CVE-2026-63909

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().

Weakness

The product reads data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
• When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
• Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
• To reduce the likelihood of introducing an out-of-bounds read, ensure that you validate and ensure correct calculations for any length argument, buffer size calculation, or offset. Be especially careful of relying on a sentinel (i.e. special character such as NUL) in untrusted inputs.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/540.html

References

• https://git.kernel.org/stable/c/0e60dafe97eca61721f3db456f97d97a80c6c8ae
• https://git.kernel.org/stable/c/0fe08c5776a798f46df1fd74b331be26bdd644d6
• https://git.kernel.org/stable/c/4f7c131d2bdd7cd64b96f60d10be5ea72253f520
• https://git.kernel.org/stable/c/5500ba1d410aed1eded3eb04a76b10cfb4409334
• https://git.kernel.org/stable/c/94215d55b09445993929f4fc966061d61de74929
• https://git.kernel.org/stable/c/d333af32e4451285e427f2d9c29de3a39f6f6d48
• https://git.kernel.org/stable/c/f6324b4240cf0b26a84c33f68a1222d727ff4af2