CVE-2026-25516

Description

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.

Proof of Concept

from nicegui import ui

# User-controlled input containing malicious payload
user_input = 'Hello! <img src=x onerror="alert(\'XSS\')">'

ui.markdown(user_input)  # XSS executes when page loads

ui.run()

When this page loads, the JavaScript in the onerror handler executes, potentially allowing an attacker to:

• Steal session cookies or authentication tokens
• Perform actions on behalf of the user
• Redirect users to malicious sites
• Modify page content

For Users (Immediate Workaround)

Until a fix is released, do not pass untrusted content directly to ui.markdown(). Instead, use one of these approaches:

Option 1: Convert and sanitize manually using ui.html()

import markdown2
from html_sanitizer import Sanitizer

sanitizer = Sanitizer()

def safe_markdown(content: str) -> None:
    """Render markdown with HTML sanitization."""
    html = markdown2.markdown(content)
    ui.html(sanitizer.sanitize(html), sanitize=False)

# Usage
safe_markdown(user_input)

Option 2: Escape HTML before markdown conversion (if raw HTML not needed)

import html

# Escape HTML entities - prevents any HTML from being interpreted
ui.markdown(html.escape(user_input))

Proposed Fix

Add a sanitize parameter to ui.markdown() consistent with other HTML-rendering components, and/or add an escape_html parameter.

Applications that render user-provided content through ui.markdown() are vulnerable to stored or reflected XSS attacks. This is particularly concerning for:

• Chat applications displaying user messages
• CMS or documentation systems with user-editable content
• Any application that displays markdown from untrusted sources

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-2026-25516 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 (3.7.0); upgrading removes the vulnerable code path.

Kodem intelligence

A release has been published in version 3.7.0.

1. What is CVE-2026-25516? CVE-2026-25516 is a medium-severity cross-site scripting (XSS) vulnerability in nicegui (pip), affecting versions <= 3.6.1. It is fixed in 3.7.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-25516? CVE-2026-25516 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-2026-25516? nicegui (pip) versions <= 3.6.1 is affected.
4. Is there a fix for CVE-2026-25516? Yes. CVE-2026-25516 is fixed in 3.7.0. Upgrade to this version or later.
5. Is CVE-2026-25516 exploitable, and should I be worried? Whether CVE-2026-25516 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-2026-25516 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-2026-25516? Upgrade nicegui to 3.7.0 or later.