CVE-2026-21873
📋 TL;DR
This vulnerability in NiceGUI allows attackers to manipulate URL fragment identifiers via cross-site iframe attacks, potentially enabling UI manipulation or client-side attacks. It affects NiceGUI versions 2.22.0 through 3.4.1. Users of affected versions who expose NiceGUI applications to untrusted users are at risk.
💻 Affected Systems
- NiceGUI
📦 What is this software?
Nicegui by Zauberzeug
⚠️ Risk & Real-World Impact
Worst Case
Attacker could manipulate the UI to trick users into performing unintended actions, potentially leading to data exposure or unauthorized operations.
Likely Case
UI manipulation allowing attackers to modify page content or redirect users to malicious sites.
If Mitigated
Limited impact with proper content security policies and input validation in place.
🎯 Exploit Status
Exploitation requires attacker to host malicious iframe targeting vulnerable NiceGUI application.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 3.5.0
Vendor Advisory: https://github.com/zauberzeug/nicegui/security/advisories/GHSA-mhpg-c27v-6mxr
Restart Required: Yes
Instructions:
1. Update NiceGUI to version 3.5.0 or later using pip: pip install nicegui>=3.5.0
2. Restart your NiceGUI application
3. Verify the update was successful
🔧 Temporary Workarounds
Implement Content Security Policy
allAdd frame-ancestors directive to prevent iframe embedding
Add 'Content-Security-Policy: frame-ancestors 'self';' to HTTP headers
🧯 If You Can't Patch
- Disable or restrict ui.sub_pages functionality if not required
- Implement strict input validation and sanitization for URL fragment handling
🔍 How to Verify
Check if Vulnerable:
Check NiceGUI version in Python environment: python -c "import nicegui; print(nicegui.__version__)"Check Version:
python -c "import nicegui; print(nicegui.__version__)"Verify Fix Applied:
Verify version is 3.5.0 or higher and test UI functionality with sub_pages📡 Detection & Monitoring
Log Indicators:
- Unusual iframe requests to NiceGUI endpoints
- Multiple fragment identifier manipulations
Network Indicators:
- Cross-origin iframe requests targeting NiceGUI applications
SIEM Query:
http.url:*nicegui* AND http.referer:*iframe*