smb: client: fix OOB read/write from unvalidated DataOffset in coalesce_t2()
CVE-2026-89633 · published 11 Sept 2026
How serious is it
- Severity: critical, CVSS score 9.8 out of 10.
- No exploit prediction (EPSS) score yet.
- Not on the CISA list of exploited vulnerabilities.
- No public exploit in Exploit-DB.
Is your app affected by CVE-2026-89633?
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Who is affected
- Linux Linux: e4eb295d38b57f4d4b956942a48887eb252d97c6 to < 6eb70b78e48b0031bd1a05fe1b58959a48e87739, e4eb295d38b57f4d4b956942a48887eb252d97c6 to < 36d01415d881d5326a0dd3384f781cd7e69aff2d, e4eb295d38b57f4d4b956942a48887eb252d97c6 to < 672cf86aa6aa0fb4012ce4c3b3498df42ad67a4e, e4eb295d38b57f4d4b956942a48887eb252d97c6 to < 033bc80019f07d158630df4e69b19a49010f54f1
- Linux Linux: 2.6.12
How to check
- Find out whether you run the affected product and which version, then compare it with the versions above.
How to fix
Copies a prompt for Claude Code, Cursor or Codex that asks it to check whether you run this product.
- Apply the vendor's update or mitigation from the references below. If there is none yet, limit exposure of the affected service until there is.
- If a vulnerable version was live, check your logs and rotate the secrets that server could read.
The advisory
In the Linux kernel, the following vulnerability has been resolved:
smb: client: fix OOB read/write from unvalidated DataOffset in coalesce_t2()
coalesce_t2() computes data pointers directly from server-supplied
DataOffset fields with no validation against buffer bounds:
data_area_of_tgt = (char *)&pSMBt->hdr.Protocol +
get_unaligned_le16(&pSMBt->t2_rsp.DataOffset);
data_area_of_src = (char *)&pSMBs->hdr.Protocol +
get_unaligned_le16(&pSMBs->t2_rsp.DataOffset);
data_area_of_tgt += total_in_tgt;
...
memcpy(data_area_of_tgt, data_area_of_src, total_in_src);
A small DataOffset can push a pointer below the actual byte area,
overwriting header fields; a large one can push it past the buffer
end, causing out-of-bounds heap reads (source) or writes (target).
The BCC overflow guard does not prevent this: BCC reflects how much
data is present, while DataOffset controls where in the buffer it
starts.
The "validate target area" comment present since the function was
first written in 2005 was a placeholder that was never implemented.
Add lower- and upper-bound checks for both data pointers before the
memcpy, and before any target header fields are modified.
Check your whole app
Dependencies are only half of it. Free and read-only: before you push, run npx shippable-dev scan localhost:3000 in your project folder, no account needed. For your live app, you prove it's yours with a one-line tag first.
References
- git.kernel.org/stable/c/6eb70b78e48b0031bd1a05fe1b58959a48e87739
- git.kernel.org/stable/c/36d01415d881d5326a0dd3384f781cd7e69aff2d
- git.kernel.org/stable/c/672cf86aa6aa0fb4012ce4c3b3498df42ad67a4e
- git.kernel.org/stable/c/033bc80019f07d158630df4e69b19a49010f54f1
- git.kernel.org/stable/c/6343c1da561962688f203362d80d6a3bfa39fa1b
Merged from CVE List. Last updated 26 Oct 2026. All vulnerabilities · RSS · JSON