# CVE-2026-23225: sched/mmcid: Don't assume CID is CPU owned on mode switch

> A use-after-free vulnerability in the Linux kernel scheduler allows local attackers to execute arbitrary code or escalate privileges via an out-of-bounds access.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 Feb 2026
- **Updated:** 5 Aug 2026
- **Also known as:** GHSA-p525-h9pq-233r, ROOT-OS-DEBIAN-11-CVE-2026-23225, ROOT-OS-DEBIAN-12-CVE-2026-23225, ROOT-OS-DEBIAN-13-CVE-2026-23225, UBUNTU-CVE-2026-23225

## What is CVE-2026-23225?

CVE-2026-23225 is a high-severity use after free vulnerability in Linux, published on 18 Feb 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:

sched/mmcid: Don't assume CID is CPU owned on mode switch

Shinichiro reported a KASAN UAF, which is actually an out of bounds access
in the MMCID management code.

   CPU0						CPU1
   						T1 runs in userspace
   T0: fork(T4) -> Switch to per CPU CID mode
         fixup() set MM_CID_TRANSIT on T1/CPU1
   T4 exit()
   T3 exit()
   T2 exit()
						T1 exit() switch to per task mode
						 ---> Out of bounds access.

As T1 has not scheduled after T0 set the TRANSIT bit, it exits with the
TRANSIT bit set. sched_mm_cid_remove_user() clears the TRANSIT bit in
the task and drops the CID, but it does not touch the per CPU storage.
That's functionally correct because a CID is only owned by the CPU when
the ONCPU bit is set, which is mutually exclusive with the TRANSIT flag.

Now sched_mm_cid_exit() assumes that the CID is CPU owned because the
prior mode was per CPU. It invokes mm_drop_cid_on_cpu() which clears the
not set ONCPU bit and then invokes clear_bit() with an insanely large
bit number because TRANSIT is set (bit 29).

Prevent that by actually validating that the CID is CPU owned in
mm_drop_cid_on_cpu().

## Which versions are affected by CVE-2026-23225?


## How do I fix CVE-2026-23225?

Apply the vendor's patch.

## Is CVE-2026-23225 being exploited?

There is no known exploitation of CVE-2026-23225 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-23225 and fix it if it is affected.

What it is: CVE-2026-23225 is a high-severity use after free vulnerability in Linux, published on 18 Feb 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-23225 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-23225/ (Markdown: https://insights.sbomify.com/CVE-2026-23225/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23225
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23225
- OSV: https://osv.dev/vulnerability/CVE-2026-23225

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23225/
Summaries are generated and may contain mistakes; the official description above is the record.