CVE-2026-31611
Published: 24 April 2026
Summary
CVE-2026-31611 is a high-severity an unspecified weakness vulnerability in Linux Linux Kernel. Its CVSS base score is 8.6 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 28.6th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 SI-10 (Information Input Validation) and SI-2 (Flaw Remediation).
Deeper analysis
CVE-2026-31611 is a vulnerability in the Linux kernel's ksmbd module, which implements the SMB server protocol. The issue lies in the parse_dacl() function, where Access Control Entry (ACE) Security Identifiers (SIDs) are compared against sid_unix_NFS_mode. If the prefix S-1-5-88-3 matches with num_subauth set to 2, a client-provided SID with only two sub-authorities can match. When such an ACE is positioned at the end of the security descriptor, accessing sid.sub_auth[2] reads four bytes past the end of the ACL, and these out-of-bounds bytes are masked to the low nine bits and applied as the file's POSIX mode.
A remote, unauthenticated attacker with network access can exploit this vulnerability with low complexity and no user interaction, as indicated by its CVSS v3.1 base score of 8.6 (AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H). By crafting a malicious SMB request with a specially constructed SID that matches the prefix but lacks a third sub-authority, and placing the ACE at the end of the descriptor, the attacker can cause the kernel to read out-of-bounds memory. This allows setting arbitrary low nine bits of a file's POSIX permissions, potentially leading to low confidentiality and integrity impacts alongside high availability disruption through malformed modes.
Kernel stable branch patches address the issue by enforcing the presence of three sub-authorities in the SID before accessing sub_auth[2], preventing the out-of-bounds read. Relevant commits include 08f9e6d899b5c834bbcc239eae1bed58d9b15d2c, 46bbcd3ebfb3549c8da1838fc4493e79bd3241e7, 53370cf9090777774e07fd9a8ebce67c6cc333ab, 9401f86a224f37b50e6a3ccf1d46a70d5ef8af0a, and b5b5d5936a50497fb151c0b122899a6894721c2b, available in the Linux kernel stable repository. Security practitioners should ensure affected kernels are updated to incorporate these fixes.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-25504
Vulnerability details
In the Linux kernel, the following vulnerability has been resolved: ksmbd: require 3 sub-authorities before reading sub_auth[2] parse_dacl() compares each ACE SID against sid_unix_NFS_mode and on match reads sid.sub_auth[2] as the file mode. If sid_unix_NFS_mode is the prefix S-1-5-88-3 with…
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num_subauth = 2 then compare_sids() compares only min(num_subauth, 2) sub-authorities so a client SID with num_subauth = 2 and sub_auth = {88, 3} will match. If num_subauth = 2 and the ACE is placed at the very end of the security descriptor, sub_auth[2] will be 4 bytes past end_of_acl. The out-of-band bytes will then be masked to the low 9 bits and applied as the file's POSIX mode, probably not something that is good to have happen. Fix this up by forcing the SID to actually carry a third sub-authority before reading it at all.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Remote unauthenticated network exploit against ksmbd SMB server directly matches T1190; resulting unauthorized modification of file POSIX mode bits directly matches T1222.002.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Directly mitigates the vulnerability by requiring timely remediation of the kernel flaw through application of stable branch patches that enforce checking for three sub-authorities before accessing sid.sub_auth[2].
Requires validation of SMB protocol inputs including SID num_subauth and structure to prevent out-of-bounds reads from client-crafted security descriptors.
Ensures secure error handling during ACL parsing to avoid applying out-of-bounds memory as file POSIX modes when SIDs lack sufficient sub-authorities.