NiceGUI’s ui.markdown() component converts Markdown to HTML with markdown2, which permits raw HTML to pass through unchanged. When an application renders user‑controlled content via ui.markdown(), an attacker can embed malicious HTML containing JavaScript event handlers. The resulting vulnerability is a classic XSS flaw (CWE‑79) that lets attackers execute arbitrary code in the victim’s browser, potentially stealing data or hijacking sessions.

The flaw exists in all versions of the NiceGUI Python UI framework released before the 3.7.0 update. Any deployment that accepts or displays Markdown input via ui.markdown() is affected.

The CVSS score of 6.1 indicates a moderate severity. EPSS shows a probability of exploitation below 1 %, suggesting attacks are not yet common, and the vulnerability is not listed in the CISA KEV catalog. Attackers can exploit this vector by providing malicious Markdown content through any user‑input channel that the application forwards to ui.markdown(). The described attack surface is inferred from the fact that user input is passed directly to the component; no additional privileges or network access are required.