CVE-2026-41149 is an improper control of code generation vulnerability (CWE-94) in the mermaid-js mermaid tool. The vulnerability arises from the classDef directive in state diagrams allowing DOM injection that escapes the SVG context, leading to HTML injection. Although <script> tags are removed, preventing direct cross-site scripting, the injection could still affect the DOM. Affected versions include all versions up to 10.9.5 and from 11.0.0-alpha.1 up to but not including 11.15.0. The vulnerability has been addressed in versions 10.9.6 and 11.15.0. Developers unable to upgrade immediately can mitigate the issue by enabling the "securityLevel": "sandbox" setting, which confines rendering to a sandboxed iframe.

The vulnerability allows HTML injection through the classDef directive, which can lead to DOM manipulation outside the SVG context. While direct script injection is prevented by stripping <script> tags, the injection could still affect the integrity of the rendered content. The CVSS 4.0 score is 5.3 (medium severity), indicating a moderate risk with network attack vector, no privileges required, and user interaction needed.

A fix is available in mermaid versions 10.9.6 and 11.15.0. Users should upgrade to these or later versions to remediate the vulnerability. If immediate upgrading is not feasible, setting the configuration option "securityLevel" to "sandbox" provides a temporary workaround by rendering diagrams in a sandboxed iframe, preventing the injection issue.