CVE-2025-54881 is a medium severity cross-site scripting (XSS) vulnerability affecting the mermaid-js mermaid library versions from 10.9.0-rc.1 up to and including 11.9.0. Mermaid is a JavaScript-based diagramming and charting tool that allows users to create complex diagrams using Markdown-inspired text definitions. The vulnerability arises because user-supplied input for sequence diagram labels is directly passed to the innerHTML property during the calculation of element sizes. This improper neutralization of input (CWE-79) allows an attacker to inject malicious scripts into the rendered diagrams. Since innerHTML is used without sanitization or encoding, any embedded script or HTML can execute in the context of the web page using the vulnerable mermaid version. The CVSS 4.0 base score is 5.3 (medium), reflecting that the attack vector is network-based (AV:N), requires no privileges (PR:N), no authentication (AT:N), but does require user interaction (UI:P). The vulnerability does not affect confidentiality, integrity, or availability directly but can lead to session hijacking, credential theft, or other client-side attacks if exploited. There are no known exploits in the wild at the time of publication, and no official patches have been linked yet. The vulnerability affects mermaid versions commonly used in web applications, documentation tools, and developer platforms that embed dynamic diagrams, potentially exposing users to malicious script execution when viewing crafted diagrams.

For European organizations, this vulnerability poses a risk primarily to web applications and internal tools that utilize the mermaid-js library for rendering diagrams. Organizations in sectors such as software development, technical documentation, education, and any enterprise using mermaid for visualization in intranet portals or public-facing websites could be impacted. Exploitation could lead to client-side attacks including session hijacking, phishing, or malware delivery through injected scripts. This could compromise user accounts, leak sensitive information, or facilitate lateral movement within corporate networks if internal tools are affected. The impact is heightened in environments where mermaid diagrams are embedded in collaborative platforms or developer portals accessed by many users. Since the vulnerability requires user interaction (viewing a malicious diagram), social engineering or phishing campaigns could be used to trigger exploitation. The medium severity score suggests a moderate risk, but the widespread use of mermaid in European tech ecosystems means the attack surface is significant. Organizations handling sensitive or regulated data should be particularly cautious due to potential data confidentiality and compliance implications.

1. Immediate mitigation involves upgrading the mermaid-js library to a version beyond 11.9.0 once an official patch is released that addresses the XSS vulnerability. 2. Until a patch is available, organizations should sanitize or validate all user-supplied input used in sequence diagram labels to ensure no executable HTML or script content is included. 3. Employ Content Security Policy (CSP) headers to restrict script execution sources and reduce the impact of any injected scripts. 4. Review and restrict access to tools or platforms that allow users to input mermaid diagrams, implementing input filtering or approval workflows. 5. Educate users and developers about the risks of opening untrusted diagrams and encourage caution with links or files containing mermaid content. 6. Monitor web application logs and user activity for signs of attempted exploitation or unusual script execution. 7. Consider isolating or sandboxing rendering environments where mermaid diagrams are processed to limit potential damage from XSS. These steps go beyond generic advice by focusing on input validation, environment hardening, and user awareness specific to the mermaid-js context.