CVE-2026-63909 ksmbd: OOB read regression in smb_check_perm_dacl() ACE-walk loops
An out-of-bounds read in the Linux kernel ksmbd module allows attackers to read heap memory or cause a crash during SMB2 operations.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-63909?
CVE-2026-63909 is a high-severity memory corruption vulnerability in Linux, published on 19 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official 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().
Which versions are affected by CVE-2026-63909?
How do I fix CVE-2026-63909?
Apply the vendor's patch.
Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.
Is CVE-2026-63909 being exploited?
There is no known exploitation of CVE-2026-63909 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-63909, DEBIAN-CVE-2026-63909, GHSA-77x3-ghg7-w5wj, UBUNTU-CVE-2026-63909.