor subsystem. The original implementation assumed that user namespace limits would indirectly constrain policy namespace creation, but this assumption proved incorrect since policy namespaces can be created and nested independently of user namespaces.

Attack Vector

An attacker with local access and sufficient privileges to create AppArmor policy namespaces can exploit this vulnerability by programmatically creating deeply nested namespace hierarchies. Each nested namespace consumes kernel memory and associated data structures. By creating thousands or millions of nested levels, an attacker can exhaust available system memory, leading to system instability, kernel panics, or denial of service conditions affecting all users and processes on the system.

The attack requires local access but can be particularly impactful in multi-tenant environments where users have limited namespace creation capabilities, or in containerized environments where AppArmor is used for workload isolation.

Detection Methods for CVE-2026-23405

Indicators of Compromise

• Unusual memory consumption patterns in kernel space without corresponding userspace activity
• High frequency of AppArmor namespace creation operations in audit logs
• System performance degradation or instability without apparent cause
• Kernel log messages related to AppArmor namespace operations or memory allocation failures

Detection Strategies

• Monitor dmesg and kernel logs for AppArmor-related warnings or errors
• Implement auditd rules to track namespace creation system calls
• Use process monitoring to detect applications creating excessive namespace hierarchies
• Deploy kernel-level resource monitoring to detect abnormal memory consumption patterns

Monitoring Recommendations

• Enable AppArmor audit logging to track policy namespace creation events
• Configure resource usage alerts for kernel memory consumption thresholds
• Implement baseline monitoring for normal namespace creation patterns in your environment
• Review container orchestration logs for unusual namespace activity

How to Mitigate CVE-2026-23405

Immediate Actions Required

• Update the Linux kernel to a patched version that includes the namespace depth limit
• Review and restrict permissions for creating AppArmor policy namespaces
• Monitor systems for signs of exploitation attempts
• Consider temporarily limiting unprivileged namespace creation via sysctl if exposure is high

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability by implementing a hard cap on policy namespace depth, aligning it with user namespace limits. The fix has been committed to the stable kernel branches. Relevant kernel commits include:

• Kernel Commit 30603941
• Kernel Commit 3f8699b3
• Kernel Commit 7b6495ea
• Kernel Commit 853ce31c
• Kernel Commit d42b2b6b

Organizations should prioritize kernel updates through their distribution's package management system.

Workarounds

• Restrict unprivileged user namespace creation using sysctl kernel.unprivileged_userns_clone=0 where applicable
• Limit access to AppArmor policy management interfaces to trusted administrators only
• Implement resource quotas and cgroups to contain potential resource exhaustion
• Use SELinux or other MAC frameworks as alternatives if patching is delayed

# Restrict unprivileged user namespace creation (temporary mitigation)
sysctl -w kernel.unprivileged_userns_clone=0

# Make the change persistent
echo "kernel.unprivileged_userns_clone=0" >> /etc/sysctl.d/99-security.conf
sysctl --system