CVE-2026-25783 in Mattermost
Summary
by MITRE • 03/16/2026
Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to properly validate User-Agent header tokens which allows an authenticated attacker to cause a request panic via a specially crafted User-Agent header. Mattermost Advisory ID: MMSA-2026-00586
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Analysis
by VulDB Data Team • 03/20/2026
This vulnerability resides in the Mattermost collaboration platform where improper validation of User-Agent header tokens creates a path for authenticated attackers to trigger request panics through crafted header values. The issue affects specific version ranges including 11.3.0 and earlier 11.3.x releases, 11.2.2 and earlier 11.2.x versions, and 10.11.10 and earlier 10.11.x versions. The flaw represents a classic input validation weakness that can escalate to denial of service conditions when exploited properly.
The technical implementation of this vulnerability stems from inadequate sanitization and validation of the User-Agent header parameter within Mattermost's request processing pipeline. When an authenticated user submits a specially crafted User-Agent header value, the application fails to properly parse or validate the input before processing it in subsequent operations. This lack of proper validation allows malicious input to bypass security checks and potentially cause the application to panic or crash during request handling. The vulnerability manifests as a request panic condition that disrupts normal application operation.
The operational impact of this vulnerability is significant for organizations relying on Mattermost for team collaboration and communication. An authenticated attacker with access to the system can leverage this weakness to cause service disruption through denial of service attacks. The panic condition affects the application's stability and can lead to temporary unavailability of the Mattermost service, impacting team communications and productivity. Given that Mattermost is used for critical business communications, such disruptions can have cascading effects on operational continuity and business processes.
This vulnerability aligns with CWE-20, which describes improper input validation, and demonstrates how insufficient header validation can lead to application instability. The attack vector follows ATT&CK technique T1210, which involves exploitation of weaknesses in remote services, specifically targeting application-level vulnerabilities in the web server stack. Organizations should prioritize patching affected versions to prevent potential exploitation. The recommended mitigation includes upgrading to patched versions of Mattermost where the User-Agent header validation has been strengthened and properly implemented. Additionally, implementing network-level monitoring and intrusion detection systems can help identify suspicious header patterns that may indicate exploitation attempts.