# CVE-2026-31611: ksmbd: require 3 sub-authorities before reading sub_auth[2]

CVE-2026-31611 is fixed in linux on 1 platform, first by DEBIAN-CVE-2026-31611, released May 15, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-31611
- Priority: High
- CVSS: 8.6 (Moderate)
- EPSS: 0.0038
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Apr 24, 2026
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-31611

## Description

In the Linux kernel, the following vulnerability has been resolved:  ksmbd: require 3 sub-authorities before reading sub_auth[2]  parse_dacl() compares each ACE SID against sid_unix_NFS_mode and on match reads sid.sub_auth[2] as the file mode.  If sid_unix_NFS_mode is the prefix S-1-5-88-3 with num_subauth = 2 then compare_sids() compares only min(num_subauth, 2) sub-authorities so a client SID with num_subauth = 2 and sub_auth = {88, 3} will match.  If num_subauth = 2 and the ACE is placed at the very end of the security descriptor, sub_auth[2] will be  4 bytes past end_of_acl.  The out-of-band bytes will then be masked to the low 9 bits and applied as the file's POSIX mode, probably not something that is good to have happen.  Fix this up by forcing the SID to actually carry a third sub-authority before reading it at all.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 12.15 (bookworm) | 6.1.176-1 | [DEBIAN-CVE-2026-31611](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31611.md) | May 15, 2026 |
| Linux | linux Debian 13.5 (trixie) | 6.12.85-1 | [DEBIAN-CVE-2026-31611](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31611.md) | May 15, 2026 |
| Linux | linux Debian 14 | 6.19.14-1 | [DEBIAN-CVE-2026-31611](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31611.md) | May 15, 2026 |

---

Data as of 2026-11-09T13:31:49Z 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 Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

Free to use with attribution: show "Data: Patchblox Patch Radar" with a link to https://patchblox.com/security/patch-radar. Terms, including what needs a written agreement: https://patchblox.com/security/patch-radar/use-the-data

Product names, logos and trademarks shown on Patch Radar belong to their respective owners. They are used only to identify the software each security fix applies to and to help readers find their platform. Their use does not imply any affiliation with, or endorsement by, those owners, and Patchblox is not affiliated with any of them.