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

High 7.8 Published 16 May 2025 Updated 5 Aug 2026

A use-after-free flaw in the Linux kernel's HFSC scheduler allows local attackers to execute arbitrary code or escalate privileges.

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

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

Which versions are affected by CVE-2025-37890?

It affects 2 products.

Products include Debian Debian Linux, Linux kernel.

How do I fix CVE-2025-37890?

Apply the vendor's patch.

Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.

Is CVE-2025-37890 being exploited?

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

All Debian Debian Linux vulnerabilities

Sources

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 and 1 more.