CVE-2026-86349 in Mattermost
Summary
by MITRE • 09/14/2026
Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.8, 10.11.x <= 10.11.22 fail to limit the nesting depth in the server-side Markdown parser which allows an authenticated attacker to cause a denial of service (CPU resource exhaustion) via a crafted post containing deeply nested blockquotes or list items.. Mattermost Advisory ID: MMSA-2026-00707
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Analysis
by VulDB Data Team • 09/14/2026
The vulnerability identified in Mattermost versions 11.9.x through 11.8.4, as well as earlier branches including 11.7.x up to 11.7.8 and 10.11.x up to 10.11.22, represents a significant server-side resource exhaustion flaw rooted in the application's Markdown parsing logic. This issue is formally cataloged under Common Weakness Enumeration as CWE-400, which describes uncontrolled resource consumption, specifically manifesting here through excessive nesting depth within blockquotes or list items. The core technical failure lies in the absence of a recursive depth limit during the server-side processing of user-supplied Markdown content. When an authenticated user submits a post containing deeply nested structural elements such as lists inside quotes or vice versa, the parser enters into a prolonged computational loop without terminating early based on complexity thresholds. This behavior allows for the creation of inputs that require disproportionate CPU cycles to parse and render, effectively turning standard text formatting features into vectors for denial of service attacks against the application infrastructure.
From an operational perspective, this vulnerability enables authenticated attackers to degrade or completely halt Mattermost services by triggering high CPU utilization across one or more server nodes involved in processing the malicious payload. Because the attack requires authentication, it is classified as a privilege-dependent exploitation scenario rather than a remote unauthenticated compromise. However, given that many enterprise deployments allow internal users with varying levels of access to create posts and channels, the blast radius can be substantial if an attacker gains foothold within the organization's network or compromises user credentials. The resulting denial of service impacts not only the availability of the chat platform for legitimate users but also consumes system resources such as memory and thread pools associated with the web server processes handling Markdown conversion. This aligns closely with MITRE ATT&CK technique T1496, which covers Resource Hijacking where an adversary uses computing resources to perform actions that are not beneficial to the victim or their organization, often leading to service degradation for other users sharing those resources.
The mitigation strategy must address both immediate remediation and long-term architectural resilience. The primary defense is the urgent application of vendor-provided patches that update the Markdown parser implementation in all affected version branches listed above. Organizations should verify their deployment versions against the advisory MMSA-2026-00707 to ensure compliance with security baselines. Beyond patching, implementing input validation and sanitization layers at the API gateway or reverse proxy level can provide an additional defense-in-depth measure by rejecting payloads that exceed predefined structural complexity limits before they reach the application server. Furthermore, configuring resource quotas and rate limiting on user endpoints can mitigate the impact of such attacks by capping the computational resources any single session can consume. Regular security audits focusing on parser implementations are recommended to prevent similar unbounded recursion issues in other text processing components within the ecosystem.