CVE-2026-40112: PraisonAI has Stored XSS via Unsanitized Agent Output in HTML Rendering (nh3 Not a Required Dependency)
Summary
The Flask API endpoint in src/praisonai/api.py renders agent output as HTML without effective sanitization. The sanitizehtml function relies on the nh3 library, which is not listed as a required or optional dependency in pyproject.toml. When nh3 is absent (the default installation), the sanitizer is a no-op that returns HTML unchanged. An attacker who can influence agent input (via RAG data poisoning, web scraping results, or prompt injection) can inject arbitrary JavaScript that executes in the browser of anyone viewing the API output.
Details
In src/praisonai/api.py, lines 6-14 define the sanitizer with a try/except ImportError fallback:
python try: import nh3 def sanitizehtml(html: str) -> str: return nh3.clean(html) except ImportError: def sanitizehtml(html: str) -> str: """Fallback: no nh3, return as-is (install nh3 for XSS protection).""" return html
The home() route at lines 21-25 converts agent output to HTML via markdown.markdown() (which preserves raw HTML tags by default) and embeds it in an HTML response using an f-string — bypassing Flask's Jinja2 auto-escaping:
python @app.route('/') def home(): output = basic() htmloutput = sanitizehtml(markdown.markdown(str(output))) return f'<html><body>{htmloutput}</body></html>'
Since nh3 is not in any dependency list (pyproject.toml core deps, optional deps, or requirements files), a standard installation will always hit the fallback path. The markdown library's default behavior passes through raw HTML tags in input text, so any <script> or event handler attributes in the agent output flow directly into the response.
Additionally, deploy.py:76-91 generates a deployment version of api.py that has no sanitization at all — it directly calls markdown.markdown(output) without any sanitizehtml wrapper.
PoC
1. Set up a PraisonAI instance with an agent that processes external content (e.g., web scraping or RAG retrieval):
yaml agents.yaml framework: crewai topic: test roles: researcher: role: Researcher goal: Process user-provided content backstory: You process content exactly as given tasks: process: description: "Return this exact text: <img src=x onerror=alert(document.cookie)>" expectedoutput: The text as-is
2. Verify nh3 is not installed (default): bash pip show nh3 2>&1 | grep -c "not found" Returns 1 (not installed)
3. Start the API: bash python src/praisonai/api.py
4. Access the endpoint: bash curl http://localhost:5000/
5. Response contains unsanitized HTML: html <html><body><p><img src=x onerror=alert(document.cookie)></p></body></html>
6. Opening this in a browser executes the JavaScript payload.
Impact
- Session hijacking: An attacker can steal cookies or session tokens from users viewing the API output. - Credential theft: Injected scripts can present fake login forms or exfiltrate data to attacker-controlled servers. - Actions on behalf of users: Malicious JavaScript can perform actions in the context of the victim's browser session.
The attack surface includes any scenario where agent output contains attacker-influenced content: RAG retrieval from poisoned documents, web scraping of malicious pages, processing of adversarial user prompts, or multi-agent communication where one agent's output is tainted.
Recommended Fix
Make nh3 a required dependency when using the API, and remove the silent fallback:
python Option 1: Make nh3 required in pyproject.toml under the "api" optional dependency In pyproject.toml: api = [ "flask>=3.0.0", ... "nh3>=0.2.14", ]
Option 2: Use markdown's built-in HTML stripping as a safe default import markdown
def sanitizehtml(html: str) -> str: try: import nh3 return nh3.clean(html) except ImportError: import re return re.sub(r'<[^>]+>', '', html) # Strip all HTML tags as fallback
Option 3 (preferred): Use Flask's Jinja2 templating with auto-escaping instead of f-string interpolation, or use markupsafe.escape() from markupsafe import Markup
@app.route('/') def home(): output = basic() # Use markdown with safe extensions only htmloutput = markdown.markdown(str(output), extensions=[]) try: import nh3 htmloutput = nh3.clean(htmloutput) except ImportError: raise RuntimeError("nh3 is required for safe HTML rendering. Install with: pip install nh3") return f'<html><body>{htmloutput}</body></html>'
Also fix deploy.py:76-91 to include sanitization in the generated api.py.
Other sources
PraisonAI is a multi-agent teams system. Prior to 4.5.128, the Flask API endpoint in src/praisonai/api.py renders agent output as HTML without effective sanitization. The sanitizehtml function relies on the nh3 library, which is not listed as a required or optional dependency in pyproject.toml. When nh3 is absent (the default installation), the sanitizer is a no-op that returns HTML unchanged. An attacker who can influence agent input (via RAG data poisoning, web scraping results, or prompt injection) can inject arbitrary JavaScript that executes in the browser of anyone viewing the API output. This vulnerability is fixed in 4.5.128.
— MITRE