CVE-2026-99275
smb: client: reject short SESSION_SETUP responses
What happened?
In the Linux kernel, the following vulnerability has been resolved:
smb: client: reject short SESSION_SETUP responses
smb2_check_message() accepts frames shorter than struct smb2_pdu when the SMB2 status is non-zero, before the minimum-PDU check added by b4694f269e66d. A 64-byte STATUS_MORE_PROCESSING_REQUIRED SESSION_SETUP response is therefore accepted even though it cannot contain the SecurityBufferOffset and SecurityBufferLength fields.
The parser then reads those fields from stale bytes left in the reused receive buffer by an earlier frame. With a long stale SecurityBufferLength, decode_ntlmssp_challenge() copies up to about 64 KB from beyond the receive object and sends the data back to the server in the NTLMv2 AUTHENTICATE target info.
Reject short MPR SESSION_SETUP responses before the short-error path, and validate SecurityBufferOffset + SecurityBufferLength against the received response length before decoding the challenge.
The ordering bypass is not new: the short-error early return predates b4694f269e66d, and the unvalidated SecurityBufferLength read predates the 2016 raw NTLMSSP split in 166cea4dc3a4, so all supported stable trees are affected. Reproduced on 6.6.87.2; the path is unchanged on mainline.
Affected software and versions
| Software | Version information | Fixed version |
|---|---|---|
| LinuxLinux | affected: 5478f9ba9a34d660eb3227dcd16314689c51f946 to < df63e01933886153e40706483b9635b2456ad91f; affected: 5478f9ba9a34d660eb3227dcd16314689c51f946 to < 4d8930afa3446041358a7db33e7b23a0bb9459b5 | See vendor advisory |
| LinuxLinux | affected: 3.6; unaffected: 0 to < 3.6; unaffected: 7.3.2 to ≤ 7.3.*; unaffected: 7.4-rc1 to ≤ * | See vendor advisory |
Version information comes from the source record and can include unaffected products and exceptions. Distribution packages may contain backported fixes: an older upstream version alone does not prove a server is affected.
Is it being exploited?
This CVE was not listed in the CISA catalogue at our last retrieval. This does not establish that exploitation is absent.
How to respond
- Confirm the product, installed package version, and any configuration requirements in the vendor advisory.
- Follow the vendor’s remediation or workaround guidance in the references below. Structured fixed-version data is unavailable here; the publisher’s description may include a fix or workaround.
- Restart affected services if required, then verify the running version. Known exploitation may warrant investigation as well as patching.