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The vulnerability in Keycloak stems from an incorrect sequence of operations when handling the Authorization header in HTTP requests. According to RFC 6750, the Bearer authentication scheme must be strictly parsed, with the scheme name “Bearer” being case-insensitive and followed by a single space before the token. However, Keycloak performs authorization checks before fully parsing and canonicalizing the header. This means that the system evaluates access based on the raw header input, which may contain non-standard characters. For example, the parser accepts tab characters or multiple spaces as separators between “Bearer” and the token, and it tolerates case variations like “bearer” or “BeArEr”. This permissive parsing can lead to misinterpretation of the token value. Attackers can craft requests with these malformed headers to potentially bypass authentication mechanisms. The flaw occurs because the authorization logic does not enforce RFC-compliant normalization first, allowing ambiguous headers to be processed incorrectly. This could result in the system granting access when it should deny it, or causing downstream components to handle tokens inconsistently. The issue is particularly critical in scenarios where strict token validation is required for security policies.
Platform: Keycloak
Version: Not specified
Vulnerability: Authorization Parsing Flaw
Severity: Moderate
date: Jan 8, 2026
Prediction: Patch expected soon
What Undercode Say:
Analytics:
curl -H “Authorization: Bearer\tmytoken” http://localhost:8080
curl -H “Authorization: bearer mytoken” http://localhost:8080
curl -H “Authorization: Bearer mytoken” http://localhost:8080
python3 -c “import requests; headers={‘Authorization’: ‘Bearer\tmytoken’}; r=requests.get(‘http://localhost:8080’, headers=headers)”
cat > test_vuln.sh << 'EOF'
!/bin/bash
for header in “Bearer\tmytoken” “bearer mytoken” “BeArEr mytoken”; do
curl -H “Authorization: $header” http://localhost:8080
done
EOF
How Exploit:
Craft HTTP requests with non-standard Authorization headers using tabs, multiple spaces, or case variations in “Bearer” to bypass authentication or cause parsing errors.
Protection from this CVE:
Update Keycloak to patched version. Validate headers per RFC 6750. Implement input sanitization.
Impact:
Authorization bypass possible. Unauthorized access to resources. Inconsistent token handling.
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Sources:
Reported By: github.com
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