CVE-2026-23407: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-23407 Overview

A bounds check bypass vulnerability has been discovered in the Linux kernel's AppArmor security module, specifically in the verify_dfa() function. The vulnerability occurs when processing differentially encoded DFA (Deterministic Finite Automaton) tables, where the DEFAULT_TABLE entries are not properly validated. This missing bounds check allows a malformed DFA with DEFAULT_TABLE[j] >= state_count to cause out-of-bounds reads and writes in kernel memory.

Critical Impact

Local attackers with limited privileges can exploit this vulnerability to read or write arbitrary kernel memory, potentially leading to privilege escalation, information disclosure, or system compromise.

Affected Products

• Linux kernel versions with AppArmor enabled
• Systems running affected kernel versions with AppArmor security profiles loaded
• Linux distributions using AppArmor as the default security module (Ubuntu, SUSE, etc.)

Discovery Timeline

• 2026-04-01 - CVE-2026-23407 published to NVD
• 2026-04-02 - Last updated in NVD database

Technical Details for CVE-2026-23407

Vulnerability Analysis

This vulnerability exists in the AppArmor DFA verification logic within the Linux kernel. The verify_dfa() function is responsible for validating DFA tables used by AppArmor for policy enforcement. When processing differential encoding chains, the function reads values from DEFAULT_TABLE and uses them as array indices without proper validation.

The KASAN (Kernel Address Sanitizer) report clearly identifies the issue as a slab-out-of-bounds read in the verify_dfa() function at offset 0x59a/0x660. The call trace shows the vulnerability is triggered through the policy loading path: aa_replace_profiles() → aa_unpack() → unpack_pdb() → aa_dfa_unpack() → verify_dfa().

The vulnerability can be exploited locally by loading a malformed AppArmor profile containing a crafted DFA structure with invalid DEFAULT_TABLE entries. Since AppArmor profiles can be loaded by processes with CAP_MAC_ADMIN capability or through certain system configurations, local privilege escalation is achievable.

Root Cause

The root cause is a conditional bounds check in the verify_dfa() function that only validates DEFAULT_TABLE bounds when the state is not differentially encoded. When the verification loop traverses the differential encoding chain, it reads k = DEFAULT_TABLE[j] and uses k as an array index without validating that k is within the valid state count range. This allows malformed DFAs to specify out-of-bounds indices that lead to memory corruption.

The fix removes the MATCH_FLAG_DIFF_ENCODE condition to validate all DEFAULT_TABLE entries unconditionally, ensuring that every index accessed is within bounds regardless of the encoding method used.

Attack Vector

The attack requires local access and the ability to load AppArmor profiles. An attacker can craft a malicious AppArmor profile containing a DFA with DEFAULT_TABLE entries pointing beyond the allocated state array. When this profile is loaded and verified, the kernel reads and writes memory outside the intended buffer, which can be leveraged for:

1. Reading sensitive kernel memory (information disclosure)
2. Corrupting adjacent slab objects (privilege escalation)
3. Causing kernel crashes (denial of service)

The exploitation path follows the vfs_write() → profile_replace() → policy_update() → aa_replace_profiles() chain, making it exploitable through the standard policy loading interface.

Detection Methods for CVE-2026-23407

Indicators of Compromise

• KASAN warnings or kernel oops messages referencing verify_dfa() in kernel logs
• Unusual AppArmor profile loading attempts from non-administrative processes
• Kernel crash dumps showing corruption in AppArmor-related slab caches
• Audit logs showing attempts to load profiles with unusual or malformed content

Detection Strategies

• Monitor kernel logs for KASAN slab-out-of-bounds errors related to verify_dfa, aa_dfa_unpack, or unpack_pdb functions
• Implement audit rules to track AppArmor profile loading operations via /sys/kernel/security/apparmor/ interfaces
• Deploy runtime kernel monitoring to detect anomalous memory access patterns in security subsystems
• Use kernel live patching detection tools to verify kernel integrity

Monitoring Recommendations

• Enable KASAN in development and testing environments to detect exploitation attempts
• Configure auditd to log all writes to AppArmor policy interfaces: /sys/kernel/security/apparmor/.replace and /sys/kernel/security/apparmor/.load
• Monitor for processes with CAP_MAC_ADMIN capability performing unusual file operations
• Implement syslog alerting for any kernel messages containing verify_dfa or aa_dfa_unpack

How to Mitigate CVE-2026-23407

Immediate Actions Required

• Update the Linux kernel to a patched version that includes the bounds check fix
• Restrict access to AppArmor profile loading interfaces to trusted administrators only
• Review and limit which processes have CAP_MAC_ADMIN capability
• Monitor systems for signs of exploitation while awaiting patch deployment

Patch Information

Multiple kernel patches have been released to address this vulnerability. The fix removes the conditional bounds checking and validates all DEFAULT_TABLE entries unconditionally. The following git commits contain the security fix:

• Kernel Git Commit 5a68e46
• Kernel Git Commit 76b4d36
• Kernel Git Commit 7c7cf05
• Kernel Git Commit d352873
• Kernel Git Commit f39e126

Workarounds

• If immediate patching is not possible, disable AppArmor temporarily using systemctl stop apparmor and systemctl disable apparmor (note: this removes AppArmor security protections)
• Restrict write access to /sys/kernel/security/apparmor/ to prevent unauthorized profile loading
• Implement additional access controls using SELinux or other security modules if AppArmor must be disabled
• Use container isolation to limit potential impact of exploitation on multi-tenant systems

# Restrict AppArmor profile loading interface permissions
chmod 600 /sys/kernel/security/apparmor/.replace
chmod 600 /sys/kernel/security/apparmor/.load

# Monitor for AppArmor-related kernel errors
dmesg -w | grep -E "(verify_dfa|aa_dfa_unpack|KASAN.*apparmor)"

# Audit AppArmor policy operations
auditctl -w /sys/kernel/security/apparmor/ -p wa -k apparmor_policy