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 in the notebook page where the file query parameter is improperly escaped. This allows unauthenticated attackers to inject JavaScript code that executes in the context of the victim's browser without Content-Security-Policy restrictions. The exploit involves crafting a malicious URL with a payload beginning with __new__ to bypass a 404 check and trigger the injection.

Successful exploitation enables an attacker to execute arbitrary JavaScript in the context of the victim's browser session on the marimo server. This can lead to theft of sensitive information, session hijacking, or other malicious actions performed with the victim's privileges. The vulnerability does not require authentication and has a medium severity score (CVSS 5.1). There are no known exploits in the wild at this time.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is available, users should avoid exposing vulnerable versions to untrusted networks or users. Implementing web application firewalls or input validation may provide temporary mitigation but are not guaranteed to fully prevent exploitation.