CVE-2026-23409 apparmor: fix differential encoding verification

What is CVE-2026-23409?

CVE-2026-23409 is a medium-severity denial of service vulnerability in Linux kernel, published on 1 Apr 2026. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

In the Linux kernel, the following vulnerability has been resolved:

apparmor: fix differential encoding verification

Differential encoding allows loops to be created if it is abused. To prevent this the unpack should verify that a diff-encode chain terminates.

Unfortunately the differential encode verification had two bugs.

1. it conflated states that had gone through check and already been marked, with states that were currently being checked and marked. This means that loops in the current chain being verified are treated as a chain that has already been verified.

2. the order bailout on already checked states compared current chain check iterators j,k instead of using the outer loop iterator i. Meaning a step backwards in states in the current chain verification was being mistaken for moving to an already verified state.

Move to a double mark scheme where already verified states get a different mark, than the current chain being kept. This enables us to also drop the backwards verification check that was the cause of the second error as any already verified state is already marked.

Weakness: CWE-835

Which versions are affected by CVE-2026-23409?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2026-23409?

Apply the vendor's patch.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-23409 being exploited?

There is no known exploitation of CVE-2026-23409 in the wild and no public exploit code.

Other vulnerabilities in Linux kernel

• CVE-2026-53266 Linux Kernel Out-of-Bounds Write Exploited High 8.8
• CVE-2013-6282 Linux Kernel Improper Input Validation Exploited High 8.8
• CVE-2022-0185 Linux Kernel Heap-Based Buffer Overflow Exploited High 8.4
• CVE-2013-2094 Linux Kernel Privilege Escalation Exploited High 8.4
• CVE-2026-53362 Linux Kernel Unspecified Exploited High 7.8

All Linux kernel vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as AZL-82031, BELL-CVE-2026-23409, DEBIAN-CVE-2026-23409, GHSA-53jf-v56h-xgqg, UBUNTU-CVE-2026-23409.