CVE-2026-116516 in Mattermost
Summary
by MITRE • 10/26/2026
Mattermost versions <=11.9 11.1.9 11.5.8 11.10.7.0 fail to validate Zoom chat line length before slicing which allows an authenticated attacker to crash the Agents plugin via summarizing a Zoom chat transcription containing short continuation lines. Mattermost Advisory ID: MMSA-2026-00723
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Analysis
by VulDB Data Team • 10/26/2026
The vulnerability identified in Mattermost versions up through 11.9, 11.1.9, 11.5.8, and 11.10.7.0 represents a critical failure in input validation within the integration logic for Zoom chat transcriptions. Specifically, the Agents plugin responsible for processing these transcripts does not adequately validate the length of individual lines before attempting to slice or process them. This oversight creates a scenario where an authenticated attacker can trigger a denial-of-service condition by crafting maliciously formatted data payloads that exploit this boundary check failure. The core technical flaw lies in the assumption that incoming text segments from external integrations will conform to expected structural norms, leading to unhandled exceptions when short continuation lines are processed without proper length verification or bounds checking prior to array slicing operations.
From a technical perspective, this issue aligns with CWE-20 Improper Input Validation and potentially CWE-134 Use of Externally-Controlled Format String if the error manifests through string formatting routines that fail on unexpected input lengths. The vulnerability is classified under ATT&CK technique T1499 Endpoint Denial of Service, as it allows an authenticated user to disrupt service availability by causing a crash in the Agents plugin component. When the system attempts to slice a line that does not meet internal length expectations or contains malformed continuation markers, the application fails to catch the resulting exception, leading to a process termination or severe instability within the Mattermost server environment. This is particularly dangerous because it requires only authentication, which lowers the barrier for exploitation compared to unauthenticated attacks, yet still poses significant risk to operational continuity and data integrity during active sessions.
The operational impact of this vulnerability is primarily centered on service availability and reliability. An attacker who has valid credentials can repeatedly send Zoom chat transcriptions with specifically crafted short continuation lines to crash the Agents plugin. This results in a denial-of-service condition where affected users may experience interrupted conversations, lost message history if not persisted correctly before the crash, or complete unavailability of Mattermost services depending on how the failure propagates through the application architecture. In enterprise environments relying on continuous communication platforms like Mattermost for critical operations, such instability can lead to significant productivity losses and potential security gaps as administrators may be forced into emergency maintenance modes to restore service stability.
Mitigation strategies should focus primarily on immediate patching to versions of Mattermost that address this advisory, specifically those released after the identification of MMSA-2026-00723. Until patches are applied, organizations can implement network-level controls or application firewalls to inspect and sanitize incoming webhook data from Zoom integrations, ensuring that line lengths conform to expected parameters before they reach the Mattermost backend. Additionally, enforcing strict role-based access control for users who have permission to trigger automated summarization processes can reduce the attack surface by limiting which authenticated accounts can interact with the vulnerable Agents plugin components. Regular auditing of integration logs and monitoring for unusual spikes in error reports related to transcription processing will also aid in early detection of exploitation attempts while long-term remediation efforts are underway.