medium CVSS 6.1 EPSS 0.24%

CVE-2026-25516·Python vulnerability

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.

Severity
medium
Software
Python
Fixed in
3.7.0
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at 0.24% — higher than 15% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Affected versions

Until: 3.7.0

Fixed in: 3.7.0

How to fix this CVE

Update NiceGUI to version 3.7.0 or later to resolve the XSS vulnerability in the ui.markdown() component. The vulnerability allows attackers to inject malicious HTML and JavaScript through markdown content when user input is rendered without sanitization. Ensure all Python environments running NiceGUI applications are patched immediately, especially those processing untrusted markdown content from end users.

sudo dnf update python3-nicegui

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST requests containing markdown payloads with embedded HTML tags such as <img src=x onerror=alert()>, <svg onload=>, or <iframe> elements directed at ui.markdown() endpoints; browser console errors showing script execution from markdown-rendered content

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement input validation to strip or escape HTML tags in markdown content before submission to ui.markdown() endpoints. Deploy WAF rules to block requests containing event handler attributes (onerror, onclick, onload, etc.) in markdown payloads. Consider implementing Content Security Policy (CSP) headers with script-src directives to prevent inline script execution.

How to check if you are affected

  1. Check NiceGUI version: pip show nicegui | grep Version
  2. Identify markdown usage in your codebase: grep -r 'ui.markdown' /path/to/application --include='*.py'
  3. Review application logs for suspicious HTML/JavaScript patterns in markdown inputs: grep -i '<script\|onerror\|onclick\|onload' /var/log/application.log
  4. Verify the patch: pip show nicegui | grep -q 'Version: 3.7.0' && echo 'Patched' || echo 'Vulnerable'

FAQ

What is CVE-2026-25516?

This is a cross-site scripting (XSS) vulnerability in NiceGUI's ui.markdown() component that fails to sanitize HTML content before rendering it in the browser. Attackers can inject malicious JavaScript payloads through markdown content that will execute in the context of the application.

Is CVE-2026-25516 being actively exploited?

No, this vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploit code is currently available.

What versions of NiceGUI are affected by CVE-2026-25516?

All versions prior to 3.7.0 are affected by this vulnerability. The fix was introduced in version 3.7.0 and later.

How do I check if my server is vulnerable to CVE-2026-25516?

Run 'pip show nicegui | grep Version' to determine the installed version. If the version is below 3.7.0, your application is vulnerable. Additionally, search your code for ui.markdown() calls processing user-supplied content.

Does Defensia detect CVE-2026-25516?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If NiceGUI is installed on a monitored server, CVE-2026-25516 will appear in your dashboard with remediation steps.

Related Python CVEs

CVE-2026-25632CVSS 10EPyT-Flow is a Python package designed for the easy generation of hydraulic and water quality scenario data of water distribution networks. Prior to 0.16.1, EPyT-Flow’s REST API parses attacker-controlled JSON request bodies using a custom deserializer (my_load_from_json) that supports a type field. When type is present, the deserializer dynamically imports an attacker-specified module/class and instantiates it with attacker-supplied arguments. This allows invoking dangerous classes such as subprocess.Popen, which can lead to OS command execution during JSON parsing. This also affects the loading of JSON files. This vulnerability is fixed in 0.16.1.
CVE-2026-34938CVSS 10PraisonAI is a multi-agent teams system. Prior to version 1.5.90, execute_code() in praisonai-agents runs attacker-controlled Python inside a three-layer sandbox that can be fully bypassed by passing a str subclass with an overridden startswith() method to the _safe_getattr wrapper, achieving arbitrary OS command execution on the host. This issue has been patched in version 1.5.90.
CVE-2026-33054CVSS 10Mesop is a Python-based UI framework that allows users to build web applications. Versions 1.2.2 and below contain a Path Traversal vulnerability that allows any user supplying an untrusted state_token through the UI stream payload to arbitrarily target files on the disk under the standard file-based runtime backend. This can result in application denial of service (via crash loops when reading non-msgpack target files as configurations), or arbitrary file manipulation. This vulnerability heavily exposes systems hosted utilizing FileStateSessionBackend. Unauthorized malicious actors could interact with arbitrary payloads overwriting or explicitly removing underlying service resources natively outside the application bounds. This issue has been fixed in version 1.2.3.
CVE-2026-28505CVSS 10Tautulli is a Python based monitoring and tracking tool for Plex Media Server. Prior to version 2.17.0, the str_eval() function in notification_handler.py implements a sandboxed eval() for notification text templates. The sandbox attempts to restrict callable names by inspecting code.co_names of the compiled code object. However, co_names only contains names from the outer code object. When a lambda expression is used, it creates a nested code object whose attribute accesses are stored in code.co_consts, NOT in code.co_names. The sandbox never inspects nested code objects. This issue has been patched in version 2.17.0.
CVE-2026-26030CVSS 9.9Semantic Kernel, Microsoft's semantic kernel Python SDK, has a remote code execution vulnerability in versions prior to 1.39.4, specifically within the `InMemoryVectorStore` filter functionality. The problem has been fixed in version `python-1.39.4`. Users should upgrade this version or higher. As a workaround, avoid using `InMemoryVectorStore` for production scenarios.

References

Track CVEs across your fleet automatically

Defensia scans your Linux servers and tells you exactly which ones are running vulnerable versions — including CVE-2026-25516. Free for 1 server.

Get started free