# CVE-2026-31610: ksmbd: fix mechToken leak when SPNEGO decode fails after token alloc

CVE-2026-31610 is fixed in linux on 1 platform, first by DEBIAN-CVE-2026-31610, released May 15, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-31610
- Priority: Routine
- CVSS: 5.5 (Moderate)
- EPSS: 0.0014
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Apr 24, 2026
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-31610

## Description

In the Linux kernel, the following vulnerability has been resolved:  ksmbd: fix mechToken leak when SPNEGO decode fails after token alloc  The kernel ASN.1 BER decoder calls action callbacks incrementally as it walks the input.  When ksmbd_decode_negTokenInit() reaches the mechToken [2] OCTET STRING element, ksmbd_neg_token_alloc() allocates conn->mechToken immediately via kmemdup_nul().  If a later element in the same blob is malformed, then the decoder will return nonzero after the allocation is already live.  This could happen if mechListMIC [3] overrunse the enclosing SEQUENCE.  decode_negotiation_token() then sets conn->use_spnego = false because both the negTokenInit and negTokenTarg grammars failed.  The cleanup at the bottom of smb2_sess_setup() is gated on use_spnego:  	if (conn->use_spnego && conn->mechToken) { 		kfree(conn->mechToken); 		conn->mechToken = NULL; 	}  so the kfree is skipped, causing the mechToken to never be freed.  This codepath is reachable pre-authentication, so untrusted clients can cause slow memory leaks on a server without even being properly authenticated.  Fix this up by not checking check for use_spnego, as it's not required, so the memory will always be properly freed.  At the same time, always free the memory in ksmbd_conn_free() incase some other failure path forgot to free it.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 12.15 (bookworm) | 6.1.180-1 | [DEBIAN-CVE-2026-31610](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31610.md) | May 15, 2026 |
| Linux | linux Debian 13.5 (trixie) | 6.12.85-1 | [DEBIAN-CVE-2026-31610](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31610.md) | May 15, 2026 |
| Linux | linux Debian 14 | 6.19.14-1 | [DEBIAN-CVE-2026-31610](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31610.md) | May 15, 2026 |

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Data as of 2026-11-09T07:25:28Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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