CVE-2025-21703

Name of the Vulnerable Software and Affected Versions: Linux Kernel versions prior to the fixed version

Description: A vulnerability in the Linux kernel has been resolved, specifically in the netem module. The issue arises from the qdisc tree reduce backlog() function not notifying the parent qdisc when the child qdisc becomes empty, unless the backlog of the child qdisc is reduced beforehand. This oversight leads to a missed opportunity to call cops->qlen notify(), resulting in a use-after-free (UAF) error in the case of the DRR (Deficit Round Robin) qdisc. The DRR relies on ->qlen notify() to maintain its active list.

Recommendations: For Linux Kernel versions prior to the fixed version, update to the latest version to resolve the issue. As a temporary workaround, consider disabling the qdisc tree reduce backlog() function until a patch is available. Restrict access to the netem module to minimize the risk of exploitation.