Description

Mattermost versions <=11.9 11.0.9 11.4.8 11.7.7 10.22.11.0 fail to recover from handler panics, which allows an authenticated user to crash the plugin via a post-action request with an unexpected field type.. Mattermost Advisory ID: MMSA-2026-00701

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "4.3",
  "pubDate": "2026-09-14T14:17:15.570Z",
  "pubdate": "2026-09-14T14:17:15.570Z",
  "executiveSummary": "This vulnerability involves a failure to adequately handle panics within the Mattermost plugin framework, specifically during the processing of post-action requests.\nThe issue is classified as a Denial of Service (DoS) vulnerability triggered by improper error handling when encountering unexpected data types in input fields.\nAffected versions include Mattermost <=11.9, 11.0.9, 11.4.8, 11.7.7, and 10.22.11.0.\nAn authenticated user can remotely exploit this flaw to crash the affected plugin, thereby disrupting associated functionalities.\nSuccessful exploitation requires the attacker to possess authenticated access to the system. The primary risk implication is the forced termination of plugin processes, which leads to service unavailability and potential disruption of automated workflows dependent on the plugin.\nThis vulnerability highlights a lack of robustness in the plugin's input validation and exception management layers, allowing malicious or malformed input to propagate unhandled exceptions.",
  "technicalDetails": "The vulnerability resides within the Mattermost plugin handler mechanism. The root cause is the failure to implement appropriate recover deferred functions or comprehensive error-checking routines when processing incoming post-action requests. When the application receives an unexpected field type within these requests, the plugin's execution flow encounters an unhandled runtime panic.\nBecause the plugin environment does not capture these panics, the specific plugin process terminates abruptly. The attack flow is initiated by an authenticated user who crafts a malicious post-action request containing an invalid or unexpected data type in a specific field. When this request is parsed and handled by the plugin, the application logic fails to validate the type, leading to a type-mismatch error that forces a crash.\nThis vulnerability is triggered through the standard post-action request interface, meaning that the attacker must have network access to the Mattermost instance and possess a valid session token to reach the vulnerable handler. By intentionally sending malformed data structures that the plugin expects to be of a specific type (e.g., string instead of map or integer), an attacker can effectively kill the plugin's background process.\nAffected versions are identified as <=11.9, 11.0.9, 11.4.8, 11.7.7, and 10.22.11.0. Because the panic occurs at the plugin level, the impact is strictly limited to the availability of the specific plugin being targeted. While this does not typically lead to remote code execution (RCE) or direct memory corruption that would compromise the host operating system, it constitutes a reliable Denial of Service condition for all users relying on the affected plugin's services.\nThe exploit does not require specialized privileges beyond basic user authentication, assuming the user has permission to trigger post-actions. The post-exploitation state results in a state of 'Plugin Crashed' or 'Plugin Stopped', necessitating administrative intervention to restart the service. This vulnerability underscores the necessity for plugin developers to strictly implement recovery mechanisms, such as deferred functions with `recover()` in Go, to prevent crashes when interacting with untrusted client-provided data."
}