# 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.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 19 Jul 2026
- **Updated:** 15 Oct 2026
- **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, ROOT-OS-UBUNTU-2204-CVE-2026-63983, UBUNTU-CVE-2026-63983

## 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.

## Is CVE-2026-63983 being exploited?

There is no known exploitation of CVE-2026-63983 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63983 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-63983 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-63983/ (Markdown: https://insights.sbomify.com/CVE-2026-63983/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63983
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63983
- OSV: https://osv.dev/vulnerability/CVE-2026-63983

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63983/
Summaries are generated and may contain mistakes; the official description above is the record.