# CVE-2026-23409: apparmor: fix differential encoding verification

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2026-04-01 · Updated: 2026-05-11

## Affected

- Linux: version range not reported
- Linux: 4.17

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory per the references below.
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  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-23409

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/ff7c73744fafe944ed9a7b2b7cf6c8d5557a3d84
- https://git.kernel.org/stable/c/1439150cd3c411228b387ab5efca92199d2a659a
- https://git.kernel.org/stable/c/0fab44285445e9012674396d5c1236a67da518e0
- https://git.kernel.org/stable/c/f90e3ecd9e1ed69f1a370f866ceed1f104f3ab4a
- https://git.kernel.org/stable/c/34fc60b125ed1d4eb002c76b0664bf0619492167
- https://git.kernel.org/stable/c/623a9d211bbbb031bb1cbdb38b23487648167f8a
- https://git.kernel.org/stable/c/1ff4857fac56ac5a90ee63b24db05fa5e91a45aa
- https://git.kernel.org/stable/c/39440b137546a3aa383cfdabc605fb73811b6093

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Page: https://cve.now/CVE-2026-23409
JSON: https://cve.now/api/v1/entries/CVE-2026-23409
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