CVE-2026-63983 net/sched: fix packet loop on netem when duplicate is on
A remote attacker can cause a denial of service by triggering infinite packet duplication in the Linux kernel netem scheduler when configured with duplication enabled.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-63983?
CVE-2026-63983 is a denial of service vulnerability in Linux, published on 19 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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 packet it re-enqueues the copy at the root qdisc. If another netem sits in the tree the copy can be duplicated again, recursing until the stack or memory is exhausted.
The original duplication guard temporarily zeroed q->duplicate around the re-enqueue, but that does not cover all cases because it is per-qdisc state shared across all concurrent enqueue paths and is not safe without additional locking.
Use the skb tc_depth field introduced in an earlier patch: - increment it on the duplicate before re-enqueue - skip duplication for any skb whose tc_depth is already non-zero.
This marks the packet itself rather than mutating qdisc state, therefore it is safe regardless of tree topology or concurrency.
Which versions are affected by CVE-2026-63983?
How do I fix CVE-2026-63983?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-63983 being exploited?
There is no known exploitation of CVE-2026-63983 in the wild and no public exploit code.
Sources
Also known as AZL-92772, BELL-CVE-2026-63983, DEBIAN-CVE-2026-63983, GHSA-rj36-qxvm-r8rm, ROOT-OS-DEBIAN-11-CVE-2026-63983, ROOT-OS-DEBIAN-12-CVE-2026-63983 and 2 more.