# CVE-2026-90054: tcp: fix corruption of urgent data on multi-segment retransmit

> A flaw in Linux kernel TCP retransmission corrupts urgent data, causing out-of-band bytes to be delivered as in-band data, potentially disrupting application logic.

- **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:** 17 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-102498, BELL-CVE-2026-90054, DEBIAN-CVE-2026-90054, GHSA-cwq3-366f-g758, UBUNTU-CVE-2026-90054

## What is CVE-2026-90054?

CVE-2026-90054 is a vulnerability in Linux, published on 17 Sep 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:

tcp: fix corruption of urgent data on multi-segment retransmit

On the normal xmit path, while in urgent mode we refuse to build a
multi-segment TSO packet, so every segment gets its own urg_ptr:

	/* tcp_write_xmit() */
	limit = mss_now;
	if (tso_segs > 1 && !tcp_urg_mode(tp))
		limit = tcp_mss_split_point(...);

The retransmit path has no such guard. __tcp_retransmit_skb() builds a
segs > 1 skb and hands it to the GSO layer, which only advances th->seq
per segment and copies urg_ptr verbatim:

	/* __tcp_retransmit_skb() */
	len = cur_mss * segs;		/* segs > 1, no urg_mode check */
	...
	/* tcp_gso_segment(): bumps seq only, urg_ptr is copied */

urg_ptr is an offset from the segment's own seq, so a copied value points
at a different place on each segment. The receiver rebuilds the absolute
urgent seq as seg.seq + urg_ptr, so it walks a moving urgent point instead
of the one OOB byte:

	seg1  seq 1     urg_ptr 5001 -> urgent @ 5001   (ok)
	seg2  seq 1001  urg_ptr 5001 -> urgent @ 6001   (wrong, +MSS)
	seg3  seq 2001  urg_ptr 5001 -> urgent @ 7001   (wrong, +2*MSS)

The real OOB byte is never pointed at, so the receiver stops splicing it
out and delivers it as normal in-band data, corrupting the stream.

Guard the retransmit length like the xmit path: keep segs = 1 while in
urgent mode.

## Which versions are affected by CVE-2026-90054?


## How do I fix CVE-2026-90054?

Apply the vendor's patch.

## Is CVE-2026-90054 being exploited?

There is no known exploitation of CVE-2026-90054 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90054 and fix it if it is affected.

What it is: CVE-2026-90054 is a vulnerability in Linux, published on 17 Sep 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-2026-90054 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-2026-90054/ (Markdown: https://insights.sbomify.com/CVE-2026-90054/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90054
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90054
- OSV: https://osv.dev/vulnerability/CVE-2026-90054

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90054/
Summaries are generated and may contain mistakes; the official description above is the record.