CVE-2026-21873 Zero-click XSS in all NiceGUI apps which uses `ui.sub_pages`

What is CVE-2026-21873?

CVE-2026-21873 is a high-severity cross-site scripting vulnerability in zauberzeug nicegui, published on 8 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

NiceGUI is a Python-based UI framework. From versions 2.22.0 to 3.4.1, an unsafe implementation in the pushstate event listener used by ui.sub_pages allows an attacker to manipulate the fragment identifier of the URL, which they can do despite being cross-site, using an iframe. This issue has been patched in version 3.5.0.

Weakness: CWE-79

Which versions are affected by CVE-2026-21873?

3 open source packages ship an affected version, including nicegui. It affects 1 product.

Products include Zauberzeug Nicegui.

How do I fix CVE-2026-21873?

Upgrade nicegui to 3.5.0 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-2026-21873 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-21873 exists.

Other vulnerabilities in zauberzeug nicegui

• CVE-2026-25732 NiceGUI's Path Traversal via Unsanitized FileUpload.name Enables Arbitrary File Write High 7.5
• CVE-2025-66645 NiceGUI Path Traversal Vulnerability in app.add_media_files() Allows Arbitrary File Reading High 7.5
• CVE-2026-33332 NiceGUI's unvalidated chunk size parameter in media routes can cause memory exhaustion Medium 6.9
• CVE-2026-27156 NiceGUI has XSS via Code Injection Medium 6.1
• CVE-2026-25516 NiceGUI's XSS vulnerability in ui.markdown() allows arbitrary JavaScript execution through unsanitized HTML content Medium 6.1

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-mhpg-c27v-6mxr, PYSEC-2026-1702.