# CVE-2025-39977: futex: Prevent use-after-free during requeue-PI

> A race condition in the Linux kernel futex subsystem allows a use-after-free, potentially enabling local attackers to execute arbitrary code or escalate privileges.

- **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:** 15 Oct 2025
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-68525, BELL-CVE-2025-39977, DEBIAN-CVE-2025-39977, GHSA-56h7-2ch6-v472, ROOT-OS-DEBIAN-12-CVE-2025-39977, ROOT-OS-DEBIAN-13-CVE-2025-39977, ROOT-OS-UBUNTU-2204-CVE-2025-39977, ROOT-OS-UBUNTU-2404-CVE-2025-39977, UBUNTU-CVE-2025-39977

## What is CVE-2025-39977?

CVE-2025-39977 is a high-severity use after free vulnerability in Linux, published on 15 Oct 2025. 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:

futex: Prevent use-after-free during requeue-PI

syzbot managed to trigger the following race:

   T1                               T2

 futex_wait_requeue_pi()
   futex_do_wait()
     schedule()
                               futex_requeue()
                                 futex_proxy_trylock_atomic()
                                   futex_requeue_pi_prepare()
                                   requeue_pi_wake_futex()
                                     futex_requeue_pi_complete()
                                      /* preempt */

         * timeout/ signal wakes T1 *

   futex_requeue_pi_wakeup_sync() // Q_REQUEUE_PI_LOCKED
   futex_hash_put()
  // back to userland, on stack futex_q is garbage

                                      /* back */
                                     wake_up_state(q->task, TASK_NORMAL);

In this scenario futex_wait_requeue_pi() is able to leave without using
futex_q::lock_ptr for synchronization.

This can be prevented by reading futex_q::task before updating the
futex_q::requeue_state. A reference on the task_struct is not needed
because requeue_pi_wake_futex() is invoked with a spinlock_t held which
implies a RCU read section.

Even if T1 terminates immediately after, the task_struct will remain valid
during T2's wake_up_state().  A READ_ONCE on futex_q::task before
futex_requeue_pi_complete() is enough because it ensures that the variable
is read before the state is updated.

Read futex_q::task before updating the requeue state, use it for the
following wakeup.

## Which versions are affected by CVE-2025-39977?


## How do I fix CVE-2025-39977?

Apply the vendor's patch.

## Is CVE-2025-39977 being exploited?

There is no known exploitation of CVE-2025-39977 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-39977 and fix it if it is affected.

What it is: CVE-2025-39977 is a high-severity use after free vulnerability in Linux, published on 15 Oct 2025. 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-2025-39977 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-2025-39977/ (Markdown: https://insights.sbomify.com/CVE-2025-39977/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-39977
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-39977
- OSV: https://osv.dev/vulnerability/CVE-2025-39977

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-39977/
Summaries are generated and may contain mistakes; the official description above is the record.