CVE-2026-43490: ksmbd: validate inherited ACE SID length

Vulnerability summary

CVE-2026-43490 is a high (CVSS 8.8) Out-of-bounds Write vulnerability in linux. EPSS estimates a 0.43% chance of exploitation in the next 30 days, higher than 37.8% of scored vulnerabilities.

Timeline

1. 15-05-2026
   PublishedVulnerability first disclosed.
2. 30-10-2026
   Last modifiedVulnerability information updated.

Description

In the Linux kernel, the following vulnerability has been resolved: ksmbd: validate inherited ACE SID length smb_inherit_dacl() walks the parent directory DACL loaded from the security descriptor xattr. It verifies that each ACE contains the fixed SID header before using it, but does not verify that the variable-length SID described by sid.num_subauth is fully contained in the ACE. A malformed inheritable ACE can advertise more subauthorities than are present in the ACE. compare_sids() may then read past the ACE. smb_set_ace() also clamps the copied destination SID, but used the unchecked source SID count to compute the inherited ACE size. That could advance the temporary inherited ACE buffer pointer and nt_size accounting past the allocated buffer. Fix this by validating the parent ACE SID count and SID length before using the SID during inheritance. Compute the inherited ACE size from the copied SID so the size matches the bounded destination SID. Reject the inherited DACL if size accumulation would overflow smb_acl.size or the security descriptor allocation size.

• CWE-787Out-of-bounds WriteThe product writes data past the end, or before the beginning, of the intended buffer.