CVE-2026-25783 Overview
CVE-2026-25783 is a denial of service vulnerability affecting Mattermost Server that stems from improper validation of User-Agent header tokens. When an authenticated attacker sends a specially crafted User-Agent header, the application fails to properly parse the malformed input, resulting in a request panic that can disrupt service availability.
This vulnerability is classified under CWE-1287 (Improper Validation of Specified Type of Input), indicating that the application does not adequately verify that User-Agent header values conform to expected formats before processing them. The flaw allows authenticated users to trigger application crashes through malformed HTTP headers.
Critical Impact
Authenticated attackers can cause denial of service conditions by triggering request panics through malformed User-Agent headers, potentially disrupting team communications and collaboration workflows.
Affected Products
- Mattermost Server versions 11.3.x up to and including 11.3.0
- Mattermost Server versions 11.2.x up to and including 11.2.2
- Mattermost Server versions 10.11.x up to and including 10.11.10
Discovery Timeline
- March 16, 2026 - CVE-2026-25783 published to NVD
- March 18, 2026 - Last updated in NVD database
Technical Details for CVE-2026-25783
Vulnerability Analysis
The vulnerability resides in Mattermost Server's HTTP request handling logic, specifically in the code responsible for parsing and validating User-Agent header tokens. The application expects User-Agent headers to follow standard formatting conventions but fails to implement robust input validation when processing these values.
When a malformed User-Agent header is received, the parser encounters unexpected character sequences or token structures that it cannot handle gracefully. Instead of returning an error response or logging the issue, the application triggers a panic condition, causing the request handler to crash.
This type of input validation failure is particularly concerning in collaborative platforms like Mattermost where continuous availability is essential for team communications. While the attack requires authentication, any legitimate user account can be leveraged to exploit this vulnerability.
Root Cause
The root cause is improper validation of specified type of input (CWE-1287). The User-Agent header parsing routine does not adequately sanitize or validate incoming header values before attempting to process them. The code likely makes assumptions about the structure of User-Agent strings that can be violated by specially crafted input, leading to unhandled exceptions or panic conditions.
Attack Vector
The attack is network-based and requires low complexity to execute. An authenticated attacker can exploit this vulnerability by:
- Authenticating to the Mattermost Server with valid credentials
- Crafting an HTTP request with a malicious User-Agent header containing specially formed tokens
- Sending the request to the server
- Triggering a request panic that disrupts the handling of that request
The vulnerability requires authentication (low privileges) but no user interaction is needed. The scope is unchanged, meaning the impact is contained within the vulnerable component. The attack primarily affects availability with no direct impact on confidentiality or integrity.
Detection Methods for CVE-2026-25783
Indicators of Compromise
- Application logs showing repeated panic errors or stack traces related to request handling or header parsing
- Unusual User-Agent strings in access logs containing malformed or non-standard token patterns
- Service restarts or crashes correlating with specific authenticated user sessions
- HTTP requests with abnormally long or malformed User-Agent headers from authenticated users
Detection Strategies
- Monitor Mattermost Server logs for panic events and stack traces related to HTTP request processing
- Implement log analysis rules to flag requests with unusual User-Agent header patterns
- Deploy application performance monitoring to detect sudden service interruptions or crash patterns
- Configure intrusion detection systems to alert on anomalous HTTP header values in authenticated sessions
Monitoring Recommendations
- Enable verbose logging for HTTP request handling components to capture detailed header information
- Set up alerting thresholds for application restarts or panic events that exceed normal baseline
- Monitor for patterns of failed requests from specific authenticated users that may indicate exploitation attempts
- Implement real-time dashboard monitoring for service availability metrics
How to Mitigate CVE-2026-25783
Immediate Actions Required
- Upgrade Mattermost Server to the latest patched version immediately
- Review access logs for any signs of exploitation attempts using malformed User-Agent headers
- Implement a Web Application Firewall (WAF) rule to sanitize or block requests with malformed User-Agent headers
- Consider temporarily restricting access to trusted users while patching is completed
Patch Information
Mattermost has released security updates addressing this vulnerability. Organizations should upgrade to patched versions as outlined in the Mattermost Security Updates page. The advisory reference for this vulnerability is MMSA-2026-00586.
- For version 11.3.x: Upgrade to version 11.3.1 or later
- For version 11.2.x: Upgrade to version 11.2.3 or later
- For version 10.11.x: Upgrade to version 10.11.11 or later
Consult the official Mattermost security advisory for the specific fixed version numbers applicable to your deployment.
Workarounds
- Deploy a reverse proxy or WAF in front of Mattermost Server to filter and validate User-Agent headers before they reach the application
- Implement rate limiting on authenticated requests to reduce the impact of potential exploitation attempts
- Configure application-level request filtering to reject requests with User-Agent headers that exceed reasonable length limits or contain suspicious characters
- Monitor and temporarily disable accounts exhibiting suspicious request patterns until patches can be applied
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
