CVE-2025-59420
📋 TL;DR
Authlib versions before 1.6.4 fail to properly validate JWS tokens with unknown critical header parameters, violating RFC 7515 requirements. Attackers can craft tokens that bypass strict verification in mixed-language environments, potentially leading to authentication bypass or privilege escalation. This affects any Python application using Authlib for OAuth/OpenID Connect token validation.
💻 Affected Systems
- Authlib
📦 What is this software?
Authlib by Authlib
⚠️ Risk & Real-World Impact
Worst Case
Complete authentication bypass allowing attackers to impersonate any user, escalate privileges to administrative levels, or replay tokens indefinitely.
Likely Case
Policy bypass in mixed-language fleets where some services reject crafted tokens but Authlib accepts them, leading to inconsistent authorization decisions.
If Mitigated
Limited impact if all services use Authlib consistently or if additional token validation layers exist.
🎯 Exploit Status
Proof of concept available in advisory. Attack requires ability to submit crafted JWS tokens to vulnerable endpoints.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.6.4
Vendor Advisory: https://github.com/authlib/authlib/security/advisories/GHSA-9ggr-2464-2j32
Restart Required: Yes
Instructions:
1. Update Authlib: pip install --upgrade authlib==1.6.4
2. Restart all Python applications using Authlib
3. Verify no downgrades occur in dependency resolution
🔧 Temporary Workarounds
Implement custom JWS header validation
allAdd pre-validation to reject tokens with unknown critical header parameters before Authlib processes them.
# Python code to validate JWS headers before Authlib verification
import json
import base64
def validate_jws_header(token):
parts = token.split('.')
if len(parts) != 3:
return False
header = json.loads(base64.urlsafe_b64decode(parts[0] + '==').decode())
if 'crit' in header:
for param in header['crit']:
if param not in ['alg', 'jku', 'jwk', 'kid', 'x5u', 'x5c', 'x5t', 'x5t#S256', 'typ', 'cty', 'crit']:
return False
return True
🧯 If You Can't Patch
- Implement strict token validation at API gateway or load balancer level to reject malformed JWS tokens
- Deploy WAF rules to block requests containing JWS tokens with unusual critical header parameters
🔍 How to Verify
Check if Vulnerable:
Check Authlib version: python -c "import authlib; print(authlib.__version__)" - if version < 1.6.4, system is vulnerable.Check Version:
python -c "import authlib; print('Authlib version:', authlib.__version__)"Verify Fix Applied:
After update, verify version is 1.6.4+ and test with crafted JWS token containing unknown critical header - should be rejected.📡 Detection & Monitoring
Log Indicators:
- Authentication failures for tokens that previously worked
- Unusual critical header parameters in JWS tokens
- Inconsistent authentication decisions across services
Network Indicators:
- HTTP 401/403 responses followed by successful authentication with similar tokens
- Tokens with 'crit' header containing unusual values
SIEM Query:
source="auth_logs" AND ("crit" OR "critical") AND NOT ("alg" OR "kid" OR "typ")