CVE-2026-23408: Linux Kernel Use-After-Free Vulnerability

CVE-2026-23408 Overview

CVE-2026-23408 is a double free vulnerability in the Linux kernel's AppArmor security module. The flaw resides in the aa_replace_profiles() function, where the ns_name pointer can be freed twice during profile replacement. The issue is tracked under CWE-415: Double Free and affects multiple Linux kernel branches, including the 5.5 series and 7.0 release candidates.

Exploitation requires local access with low privileges and no user interaction. Successful exploitation can lead to kernel memory corruption, with potential consequences ranging from denial of service to local privilege escalation.

Critical Impact
A local attacker with the ability to load AppArmor profiles can trigger a double free in kernel memory, potentially leading to privilege escalation or system compromise.

Affected Products

Linux Kernel (multiple stable branches prior to patched commits)
Linux Kernel 5.5
Linux Kernel 7.0 release candidates (rc1 through rc7)

Discovery Timeline

2026-04-01 - CVE-2026-23408 published to NVD
2026-04-24 - Last updated in NVD database

Technical Details for CVE-2026-23408

Vulnerability Analysis

The vulnerability exists in the AppArmor profile replacement logic within the Linux kernel security subsystem. AppArmor is a Mandatory Access Control (MAC) framework used to confine programs to a limited set of resources.

During profile replacement, the aa_replace_profiles() function calls aa_unpack() to extract a namespace name into the local ns_name variable. When ns_name is returned as NULL but the profile entry ent->ns_name contains a valid string, the code reassigns ns_name = ent->ns_name. Both pointers then reference the same memory allocation.

Later in the function, aa_load_ent_free(ent) releases the memory referenced by ent->ns_name. The function then calls kfree(ns_name) on the same address, producing a double free condition.

Root Cause

The root cause is shared ownership of a single heap allocation between two pointer variables without proper transfer of ownership semantics. The fix sets ent->ns_name = NULL after the pointer is transferred to ns_name, ensuring aa_load_ent_free() does not release the same allocation.

Attack Vector

The attack vector is local. An attacker requires the ability to invoke AppArmor profile loading operations, typically through the apparmor_parser interface or direct writes to /sys/kernel/security/apparmor/.replace. Crafted profile data can trigger the conditional path where ent->ns_name is populated while aa_unpack() returns a NULL ns_name. The resulting kernel heap double free can corrupt slab allocator metadata. Skilled attackers can leverage such corruption for privilege escalation through heap layout manipulation techniques.

No public proof-of-concept exploit is currently available for this issue.

Detection Methods for CVE-2026-23408

Indicators of Compromise

Kernel panic or oops messages referencing aa_replace_profiles, aa_load_ent_free, or kfree in the call trace
SLUB or SLAB allocator warnings such as double free detected or kernel BUG at mm/slub.c
Unexpected processes invoking apparmor_parser with crafted profile payloads

Detection Strategies

Monitor dmesg and /var/log/kern.log for slab corruption signatures and AppArmor stack traces
Audit apparmor_parser invocations and writes to /sys/kernel/security/apparmor/ interfaces by non-root or unexpected users
Track loaded kernel versions across the fleet and flag hosts running unpatched kernel builds

Monitoring Recommendations

Enable kernel hardening features such as KASAN in test environments to surface double free conditions during validation
Forward kernel logs to a centralized logging platform and alert on AppArmor-related fault signatures
Inventory local users with CAP_MAC_ADMIN and review their need for AppArmor profile modification rights

How to Mitigate CVE-2026-23408

Immediate Actions Required

Apply the upstream stable kernel patches referenced in the kernel.org commits listed below
Restrict access to AppArmor profile management interfaces to trusted administrative accounts only
Validate that only signed or vetted AppArmor profiles can be loaded on production systems

Patch Information

The Linux kernel maintainers released fixes across multiple stable branches. The patch sets ent->ns_name to NULL after transferring its pointer to ns_name, eliminating the double free. Relevant commits include Kernel Patch 18b5233, Kernel Patch 35f4cae, Kernel Patch 55ef2af, Kernel Patch 5df0c44, Kernel Patch 7998ab3, Kernel Patch 86feecc, Kernel Patch c053ae3, and Kernel Patch c6347a2.

Distribution-provided kernel updates should be applied as soon as vendors publish them.

Workarounds

Restrict CAP_MAC_ADMIN to a minimal set of administrative accounts to limit who can load AppArmor profiles
Disable AppArmor profile reloading at runtime where operationally feasible until patches are deployed
Use kernel lockdown mode or LSM stacking policies to constrain profile replacement operations

bash

# Verify running kernel and identify hosts requiring patching
uname -r

# Restrict AppArmor profile interfaces to root only
ls -l /sys/kernel/security/apparmor/.replace

# After applying distribution kernel update, reboot to load patched kernel
sudo apt-get update && sudo apt-get install --only-upgrade linux-image-generic
sudo reboot