CVE-2025-37797: net_sched: hfsc: Fix a UAF vulnerability in class handling

Severity: high (CVSS 7.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-05-02 · updated 2026-07-30
Weakness: CWE-416

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.

Affected products

Debian Linux: 11.0
Linux kernel: from 4.14.1 before 5.4.293; from 5.5 before 5.10.237; from 5.11 before 5.15.181; from 5.16 before 6.1.136; from 6.2 before 6.6.89; from 6.7 before 6.12.26 — fixed in 5.10.237, 5.15.181, 5.4.293, 6.1.136, 6.12.26, 6.14.5

How to fix

Upgrade to a fixed version: Linux kernel 5.10.237, 5.15.181, 5.4.293, 6.1.136. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2025-37797 or the next vulnerability affects your stack