CVE-2026-68100

In the Linux kernel, the following vulnerability has been resolved:

ksmbd: validate num_subauth when copying ACE in set_ntacl_dacl

set_ntacl_dacl() copies each ACE from the attacker-controlled stored security descriptor verbatim into the response DACL without checking sid.num_subauth. The ACE bytes (including an unchecked num_subauth) originate from an authenticated SMB2_SET_INFO(SecInfo=DACL) that is stored raw via ksmbd_vfs_set_sd_xattr(); parse_dacl() rejects a bad ACE with break rather than an error, so parse_sec_desc() still returns success and the malformed SD reaches the xattr intact.

On a subsequent SMB2_QUERY_INFO(SecInfo=DACL) for an inode carrying a POSIX access ACL, build_sec_desc() -> set_ntacl_dacl() -> set_posix_acl_entries_dacl() walks the copied ACEs and reads

ntace->sid.sub_auth[ntace->sid.num_subauth - 1]

with num_subauth taken straight from the stored SD. Since sub_auth[] is fixed at SID_MAX_SUB_AUTHORITIES (15), a crafted num_subauth (e.g. 255) drives an out-of-bounds heap read of ~1 KB with an offset fully controlled by an authenticated client.

The sibling functions already gate this field: parse_dacl() – num_subauth == 0 || > SID_MAX_SUB_AUTHORITIES parse_sid() – num_subauth > SID_MAX_SUB_AUTHORITIES smb_copy_sid() – min_t(u8, num_subauth, SID_MAX_SUB_AUTHORITIES) set_ntacl_dacl() is the lone inconsistent path that omits the check.

Add the same num_subauth validation in set_ntacl_dacl() before copying the ACE, matching the gate already enforced by parse_dacl().

Affected Software

References

• https://git.kernel.org/stable/c/47f0b34f6bc98ed85bfdc293e8f3e432ec24958d
• https://git.kernel.org/stable/c/5acbd3012fd4a7ccfebd91ea6f784120084eb897
• https://git.kernel.org/stable/c/b6d3cc6a524416dfdb2b47e4bba2e7e20011d056
• https://git.kernel.org/stable/c/e31fada5143784bc05c7ae44c79eed9b7a2e147e
• https://git.kernel.org/stable/c/fb3dc8e6da46a1ccad1956cda57de29d9b3033e0