EUVD-2026-30753
GHSA-8r89-8w26-cq32
Severity by source
AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
Primary rating from NVD.
CVSS VectorNVD
Lifecycle Timeline
1DescriptionCVE.org
Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645
AnalysisAI
Authenticated Mattermost users can read private channel threads and direct messages they lack access to by exploiting the AI post rewrite endpoint. Versions 11.5.0 and 11.5.1 fail to verify channel membership before processing AI-assisted message rewrites, enabling privilege escalation from low-privileged authenticated users to access confidential communications. CVSS 6.5 reflects network-accessible attack with low complexity requiring only basic authentication. EPSS data not available; no public exploit or KEV listing identified at time of analysis.
Technical ContextAI
This vulnerability stems from CWE-862 (Missing Authorization) in Mattermost's AI-assisted message rewrite feature, introduced in version 11.5.x. The post rewrite endpoint processes user requests to modify message content using AI assistance but fails to validate whether the requesting user has membership or read permissions for the channel containing the target thread. The CVSS vector (AV:N/AC:L/PR:L/UI:N) indicates a network-accessible API endpoint with no rate limiting or additional security controls beyond basic authentication. Mattermost's access control model relies on explicit channel membership grants, particularly for private channels and direct messages which contain sensitive organizational communications. The authorization bypass occurs server-side during API request processing, allowing any authenticated user to craft requests referencing post IDs from channels they cannot normally view.
RemediationAI
Upgrade immediately to Mattermost version 11.5.2 or later which addresses the missing authorization check in the AI post rewrite endpoint per vendor advisory MMSA-2026-00645 at https://mattermost.com/security-updates. If immediate patching is not feasible, disable the AI-assisted message rewrite feature through Mattermost system console under AI settings, which eliminates the vulnerable attack surface but removes functionality users may rely on for productivity. Alternatively, implement network-level access controls restricting API endpoint access to trusted IP ranges, though this does not mitigate insider threats from authenticated users within the network perimeter. Review access logs for unusual POST requests to rewrite endpoints referencing post IDs from channels the requesting user does not belong to, filtering for HTTP 200 responses that should have returned 403 Forbidden errors. Audit private channel memberships and direct message participant lists to identify potential unauthorized access during the vulnerability window between 11.5.0 release and patching.
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Same weakness CWE-862 – Missing Authorization
View allSame technique Authentication Bypass
View allVendor StatusVendor
SUSE
Severity: Moderate| Product | Status |
|---|---|
| SUSE Linux Enterprise Server 16.1 | Affected |
| SUSE Linux Enterprise Server for SAP applications 16.1 | Affected |
| SUSE Linux Enterprise Module for Package Hub 15 SP5 | Affected |
| SUSE Linux Enterprise Module for Package Hub 15 SP6 | Affected |
| openSUSE Leap 15.5 | Affected |
| openSUSE Leap 15.6 | Affected |
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External POC / Exploit Code
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