CVE-2026-23407: 
Linux Kernel vulnerability analysis and mitigation

CVE-2026-23407 is a missing bounds check vulnerability in the Linux kernel's AppArmor subsystem, specifically within the verify_dfa() function. The flaw was disclosed on April 1, 2026, and affects Linux kernel versions from 4.17 through multiple stable branches up to 6.19.7, as well as Microsoft Azure Linux kernel 6.6.0.0-1. It carries a CVSS v3.1 base score of 7.8 (High), requiring only local access and low privileges to exploit (Github Advisory, Red Hat Bugzilla).

The root cause is improper validation of a specified index in the verify_dfa() function (CWE-1285, CWE-125). The function conditionally checks DEFAULT_TABLE bounds only when a DFA state is not differentially encoded; when traversing the differential encoding chain, it reads k = DEFAULT_TABLE[j] and uses k as an array index without any bounds validation. A locally-privileged attacker can craft a malformed AppArmor DFA policy where DEFAULT_TABLE[j] >= state_count, triggering both out-of-bounds reads and writes in kernel slab memory. The fix removes the MATCH_FLAG_DIFF_ENCODE condition so that all DEFAULT_TABLE entries are validated unconditionally (Github Advisory, Red Hat Bugzilla).

Successful exploitation results in kernel slab out-of-bounds reads and writes, leading to kernel information disclosure, memory corruption, and potential local privilege escalation to root. An attacker with low-privilege local access who can load AppArmor policies (via the policy_update / aa_replace_profiles path) could cause a denial of service (kernel crash) or potentially execute arbitrary code with kernel privileges. The vulnerability affects the confidentiality, integrity, and availability of the entire system (Github Advisory, Feedly).

There is no public proof-of-concept exploit and no evidence of in-the-wild exploitation as of the time of reporting (Feedly). The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. The EPSS score is approximately 0.018% (1st percentile), indicating a very low near-term probability of exploitation. Exploitation requires local access and the ability to load AppArmor policies, which limits the attack surface to privileged or semi-privileged local users.

1. Gain local access: Obtain a low-privileged local account on a system running a vulnerable Linux kernel version (4.17 through 5.10.252, 5.11–5.15.202, 5.16–6.1.168, 6.2–6.6.129, 6.7–6.12.76, 6.13–6.18.17, or 6.19–6.19.7) with AppArmor enabled.
2. Craft malformed AppArmor policy: Create a binary AppArmor DFA policy blob where the DEFAULT_TABLE contains entries with values greater than or equal to state_count, and the affected states are marked with the MATCH_FLAG_DIFF_ENCODE flag to bypass the existing bounds check.
3. Load the malicious policy: Write the crafted policy blob to the AppArmor policy interface (e.g., /sys/kernel/security/apparmor/.replace or via aa_replace_profiles), triggering the policy_update → aa_replace_profiles → aa_unpack → unpack_pdb → aa_dfa_unpack → verify_dfa call chain.
4. Trigger out-of-bounds access: The verify_dfa() function reads the out-of-bounds DEFAULT_TABLE[j] value and uses it as an array index, causing a slab out-of-bounds read/write in kernel memory.
5. Achieve privilege escalation or DoS: Depending on heap layout and kernel hardening, the memory corruption may be leveraged for local privilege escalation to root, or at minimum cause a kernel panic (denial of service) (Github Advisory, Red Hat Bugzilla).

• Logs: KASAN (Kernel Address Sanitizer) reports in kernel logs such as BUG: KASAN: slab-out-of-bounds in verify_dfa+0x59a/0x660 and Read of size 4 at addr ffff888100eadec4; kernel oops or panic messages referencing verify_dfa, aa_dfa_unpack, unpack_pdb, or aa_replace_profiles in the call trace.
• Process: Unusual processes (e.g., su, custom binaries) writing to AppArmor policy interfaces (/sys/kernel/security/apparmor/.replace or .load) with unexpected or malformed policy data.
• File System: Presence of crafted AppArmor policy files with anomalous binary DFA structures; unexpected writes to AppArmor securityfs endpoints by non-administrative users.
• Network: No direct network indicators; however, lateral movement following privilege escalation may manifest as unexpected outbound connections from the compromised host.

Apply kernel security updates to patched versions: Linux 5.10.253+, 5.15.203+, 6.1.169+, 6.6.130+, 6.12.77+, 6.18.18+, 6.19.8+, or 7.0+. Patches are available via multiple stable branch commits on git.kernel.org. Ubuntu has issued security notices (USN-8152-1, USN-8163-1, USN-8163-2, USN-8201-1, USN-8224-1, USN-8243-1) and SUSE has released advisories (SUSE-SU-2026:2111-1, SUSE-SU-2026:2195-1, SUSE-SU-2026:2215-1, SUSE-SU-2026:2238-1). As a workaround, restrict AppArmor policy loading permissions to trusted administrators only; if immediate patching is not feasible and high availability is not required, consider temporarily disabling AppArmor (Github Advisory, Red Hat Bugzilla).

Qualys published a blog post titled "CrackArmor: Critical AppArmor Flaws Enable Local Privilege Escalation to Root" covering this and related AppArmor vulnerabilities, indicating notable researcher interest in the AppArmor attack surface. The vulnerability was announced via the official Linux kernel CVE mailing list and picked up by oss-security shortly after disclosure. Multiple Linux distributions (Ubuntu, SUSE, openSUSE) issued timely security advisories, and Microsoft acknowledged the impact on Azure Linux. Community reaction has been measured, consistent with a local-only, no-PoC kernel vulnerability.