# CVE-2026-31478: ksmbd: replace hardcoded hdr2_len with offsetof() in smb2_calc_max_out_buf_len()

CVE-2026-31478 is fixed in linux on 1 platform, first by DEBIAN-CVE-2026-31478, released May 15, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-31478
- Priority: High
- CVSS: 9.8 (Important)
- EPSS: 0.0050
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Apr 22, 2026
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-31478

## Description

In the Linux kernel, the following vulnerability has been resolved:  ksmbd: replace hardcoded hdr2_len with offsetof() in smb2_calc_max_out_buf_len()  After this commit (e2b76ab8b5c9 "ksmbd: add support for read compound"), response buffer management was changed to use dynamic iov array. In the new design, smb2_calc_max_out_buf_len() expects the second argument (hdr2_len) to be the offset of ->Buffer field in the response structure, not a hardcoded magic number. Fix the remaining call sites to use the correct offsetof() value.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 12.15 (bookworm) | 6.1.170-1 | [DEBIAN-CVE-2026-31478](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31478.md) | May 15, 2026 |
| Linux | linux Debian 13.5 (trixie) | 6.12.85-1 | [DEBIAN-CVE-2026-31478](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31478.md) | May 15, 2026 |
| Linux | linux Debian 14 | 6.19.11-1 | [DEBIAN-CVE-2026-31478](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31478.md) | May 15, 2026 |

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Data as of 2026-11-08T19:12:23Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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