CVE-2025-21971: Linux Kernel

Medium severity, CVSS 5.5. EPSS: 0.2% chance of exploitation in the next 30 days.

In the Linux kernel, the following vulnerability has been resolved: net_sched: Prevent creation of classes with TC_H_ROOT The function qdisc_tree_reduce_backlog() uses TC_H_ROOT as a termination condition when traversing up the qdisc tree to update parent backlog counters. However, if a class is created with classid TC_H_ROOT, the traversal terminates prematurely at this class instead of reaching the actual root qdisc, causing parent statistics to be incorrectly maintained. In case of DRR, this could lead to a crash as reported by Mingi Cho. Prevent the creation of any Qdisc class with classid TC_H_ROOT (0xFFFFFFFF) across all qdisc types, as suggested by Jamal.

Affected products

• Linux Linux Kernel: from 2.6.26, before 5.4.292 (fixed in 5.4.292); from 5.5, before 5.10.236 (fixed in 5.10.236); from 5.11, before 5.15.180 (fixed in 5.15.180); from 5.16, before 6.1.132 (fixed in 6.1.132); from 6.2, before 6.6.84 (fixed in 6.6.84); from 6.7, before 6.12.20 (fixed in 6.12.20); …

Published 2025-04-01. Last modified 2026-07-30.