CVE-2026-54386 is a reflected cross-site scripting vulnerability in marimo before version 0.23.9. The vulnerability arises from improper neutralization of input during web page generation, specifically due to inadequate escaping of single quotes in the file query parameter reflected into an inline JavaScript string literal on the notebook page. This allows unauthenticated attackers to inject arbitrary JavaScript code that executes in the context of the victim's browser without Content-Security-Policy restrictions. The attack vector includes crafting a malicious URL with a payload beginning with __new__ to bypass the 404 error check.

Successful exploitation enables execution of arbitrary JavaScript in the context of the victim's browser session on the marimo server origin. This can lead to typical XSS impacts such as session hijacking, defacement, or other malicious actions performed with the victim's privileges. The vulnerability does not require authentication and has a medium severity rating with a CVSS 4.0 score of 5.1.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is available, users should consider restricting access to the affected notebook page or applying web application firewall rules to detect and block suspicious payloads containing __new__. Monitor vendor channels for updates regarding an official patch or mitigation.