CVE-2026-27156

Several NiceGUI APIs that execute methods on client-side elements (Element.run_method(), AgGrid.run_grid_method(), EChart.run_chart_method(), and others) use an eval() fallback in the JavaScript-side runMethod() function. When user-controlled input is passed as the method name, an attacker can inject arbitrary JavaScript that executes in the victim's browser.

Additionally, Element.run_method() and Element.get_computed_prop() used string interpolation instead of json.dumps() for the method/property name, allowing quote injection to break out of the intended string context.

Attack Vector

An attacker crafts a malicious URL with a payload as a query parameter. If the application passes this parameter as a method name to any of the affected APIs, the payload is sent to the client via WebSocket and executed via eval().

Example: /?method=alert(document.cookie) combined with application code like:

element.run_method(user_provided_method_name)

Affected Methods

1. Element.run_method()
2. Element.get_computed_prop()
3. AgGrid.run_grid_method()
4. AgGrid.run_row_method()
5. EChart.run_chart_method()
6. JsonEditor.run_editor_method()
7. Xterm.run_terminal_method()
8. Leaflet.run_map_method()
9. Leaflet.run_layer_method()
10. LeafletLayer.run_method()

Migration

Code that previously passed JavaScript functions as method names needs to use ui.run_javascript() instead:

# Before:
row = await grid.run_grid_method('g => g.getDisplayedRowAtIndex(0).data')

# After:
row = await ui.run_javascript(f'return getElement({grid.id}).api.getDisplayedRowAtIndex(0).data')

• Cookie/token theft
• DOM manipulation (phishing, fake login forms)
• Actions performed as the victim user

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-27156 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.8.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. Use json.dumps() for proper escaping of method/property names in run_method() and get_computed_prop()
2. Remove the eval() fallback from runMethod() in nicegui.js, method names that are not found on the element now raise an error instead of being evaluated as arbitrary JavaScript

1. What is CVE-2026-27156? CVE-2026-27156 is a medium-severity cross-site scripting (XSS) vulnerability in nicegui (pip), affecting versions <= 3.7.1. It is fixed in 3.8.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-27156? CVE-2026-27156 has a CVSS score of 6.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of nicegui are affected by CVE-2026-27156? nicegui (pip) versions <= 3.7.1 is affected.
4. Is there a fix for CVE-2026-27156? Yes. CVE-2026-27156 is fixed in 3.8.0. Upgrade to this version or later.
5. Is CVE-2026-27156 exploitable, and should I be worried? Whether CVE-2026-27156 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-27156 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-27156? Upgrade nicegui to 3.8.0 or later.