CVE-2026-25516 NiceGUI's XSS vulnerability in ui.markdown() allows arbitrary JavaScript execution through unsanitized HTML content
Attackers can execute arbitrary JavaScript via unsanitized HTML in NiceGUI's markdown component, affecting applications rendering user-controlled content.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-25516?
CVE-2026-25516 is a medium-severity cross-site scripting vulnerability in zauberzeug nicegui, published on 6 Feb 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
NiceGUI is a Python-based UI framework. The ui.markdown() component uses the markdown2 library to convert markdown content to HTML, which is then rendered via innerHTML. By default, markdown2 allows raw HTML to pass through unchanged. This means that if an application renders user-controlled content through ui.markdown(), an attacker can inject malicious HTML containing JavaScript event handlers. Unlike other NiceGUI components that render HTML (ui.html(), ui.chat_message(), ui.interactive_image()), the ui.markdown() component does not provide or require a sanitize parameter, leaving applications vulnerable to XSS attacks. This vulnerability is fixed in 3.7.0.
Weakness: CWE-79
Which versions are affected by CVE-2026-25516?
3 open source packages ship an affected version, including nicegui. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
nicegui PyPI | All versions | 3.7.0 |
nicegui PyPI | All versions | 3.7.0 |
nicegui PyPI | All versions | 3.7.0 |
Products include Zauberzeug Nicegui.
How do I fix CVE-2026-25516?
Upgrade nicegui to 3.7.0 or later.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-25516 being exploited?
There is no known exploitation of CVE-2026-25516 in the wild and no public exploit code.
Other vulnerabilities in zauberzeug nicegui
- CVE-2026-25732 NiceGUI's Path Traversal via Unsanitized FileUpload.name Enables Arbitrary File Write
- CVE-2025-66645 NiceGUI Path Traversal Vulnerability in app.add_media_files() Allows Arbitrary File Reading
- CVE-2026-21873 Zero-click XSS in all NiceGUI apps which uses `ui.sub_pages`
- CVE-2026-33332 NiceGUI's unvalidated chunk size parameter in media routes can cause memory exhaustion
- CVE-2026-27156 NiceGUI has XSS via Code Injection
Sources
Also known as GHSA-v82v-c5x8-w282, PYSEC-2026-2231.