CVE-2026-63909: ksmbd: OOB read regression in smb_check_perm_dacl() ACE-walk loops

Severity: high (CVSS 8.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-07-19 · updated 2026-10-30
Weakness: CWE-125

Description

In the Linux kernel, the following vulnerability has been resolved: ksmbd: OOB read regression in smb_check_perm_dacl() ACE-walk loops Commit d07b26f39246 ("ksmbd: require minimum ACE size in smb_check_perm_dacl()") introduced a transposed bounds check: if (offsetof(struct smb_ace, sid) + aces_size < CIFS_SID_BASE_SIZE) Since offsetof(..sid) is 8 and CIFS_SID_BASE_SIZE is 8, this evaluates to `aces_size < 0`. Because `aces_size` is always non-negative, this check becomes dead code and never breaks the loop. Worse, that commit removed the old 4-byte guard, meaning the loop now reads `ace->size` (offset 2) even when `aces_size` is 0-3 bytes. This re-opens a 2-byte heap out-of-bounds (OOB) read past the pntsd allocation during subsequent SMB2_CREATE operations. Fix this by properly transposing the comparison to require at least 16 bytes (8-byte offset + 8-byte SID base), matching the correct form used in smb_inherit_dacl().

Affected products

Linux kernel: from 6.6.140 before 6.6.143; from 6.12.84 before 6.12.93; from 6.18.25 before 6.18.35; from 7.0.2 before 7.0.12; 7.1-rc1, 7.1-rc2, 7.1-rc3, 7.1-rc4, 7.1-rc5 — fixed in 6.6.143, 6.12.93, 6.18.35, 7.0.12
Linux kernel: from 6.6.140 before 6.6.143; from 6.12.84 before 6.12.93; from 6.18.25 before 6.18.35; from 7.0.2 before 7.0.12 — fixed in 6.12.93, 6.18.35, 6.6.143, 7.0.12

How to fix

Upgrade to a fixed version: Linux kernel 6.6.143, 6.12.93, 6.18.35, 7.0.12; Linux kernel 6.12.93, 6.18.35, 6.6.143, 7.0.12. Check the vendor advisory for mitigations.

References

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