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.
PUBLISHED5.2CWE-79
NiceGUI's XSS vulnerability in ui.markdown() allows arbitrary JavaScript execution through unsanitized HTML content
Problem type
Affected products
zauberzeug
nicegui
< 3.7.0 - AFFECTED
References
https://github.com/zauberzeug/nicegui/security/advisories/GHSA-v82v-c5x8-w282
https://github.com/zauberzeug/nicegui/security/advisories/GHSA-v82v-c5x8-w282
https://github.com/zauberzeug/nicegui/commit/f1f7533577875af7d23f161ed3627f73584cb561
https://github.com/zauberzeug/nicegui/commit/f1f7533577875af7d23f161ed3627f73584cb561
GitHub Security Advisories
GHSA-v82v-c5x8-w282
NiceGUI's XSS vulnerability in ui.markdown() allows arbitrary JavaScript execution through unsanitized HTML content
https://github.com/advisories/GHSA-v82v-c5x8-w282Description
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
Impact
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
Remediation
A release has been published in version 3.7.0.
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.
github.com
https://github.com/zauberzeug/nicegui/security/advisories/GHSA-v82v-c5x8-w282
github.com
https://github.com/zauberzeug/nicegui/commit/f1f7533577875af7d23f161ed3627f73584cb561
nvd.nist.gov
https://nvd.nist.gov/vuln/detail/CVE-2026-25516
github.com
https://github.com/advisories/GHSA-v82v-c5x8-w282
JSON source
https://cveawg.mitre.org/api/cve/CVE-2026-25516Click to expand
{
"dataType": "CVE_RECORD",
"dataVersion": "5.2",
"cveMetadata": {
"cveId": "CVE-2026-25516",
"assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
"assignerShortName": "GitHub_M",
"dateUpdated": "2026-02-06T21:12:19.501Z",
"dateReserved": "2026-02-02T18:21:42.487Z",
"datePublished": "2026-02-06T21:12:19.501Z",
"state": "PUBLISHED"
},
"containers": {
"cna": {
"providerMetadata": {
"orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
"shortName": "GitHub_M",
"dateUpdated": "2026-02-06T21:12:19.501Z"
},
"title": "NiceGUI's XSS vulnerability in ui.markdown() allows arbitrary JavaScript execution through unsanitized HTML content",
"descriptions": [
{
"lang": "en",
"value": "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."
}
],
"affected": [
{
"vendor": "zauberzeug",
"product": "nicegui",
"versions": [
{
"version": "< 3.7.0",
"status": "affected"
}
]
}
],
"problemTypes": [
{
"descriptions": [
{
"lang": "en",
"description": "CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')",
"cweId": "CWE-79",
"type": "CWE"
}
]
}
],
"references": [
{
"url": "https://github.com/zauberzeug/nicegui/security/advisories/GHSA-v82v-c5x8-w282",
"name": "https://github.com/zauberzeug/nicegui/security/advisories/GHSA-v82v-c5x8-w282",
"tags": [
"x_refsource_CONFIRM"
]
},
{
"url": "https://github.com/zauberzeug/nicegui/commit/f1f7533577875af7d23f161ed3627f73584cb561",
"name": "https://github.com/zauberzeug/nicegui/commit/f1f7533577875af7d23f161ed3627f73584cb561",
"tags": [
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],
"metrics": [
{
"cvssV3_1": {
"version": "3.1",
"vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
"attackVector": "NETWORK",
"attackComplexity": "LOW",
"privilegesRequired": "NONE",
"userInteraction": "REQUIRED",
"scope": "CHANGED",
"confidentialityImpact": "LOW",
"integrityImpact": "LOW",
"availabilityImpact": "NONE",
"baseScore": 6.1,
"baseSeverity": "MEDIUM"
}
}
]
}
}
}