CVE-2026-93847 in Mattermost
Summary
by MITRE • 10/26/2026
Mattermost versions 11.7.x <= 11.7.10 fail to validate reserved system post types on the scheduled post creation and publication paths which allows an authenticated channel member to publish messages rendering as trusted system events via the scheduled post endpoint.. Mattermost Advisory ID: MMSA-2026-00732
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Analysis
by VulDB Data Team • 10/26/2026
The vulnerability identified in Mattermost versions 11.7.x through 11.7.10 represents a critical logic flaw within the application's message scheduling subsystem, specifically affecting how reserved system post types are validated during creation and publication workflows. In standard operation, certain post types such as join leave events or bot messages are designated as trusted system posts that carry specific visual indicators and permissions to distinguish them from regular user content. The core technical failure lies in the absence of strict input validation on the scheduled post endpoint, which allows an authenticated channel member to bypass these restrictions by manipulating request parameters to specify a reserved system post type rather than a standard user message. This oversight enables the attacker to craft requests that trick the server into treating arbitrary user-generated content as official system notifications or automated bot actions.
From an operational perspective, this flaw facilitates a sophisticated form of social engineering and reputation abuse within team communication channels. By rendering malicious messages as trusted system events, an authenticated insider can significantly increase the credibility and visibility of their posts without triggering standard security warnings that users typically associate with external links or suspicious activity. This capability undermines the integrity of channel communications by blurring the line between official organizational announcements and user-generated content. Attackers could leverage this to spread misinformation, conduct phishing campaigns disguised as system alerts, or manipulate team dynamics by simulating administrative actions such as user joins or leaves, thereby eroding trust in the platform's notification mechanisms.
The security implications of this vulnerability align closely with CWE-20 Improper Input Validation and CWE-798 Use of Hard-coded Credentials if the attacker exploits associated tokens to maintain persistent access. Furthermore, within the context of the MITRE ATT&CK framework for enterprise environments, this behavior maps directly to T1566 Phishing via Trusted Vectors or T1499 Endpoint Denial of Service through reputation manipulation, as it abuses trusted system interfaces to deceive end-users. The attack vector is classified under Initial Access and Execution phases where the attacker utilizes valid credentials to exploit a logic error in application design rather than exploiting a traditional buffer overflow or injection flaw.
Mitigation strategies must prioritize immediate patching to version 11.7.11 or later, which addresses this validation gap by enforcing strict type checking on all scheduled post requests regardless of user role. Administrators should also review audit logs for any anomalous scheduling activity that resulted in system-style notifications prior to the update. Implementing additional monitoring rules to detect rapid succession of posts with reserved types can provide early warning indicators of exploitation attempts. Long-term remediation involves adopting a principle of least privilege for API endpoints and ensuring that all user-supplied data, particularly metadata defining post classification, undergoes rigorous server-side validation against an allowlist of permissible values before processing.