The vulnerability involves a missing authorization check in the AI-assisted message rewrite endpoint of Mattermost Release 11.5.x, allowing an authenticated attacker to read private thread content in channels or direct messages they are not members of. The flaw permits leakage of confidential user conversations, compromising confidentiality for all private threads accessible through the rewrite feature. The weakness corresponds to CWE‑862, indicating an improper authorization control.

The affected product is Mattermost Version 11.5.x through 11.5.1. All Mattermost installations using these versions lack the necessary channel membership verification during AI rewrite requests. Future releases, including Mattermost 11.5.2 and any 11.6.0 version or later, contain the fix.

The CVSS score of 6.5 indicates a moderate severity vulnerability. EPSS information is not available, so the precise exploitation likelihood cannot be quantified, but the flaw is exploitable by any authenticated user who can send a crafted rewrite request. The vulnerability is not listed in CISA KEV, implying no confirmed exploits yet, yet the ability to expose private conversation content poses a significant confidentiality risk.