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Dynamic OAuth Callback URL Constructed from Host Header #93

Description

@andrewmusselman

Issue: FINDING-164 - Dynamic OAuth Callback URL Constructed from Host Header

Labels: bug, security, priority:medium, asvs-level:L1

ASVS Level(s): L1

Description:

Summary

The OAuth redirect_uri parameter is dynamically derived from the HTTP Host header (callback_host = quart.request.host_url.replace('http://', 'https://')). This violates ASVS 10.4.1 because: (1) The client sends a variable callback URL instead of a fixed, pre-registered one; (2) If the deployment violates the documented ProxyPreserveHost On assumption, an attacker controlling the Host header can craft an arbitrary callback URL; (3) If the authorization server uses pattern-based or prefix matching instead of exact string comparison, the authorization code could be redirected to an attacker-controlled domain. This could lead to authorization code theft and account takeover, contingent on AS redirect URI validation strictness and deployment configuration.

Details

The vulnerability exists in src/asfquart/generics.py lines 63-68, where the callback URL is constructed from quart.request.host_url. This creates a dynamic redirect URI that varies based on the incoming request's Host header.

Recommended Remediation

Pre-configure the callback host using an environment variable instead of deriving it from the request:

import os
import urllib.parse

# In configuration
CALLBACK_HOST = os.environ.get('OAUTH_CALLBACK_HOST', 'https://myapp.apache.org')

# Modify setup_oauth() to accept callback_host parameter
def setup_oauth(app, uri: str, callback_host: str):
    @app.route(uri)
    async def oauth_endpoint():
        state = secrets.token_urlsafe(16)
        # Use fixed callback_host instead of request.host_url
        callback_url = urllib.parse.urljoin(callback_host, f'{uri}?state={state}')
        # ... rest of implementation

This ensures the callback URL is fixed and can be pre-registered with exact string matching at the Authorization Server.

Acceptance Criteria

  • OAUTH_CALLBACK_HOST environment variable added to configuration
  • setup_oauth() modified to accept callback_host parameter
  • Callback URL constructed from fixed configuration instead of Host header
  • Unit tests verify callback URL is not affected by Host header
  • Integration tests verify OAuth flow with fixed callback URL
  • Documentation updated with required environment variable
  • Deployment guide updated to emphasize fixed callback URL registration

References

  • Source reports: L1:10.4.1.md
  • Related findings: FINDING-273
  • ASVS sections: 10.4.1
  • CWE: CWE-601

Priority

Medium

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