Mercury runs a JupyterLab server with anonymous access by default (mercury_app/main.py sets –IdentityProvider.token='' when no –token/MERCURY_TOKEN is given; when tokenless it also sets –ServerApp.disable_check_xsrf=True). It relies on a blocklist (mercury_app/block_handler.py BLOCKED_PATTERNS: /lab, /api/terminals, /api/shutdown, /files) to keep anonymous users off dangerous Jupyter endpoints.
A missing comma in mercury_app/block_handler.py's BLOCKED_PATTERNS list causes the blocklist to fail to cover the /api/terminals endpoint. As a result, any unauthenticated attacker can spawn a full, interactive OS-level shell on the host running Mercury.
Mercury turns Jupyter notebooks into shared web apps/dashboards. The mr.Markdown widget converts its input to HTML with python-markdown and no sanitization step. MarkdownWidget is an ipywidgets.HTML widget, whose value is written to the page as raw HTML (via innerHTML) with no sanitization, so the result reaches the DOM verbatim. When an app passes untrusted data (database rows, user submissions, uploaded file contents, or an external/LLM API response) to mr.Markdown, HTML/JS …