In the Linux kernel, the following vulnerability has been resolved: net/sched: fix packet loop on netem when duplicate is on When netem duplicates a… (CVE-2026-63983)

The Linux kernel vulnerability CVE-2026-63983 involves the net/sched subsystem's netem feature, which duplicates packets for network emulation. When netem duplicates a packet, it re-enqueues the copy at the root qdisc. If another netem instance is present, the copy can be duplicated again recursively, leading to packet loops that exhaust stack or memory. The original guard used a per-qdisc state variable that was unsafe under concurrency. The fix uses the skb tc_depth field to mark packets, incrementing it on duplication and skipping duplication if tc_depth is non-zero, ensuring safe operation regardless of tree topology or concurrency. Multiple Linux kernel versions prior to 7.0.0-28.28~24.04.1 and related builds are affected. The issue is fixed in updated kernel packages provided by Ubuntu and others. The vendor advisory confirms patch availability and recommends updating and rebooting systems.

The vulnerability can cause recursive packet duplication loops in the Linux kernel's network scheduler, potentially exhausting system stack or memory resources. This could lead to denial of service conditions or system instability. There are no reports of exploitation in the wild. The impact is limited to affected kernel versions running netem with packet duplication enabled in certain configurations.

A patch is available and has been released by vendors including Ubuntu. Users should apply the updated Linux kernel packages as per their distribution's security advisories and reboot the system to activate the fixes. Due to an ABI change, recompilation and reinstallation of third-party kernel modules may be required. No additional mitigation steps are indicated beyond applying the official updates.