CVE-2025-21703 netem: Update sch->q.qlen before qdisc_tree_reduce_backlog()

What is CVE-2025-21703?

CVE-2025-21703 is a high-severity use after free vulnerability in Linux kernel, published on 18 Feb 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:

netem: Update sch->q.qlen before qdisc_tree_reduce_backlog()

qdisc_tree_reduce_backlog() notifies parent qdisc only if child qdisc becomes empty, therefore we need to reduce the backlog of the child qdisc before calling it. Otherwise it would miss the opportunity to call cops->qlen_notify(), in the case of DRR, it resulted in UAF since DRR uses ->qlen_notify() to maintain its active list.

Weakness: CWE-416

Which versions are affected by CVE-2025-21703?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2025-21703?

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-2025-21703 being exploited?

There is no known exploitation of CVE-2025-21703 in the wild and no public exploit code.

Other vulnerabilities in Linux kernel

• CVE-2026-53266 Linux Kernel Out-of-Bounds Write Exploited High 8.8
• CVE-2013-6282 Linux Kernel Improper Input Validation Exploited High 8.8
• CVE-2022-0185 Linux Kernel Heap-Based Buffer Overflow Exploited High 8.4
• CVE-2013-2094 Linux Kernel Privilege Escalation Exploited High 8.4
• CVE-2026-53362 Linux Kernel Unspecified Exploited High 7.8

All Linux kernel vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as AZL-58965, BELL-CVE-2025-21703, DEBIAN-CVE-2025-21703, GHSA-cg86-m5xc-jqrm, ROOT-OS-DEBIAN-11-CVE-2025-21703, UBUNTU-CVE-2025-21703.