# CVE-2025-37890: net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc

> A use-after-free flaw in the Linux kernel's HFSC scheduler allows 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:** 16 May 2025
- **Updated:** 5 Aug 2026
- **Also known as:** BELL-CVE-2025-37890, DEBIAN-CVE-2025-37890, GHSA-fwvg-7877-39cm, ROOT-OS-DEBIAN-11-CVE-2025-37890, ROOT-OS-DEBIAN-12-CVE-2025-37890, ROOT-OS-UBUNTU-2204-CVE-2025-37890, UBUNTU-CVE-2025-37890

## What is CVE-2025-37890?

CVE-2025-37890 is a high-severity use after free vulnerability in Debian Debian Linux, published on 16 May 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:

net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc

As described in Gerrard's report [1], we have a UAF case when an hfsc class
has a netem child qdisc. The crux of the issue is that hfsc is assuming
that checking for cl->qdisc->q.qlen == 0 guarantees that it hasn't inserted
the class in the vttree or eltree (which is not true for the netem
duplicate case).

This patch checks the n_active class variable to make sure that the code
won't insert the class in the vttree or eltree twice, catering for the
reentrant case.

[1] https://lore.kernel.org/netdev/CAHcdcOm+03OD2j6R0=YHKqmy=VgJ8xEOKuP6c7mSgnp-TEJJbw@mail.gmail.com/

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2025-37890?

It affects 2 products.


Products: Debian Debian Linux, Linux kernel.

## How do I fix CVE-2025-37890?

Apply the vendor's patch.

## Is CVE-2025-37890 being exploited?

There is no known exploitation of CVE-2025-37890 in the wild and no public exploit code.

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- [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild)
- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-37890 and fix it if it is affected.

What it is: CVE-2025-37890 is a high-severity use after free vulnerability in Debian Debian Linux, published on 16 May 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-2025-37890 in the wild and no public exploit code.

Affected product: Debian Debian 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-2025-37890/ (Markdown: https://insights.sbomify.com/CVE-2025-37890/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-37890
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-37890
- OSV: https://osv.dev/vulnerability/CVE-2025-37890

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-37890/
Summaries are generated and may contain mistakes; the official description above is the record.