Fix XSS in API Responses in Sanic
XSS in Sanic APIs typically stems from improper use of response types or failing to sanitize user-controlled data reflected in the output. While Sanic is built for speed, developers often shoot themselves in the foot by returning raw strings with 'text/html' content types or using 'response.html' without escaping, allowing attackers to inject malicious scripts into the DOM.
The Vulnerable Pattern
from sanic import Sanic, responseapp = Sanic(‘VulnerableAPI’)
@app.get(‘/v1/greet’) async def vuln_handler(request): name = request.args.get(‘name’, ‘Guest’) # VULNERABLE: Direct reflection of input into HTML response return response.html(f’
Hello {name}
’)
@app.get(‘/v1/debug’) async def debug_handler(request): payload = request.args.get(‘data’) # VULNERABLE: Setting content_type to text/html for what should be JSON/Plain text return response.text(f’Debug info: {payload}’, content_type=‘text/html’)
The Secure Implementation
To kill XSS in Sanic, follow three rules: 1. Use 'response.json()' for all API endpoints. This forces the 'Content-Type: application/json' header, which instructs modern browsers not to interpret the body as HTML. 2. If you must return HTML, use 'markupsafe.escape' or a templating engine like Jinja2 that auto-escapes. 3. Hardening: Apply a global middleware to inject 'Content-Security-Policy' (CSP) and 'X-Content-Type-Options: nosniff' headers. This prevents the browser from 'sniffing' a JSON response and trying to render it as HTML if it contains script-like tags.
from sanic import Sanic, response from markupsafe import escapeapp = Sanic(‘SecureAPI’)
@app.get(‘/v1/greet’) async def secure_handler(request): name = request.args.get(‘name’, ‘Guest’) # SECURE: Explicitly escape user input before rendering return response.html(f’
Hello {escape(name)}
’)@app.get(‘/v1/data’) async def json_handler(request): user_data = request.args.get(‘input’) # SECURE: Use response.json() which sets ‘application/json’ and prevents HTML parsing return response.json({‘status’: ‘ok’, ‘received’: user_data})
@app.middleware(‘response’) async def add_csp_headers(request, response): # SECURE: Implement Content-Security-Policy as a secondary defense response.headers[‘Content-Security-Policy’] = “default-src ‘self’; script-src ‘none’;”
Your API Responses API
might be exposed to XSS
74% of API Responses apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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