Mattermost Multiple Security Issues

Mattermost 10.11.x through 11.3.x fails to validate password length, allowing unauthenticated attackers to trigger denial of service by submitting multi-megabyte passwords during login attempts that consume excessive CPU and memory resources. The vulnerability affects all versions up to 10.11.10, 11.2.2, and 11.3.0, with no patch currently available.

This vulnerability in Mattermost allows unauthenticated attackers to achieve remote code execution and exfiltrate sensitive credentials through malicious plugin installation on CI test instances that retain default admin credentials. Affected versions include Mattermost 10.11.x through 10.11.10, 11.2.x through 11.2.2, and 11.3.0, with the core issue stemming from insufficient access controls on plugin installation combined with default credential exposure. An attacker can upload a malicious plugin after modifying the import directory to gain full system compromise and access AWS and SMTP credentials stored in configuration files.

Mattermost versions 11.3.0, 11.2.2, and 10.11.10 and earlier lack proper memory bounds checking when processing DOC file uploads, enabling authenticated attackers to trigger server memory exhaustion and denial of service. An attacker with valid credentials can upload a specially crafted DOC file to exhaust available memory and crash the Mattermost server. This vulnerability currently lacks a patch and affects multiple active versions of the platform.

Mattermost fails to properly validate User-Agent header tokens in versions 11.3.0 and earlier, 11.2.2 and earlier, and 10.11.10 and earlier, allowing authenticated attackers to trigger request panics through specially crafted User-Agent headers. This denial-of-service vulnerability affects availability but requires prior authentication and results in only low-severity impact. While the CVSS score of 4.3 reflects the low severity, the practical risk depends on whether the application is exposed to untrusted authenticated users and whether automatic exploitation tools are readily available.