CVE-2026-0707: Keycloak Auth Bypass Vulnerability

CVE-2026-0707 Overview

A flaw was found in Keycloak where the Authorization header parser is overly permissive regarding the formatting of the "Bearer" authentication scheme. The parser accepts non-standard characters (such as tabs) as separators and tolerates case variations that deviate from RFC 6750 specifications. This permissive parsing behavior could potentially be exploited to bypass security controls or cause inconsistent authentication handling across different systems.

Critical Impact

Keycloak's permissive Authorization header parsing deviates from RFC 6750 specifications, potentially allowing attackers to craft malformed Bearer tokens that bypass security filters while still being accepted by Keycloak.

Affected Products

• Keycloak (specific versions not disclosed)

Discovery Timeline

• 2026-01-08 - CVE CVE-2026-0707 published to NVD
• 2026-01-08 - Last updated in NVD database

Technical Details for CVE-2026-0707

Vulnerability Analysis

This vulnerability relates to CWE-551 (Incorrect Behavior Order: Authorization Before Parsing and Canonicalization). The Keycloak Authorization header parser exhibits overly permissive behavior that deviates from the strict formatting requirements defined in RFC 6750, which specifies the Bearer authentication scheme for OAuth 2.0.

The parser incorrectly accepts non-standard separator characters, including tab characters, between the "Bearer" keyword and the token value. Additionally, it tolerates case variations in the "Bearer" scheme identifier (such as "bearer", "BEARER", or mixed-case variants) rather than enforcing the standard capitalization.

This permissive parsing can lead to security inconsistencies when Keycloak is deployed behind reverse proxies, web application firewalls (WAFs), or other security appliances that may strictly enforce RFC 6750 compliance. An attacker could potentially craft Authorization headers that bypass upstream security controls while still being accepted by Keycloak.

Root Cause

The root cause is improper input validation in the Authorization header parsing logic. The parser fails to strictly enforce RFC 6750 formatting requirements, accepting malformed or non-standard Bearer token headers instead of rejecting them. This permissive approach creates a semantic gap between how Keycloak interprets Authorization headers versus how other security components in the request chain may interpret them.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by sending HTTP requests with malformed Authorization headers containing non-standard characters or case variations. The attack scenarios include:

1. Security Filter Bypass: Crafting Authorization headers with tab separators or case variations that upstream security filters may not recognize as Bearer tokens, while Keycloak accepts them as valid
2. Request Smuggling: Using parsing discrepancies to smuggle malicious requests past security controls
3. Token Injection: Exploiting permissive parsing to inject unexpected characters or values into the authentication flow

The vulnerability can be exploited by sending HTTP requests with malformed Authorization headers. For example, headers using tab characters as separators between "Bearer" and the token, or using non-standard case variations of the "Bearer" keyword. For detailed technical information, refer to the Red Hat CVE-2026-0707 Advisory and Red Hat Bug Report #2427768.

Detection Methods for CVE-2026-0707

Indicators of Compromise

• HTTP requests containing Authorization headers with tab characters (\t) as separators between "Bearer" and the token value
• Authorization headers using non-standard case variations such as bearer, BEARER, or mixed-case like BeArEr
• Unusual patterns of authentication requests that may indicate probing for parser inconsistencies

Detection Strategies

• Implement logging and monitoring of all Authorization headers to detect non-standard formatting patterns
• Configure WAF rules to flag or block Authorization headers containing tab characters or unexpected whitespace
• Deploy application-layer intrusion detection to identify Bearer token format anomalies
• Audit authentication logs for requests with case-insensitive Bearer scheme variations

Monitoring Recommendations

• Enable detailed HTTP request logging on reverse proxies and load balancers to capture full Authorization header contents
• Create alerts for Authorization headers that contain control characters or deviate from RFC 6750 formatting
• Monitor for increased authentication failures that may indicate exploitation attempts
• Implement correlation rules to detect patterns of malformed authentication attempts from single sources

How to Mitigate CVE-2026-0707

Immediate Actions Required

• Review all Keycloak deployments for exposure and assess the risk based on your architecture
• Implement strict Authorization header validation at the WAF or reverse proxy layer to reject non-RFC-6750-compliant headers
• Monitor authentication logs for signs of exploitation attempts
• Subscribe to Red Hat security advisories for official patch announcements

Patch Information

Organizations should monitor the Red Hat CVE-2026-0707 Advisory for official patch releases. Apply vendor-supplied updates as soon as they become available. Review the Red Hat Bug Report #2427768 for additional technical details and remediation guidance.

Workarounds

• Deploy a reverse proxy or WAF in front of Keycloak configured to enforce strict RFC 6750 Authorization header formatting
• Implement custom request filters to normalize or reject malformed Authorization headers before they reach Keycloak
• Consider implementing additional authentication validation at the application layer to verify token integrity
• Use network segmentation to limit direct access to Keycloak instances

# Example nginx configuration to enforce strict Bearer token formatting
# Add to nginx location block protecting Keycloak
if ($http_authorization ~* "^(?!Bearer [A-Za-z0-9\-._~+/]+=*$)") {
    return 400;
}