CVE-2026-3471 in Desktop App
Summary
by MITRE • 05/18/2026
Mattermost Desktop App versions <=6.1 6.0.1 5.4.13.0 fail to prevent an invalid URL from loading in a pop-up window in the Mattermost Desktop App which allows a malicious server owner to repeated crash the application via calling {{window.open('javascript:alert()');}}. Mattermost Advisory ID: MMSA-2026-00618
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Analysis
by VulDB Data Team • 05/19/2026
The vulnerability identified in Mattermost Desktop App versions up to 6.1.6.0.1 and 5.4.13.0 represents a critical security flaw that undermines the application's ability to validate URL inputs within pop-up window contexts. This issue stems from inadequate input sanitization mechanisms that fail to properly validate the URL scheme and content before execution, creating a persistent vector for malicious exploitation. The vulnerability specifically manifests when the application processes javascript: URLs through window.open() calls, allowing remote attackers to inject malicious code that can trigger unintended application behavior. The flaw resides in the desktop application's security model where it does not sufficiently restrict or sanitize URL parameters passed to the window.open() function, enabling attackers to execute arbitrary javascript code within the application's context.
The technical exploitation of this vulnerability follows a well-defined pattern where malicious server owners can craft specific URL payloads that when processed through the desktop application's window.open() method trigger immediate application instability. When the javascript:alert() payload is executed, it causes the desktop application to crash repeatedly due to the malformed URL handling. This vulnerability operates at the intersection of web application security and desktop application sandboxing, where the desktop application fails to implement proper URL validation controls that should prevent execution of potentially dangerous protocols. The flaw is particularly concerning because it operates within the application's legitimate user interface components, making it difficult to distinguish between valid and malicious URL requests without proper input validation.
The operational impact of this vulnerability extends beyond simple application instability to encompass potential denial-of-service scenarios that can disrupt user productivity and compromise the overall security posture of organizations relying on Mattermost for communication. Attackers can repeatedly trigger application crashes through automated scripts or social engineering tactics, leading to persistent service disruption that affects team collaboration and communication workflows. The vulnerability affects multiple versions of the desktop application, indicating a widespread issue that has persisted across different release branches, suggesting either incomplete patching or a fundamental flaw in the application's security architecture. Organizations using affected versions face significant risk of operational disruption and potential data exposure through prolonged service unavailability.
Mitigation strategies for this vulnerability should prioritize immediate version updates to the latest stable releases that contain proper URL validation and sanitization mechanisms. System administrators should implement network-level controls to monitor and restrict access to known malicious domains that might attempt to exploit this vulnerability through crafted URLs. The implementation of Content Security Policy headers and proper input validation at the application level should be enforced to prevent javascript: URL execution in pop-up contexts. Additionally, organizations should consider implementing application whitelisting controls and user education programs to prevent accidental exploitation through phishing or malicious URL delivery methods. This vulnerability aligns with CWE-79 which addresses cross-site scripting flaws and ATT&CK technique T1211 which covers exploitation of software vulnerabilities, highlighting the need for comprehensive security controls that address both application-level and network-level protections.