CVE-2026-89633 smb: client: fix OOB read/write from unvalidated DataOffset in coalesce_t2()
A missing bounds check in the Linux kernel SMB client allows remote attackers to trigger out-of-bounds memory reads and writes, potentially leading to code execution or system crashes.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89633?
CVE-2026-89633 is a critical memory corruption vulnerability in Linux, published on 11 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:
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.
Which versions are affected by CVE-2026-89633?
How do I fix CVE-2026-89633?
Apply the vendor's patch.
Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.
Is CVE-2026-89633 being exploited?
There is no known exploitation of CVE-2026-89633 in the wild and no public exploit code.
Sources
Also known as AZL-100592, BELL-CVE-2026-89633, DEBIAN-CVE-2026-89633, GHSA-6jcv-m8xc-577h, UBUNTU-CVE-2026-89633.