CVE-2026-24661 Overview
CVE-2026-24661 is a Denial of Service (DoS) vulnerability affecting Mattermost Plugins versions <=2.1.3.0. The vulnerability arises from a failure to limit the request body size on the /changes webhook endpoint, which allows an authenticated attacker to cause memory exhaustion and denial of service by sending an oversized JSON payload.
Critical Impact
An authenticated attacker can exhaust server memory resources, causing service unavailability for all users relying on the Mattermost platform.
Affected Products
- Mattermost Plugins versions <=2.1.3.0
Discovery Timeline
- 2026-04-09 - CVE CVE-2026-24661 published to NVD
- 2026-04-09 - Last updated in NVD database
Technical Details for CVE-2026-24661
Vulnerability Analysis
This vulnerability is classified under CWE-770 (Allocation of Resources Without Limits or Throttling). The /changes webhook endpoint in affected Mattermost Plugins versions does not implement proper request body size validation. This architectural oversight allows authenticated users to submit arbitrarily large JSON payloads to the endpoint, which the server attempts to parse and store in memory without restriction.
When exploited, the server allocates memory proportional to the size of the malicious payload. Since there is no upper bound on the accepted request size, an attacker can craft a payload large enough to exhaust available server memory, leading to service degradation or complete unavailability. The attack requires network access and authentication but can be executed with relatively low complexity once an attacker has valid credentials.
Root Cause
The root cause of this vulnerability is the absence of request body size limits on the /changes webhook endpoint. The application accepts and processes incoming requests without validating the Content-Length header or implementing streaming limits. This allows unbounded memory allocation when parsing large JSON payloads, which is a classic resource exhaustion pattern. Proper input validation and resource throttling mechanisms were not implemented for this specific endpoint.
Attack Vector
The attack vector is network-based, requiring an authenticated session to the Mattermost instance. An attacker would craft an HTTP POST request to the /changes webhook endpoint containing an extremely large JSON payload. Upon receipt, the server attempts to deserialize the entire payload into memory, consuming available RAM. Repeated requests or a single sufficiently large payload can exhaust server resources, causing memory exhaustion that affects all users of the platform.
The attack flow involves:
- Authenticating to the Mattermost instance with valid credentials
- Constructing an oversized JSON payload targeting the /changes endpoint
- Sending the malicious request, forcing the server to allocate excessive memory
- Server memory becomes exhausted, resulting in denial of service
Detection Methods for CVE-2026-24661
Indicators of Compromise
- Unusual spikes in memory utilization on servers hosting Mattermost Plugins
- Large HTTP POST requests to the /changes webhook endpoint with abnormally high Content-Length values
- Server out-of-memory (OOM) errors or process crashes in application logs
- Repeated requests from single authenticated users targeting webhook endpoints
Detection Strategies
- Monitor HTTP request sizes to the /changes endpoint and alert on requests exceeding expected thresholds
- Implement network-level inspection for abnormally large payloads targeting Mattermost services
- Configure application performance monitoring (APM) to detect rapid memory consumption patterns
- Review authentication logs for suspicious activity patterns preceding memory exhaustion events
Monitoring Recommendations
- Establish baseline memory utilization metrics for Mattermost server processes
- Configure alerting for memory consumption exceeding 80% of available resources
- Log and analyze all requests to webhook endpoints, including payload sizes
- Implement rate limiting visibility to identify potential abuse patterns
How to Mitigate CVE-2026-24661
Immediate Actions Required
- Upgrade Mattermost Plugins to a version greater than 2.1.3.0 that includes the security fix
- Review Mattermost Security Updates for official patch information and advisory details
- Implement network-level request size limits as a defense-in-depth measure
- Monitor server memory utilization for signs of active exploitation attempts
Patch Information
Mattermost has acknowledged this vulnerability under advisory ID MMSA-2026-00611. Organizations should update Mattermost Plugins to the latest available version that addresses this vulnerability. For official patch information and detailed remediation guidance, refer to the Mattermost Security Updates page.
Workarounds
- Configure reverse proxy or load balancer to enforce maximum request body size limits on the /changes endpoint
- Implement web application firewall (WAF) rules to block oversized JSON payloads
- Apply rate limiting on webhook endpoints to reduce the impact of resource exhaustion attacks
- Restrict access to webhook endpoints to trusted IP ranges where feasible
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
