Description

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

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "4.3",
  "pubDate": "2026-09-14T14:17:15.687Z",
  "pubdate": "2026-09-14T14:17:15.687Z",
  "executiveSummary": "Mattermost versions 11.9.0 and below, 11.8.4 and below, 11.7.8 and below, and 10.11.22 and below are susceptible to a Denial of Service (DoS) vulnerability triggered by improper input validation within the server-side Markdown parser.\nThe vulnerability allows an authenticated attacker to exhaust CPU resources by submitting specially crafted messages containing deeply nested Markdown syntax, such as blockquotes or list items.\nThis flaw results in resource exhaustion that can degrade or completely halt service availability for the affected Mattermost instance.\nThe attack requires authentication, meaning a malicious user must have a valid account on the platform to submit the malicious payload.\nThere are no specific requirements for elevated privileges; standard user access is sufficient to exploit this defect.\nThe primary risk implication is the potential for service instability or downtime, impacting organizational communications.\nSystem administrators are advised to monitor official security advisories and update to patched versions as they become available to mitigate this risk.",
  "technicalDetails": "The vulnerability resides in the server-side Markdown rendering engine utilized by Mattermost to process user-generated content in posts.\nThe root cause of the vulnerability is the lack of a depth-limiting mechanism within the recursive parsing logic tasked with handling hierarchical Markdown elements.\nMarkdown parsers often utilize recursive descent or similar tree-traversal algorithms to interpret nested structures. In the absence of a defined recursion limit or stack depth constraint, deeply nested syntax structures force the parser to perform a disproportionately large number of operations.\nWhen a user submits a payload containing an excessive number of nested blockquotes ('>') or nested list items, the parser's recursive calls grow proportionally to the input depth.\nThe attack flow begins when an authenticated attacker constructs a malicious post containing a highly nested structure designed to exceed the parser's intended computational complexity.\nUpon submission, the Mattermost server attempts to render the message in the context of the posting process. The parser enters a state of high computational overhead as it attempts to resolve the pathological nesting.\nThis behavior leads to CPU starvation, where the rendering process consumes an excessive amount of processor cycles, delaying or blocking other concurrent requests, effectively inducing a denial-of-service state for the application.\nAffected versions include Mattermost 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.8, and 10.11.x <= 10.11.22.\nThe exploitation is non-privileged and requires only that the attacker has authenticated access to a workspace where they can post messages. The network exposure is limited by the requirement of initial authentication, but the impact is significant once access is obtained.\nPost-exploitation impact is limited to resource exhaustion of the server-side process, as the vulnerability does not inherently facilitate remote code execution or unauthorized data access; however, the resulting unavailability directly interrupts critical business communications and collaborative workflows."
}