CVE-2026-59791 in YouTrack
Summary
by MITRE • 07/10/2026
In JetBrains YouTrack before 2026.2.17012 cSS injection via Mermaid diagram rendering was possible
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Analysis
by VulDB Data Team • 07/10/2026
JetBrains YouTrack is a web-based issue tracking and project management platform that allows users to create various types of diagrams including mermaid diagrams for visualizing workflows and system structures. The vulnerability in question involves a cross-site scripting flaw that occurs during the rendering of mermaid diagrams, specifically affecting versions prior to 2026.2.17012. This type of vulnerability falls under CWE-79 which represents Cross-Site Scripting attacks where malicious code is injected into web applications and executed in the context of other users. The flaw allows attackers to inject malicious CSS or JavaScript code through mermaid diagram specifications that are then rendered on the target system. When a user views a page containing a maliciously crafted mermaid diagram, the injected code executes in their browser context, potentially leading to session hijacking, data theft, or further exploitation of the victim's privileges.
The technical implementation of this vulnerability relies on the insufficient sanitization of user-provided input within the mermaid diagram rendering engine. Mermaid diagrams are typically defined using a domain-specific language that gets parsed and converted into visual representations. In the affected YouTrack versions, the parsing process did not adequately filter or escape special characters and script tags that could be embedded within diagram definitions. Attackers could craft malicious mermaid specifications containing embedded javascript or css code that would execute when the diagram is rendered in a user's browser. This particular attack vector demonstrates how seemingly benign diagramming capabilities can become security risks when proper input validation and output encoding are not implemented.
The operational impact of this vulnerability extends beyond simple data theft as it enables attackers to perform session manipulation and privilege escalation attacks within the context of the vulnerable YouTrack instance. An attacker who can inject malicious code into a mermaid diagram could potentially steal user sessions, access sensitive project information, modify issues, or even escalate privileges if they can target administrators. The vulnerability is particularly concerning because mermaid diagrams are often used in documentation and knowledge bases where users might trust the content implicitly. This creates an attack surface where legitimate users might inadvertently execute malicious code simply by viewing a diagram that was added to a project management page. The exploitability of this vulnerability is enhanced by the fact that it does not require authentication to the YouTrack system itself, only the ability to create or modify content within the application.
Organizations using JetBrains YouTrack should immediately update to version 2026.2.17012 or later where this vulnerability has been patched. The patch likely includes improved input sanitization and output encoding mechanisms for mermaid diagram rendering that prevent malicious code injection. Security teams should also implement additional monitoring for suspicious diagram content, particularly around user-generated content in project documentation areas. Network-level protections such as web application firewalls can provide an additional layer of defense by filtering out known malicious patterns in diagram specifications. The vulnerability aligns with ATT&CK technique T1566 which covers social engineering attacks through malicious content, and also relates to T1059 which involves executing malicious code through various attack vectors including script injection. Organizations should conduct security assessments of their YouTrack instances to verify that all users cannot create potentially dangerous diagram content and implement proper access controls for diagram creation features.
This vulnerability represents a classic example of how modern web applications must consider security at every layer of functionality, particularly when rendering user-provided content. The mermaid diagram feature, while valuable for project documentation and visualization, became a potential attack vector due to inadequate input validation. The fix implemented by JetBrains addresses the core issue by ensuring that all user-supplied diagram content is properly sanitized before rendering, preventing malicious code from being executed in users' browsers. Security researchers should continue monitoring for similar vulnerabilities in other diagramming or visualization tools where user input is processed and rendered without proper security controls. The incident underscores the importance of following secure coding practices including input validation, output encoding, and principle of least privilege when implementing features that process external data.