CVE-2026-9689: Keycloak Auth Bypass Vulnerability

CVE-2026-9689 Overview

CVE-2026-9689 affects Keycloak, an open-source identity and access management (IAM) solution. The flaw enables HTTP parameter pollution when a client application is configured to accept broad redirect Uniform Resource Identifiers (URIs). A remote attacker can craft a malicious authentication link that, once clicked by a victim, causes the client application to prioritize attacker-controlled parameters over legitimate ones. Successful exploitation can bypass security controls or enable unauthorized access to protected resources. The weakness is tracked as CWE-1288 (Improper Validation of Consistency within Input).

Critical Impact

Attackers can manipulate OAuth/OIDC authentication flows through parameter pollution, potentially bypassing redirect validation and accessing protected resources when victims click crafted links.

Affected Products

• Keycloak (open-source identity and access management server)
• Red Hat Build of Keycloak distributions
• Client applications configured with broad redirect URI patterns

Discovery Timeline

• 2026-05-27 - CVE CVE-2026-9689 published to NVD
• 2026-05-27 - Last updated in NVD database

Technical Details for CVE-2026-9689

Vulnerability Analysis

The vulnerability resides in Keycloak's handling of OAuth 2.0 and OpenID Connect (OIDC) authentication flows when clients permit overly permissive redirect URI patterns. Keycloak processes authentication requests containing URL parameters such as client_id, redirect_uri, state, and response_type. When duplicate parameters appear in a single request, inconsistent parsing between Keycloak and downstream client applications creates an exploitable mismatch.

An attacker constructs an authorization URL containing duplicated parameters. Keycloak validates one instance while the client application later consumes another. This inconsistency lets attacker-controlled values override legitimate authentication context.

The attack requires user interaction. A victim must click the crafted link, which is plausible in phishing scenarios targeting authenticated users. The attack complexity is high because exploitation depends on permissive client configuration combined with specific parser disagreements.

Root Cause

The root cause is improper validation of consistency within input (CWE-1288). Keycloak and client applications do not enforce a single canonical interpretation of repeated query string parameters. Broad redirect URI patterns, such as wildcards or partial matches, further expand the attacker's ability to substitute values that still pass validation.

Attack Vector

An attacker crafts an authorization request containing polluted parameters and delivers it to a victim through phishing, malicious advertising, or compromised pages. When the user clicks the link, the authentication flow proceeds, but the client application acts on attacker-supplied values. This can redirect authorization codes, manipulate state validation, or bypass intended access restrictions. Refer to the Red Hat CVE-2026-9689 Advisory for vendor-specific technical detail.

Detection Methods for CVE-2026-9689

Indicators of Compromise

• Authorization requests containing duplicate query parameters such as multiple redirect_uri, client_id, or state values
• Authentication logs showing redirect URIs that match permitted patterns but deviate from registered values
• Unexpected authorization code exchanges originating from external referrers
• Spikes in failed or anomalous OAuth/OIDC callbacks tied to specific client identifiers

Detection Strategies

• Parse Keycloak access and event logs for repeated query string keys within a single authorization request
• Correlate redirect_uri values consumed by client applications against the values validated by Keycloak
• Alert on authorization flows where the referring host differs from expected trusted domains
• Inspect web proxy and WAF logs for HTTP parameter pollution patterns targeting /auth/realms/*/protocol/openid-connect/auth

Monitoring Recommendations

• Enable Keycloak event logging at the ERROR and WARN level for authentication and client events
• Forward Keycloak events to a centralized SIEM with normalized parsing of OAuth parameters
• Establish baselines for redirect URI usage per client and alert on deviations
• Review client registrations periodically for wildcards or overly broad redirect patterns

How to Mitigate CVE-2026-9689

Immediate Actions Required

• Audit all Keycloak client configurations and replace broad or wildcard redirect URI patterns with exact matches
• Apply vendor updates referenced in the Red Hat CVE-2026-9689 Advisory as soon as fixed builds are available
• Restrict authorization endpoints behind a WAF that normalizes duplicate query parameters
• Notify users of phishing risk and enforce step-up authentication for sensitive client applications

Patch Information

Consult the Red Hat CVE-2026-9689 Advisory and Red Hat Bug Report #2481845 for fixed package versions, errata identifiers, and upstream Keycloak release notes. Apply updates across all Keycloak nodes and restart the service to load corrected validation logic.

Workarounds

• Replace wildcard redirect URIs with full, exact URI values per client
• Configure upstream proxies or API gateways to reject requests containing duplicate OAuth parameters
• Enforce state and PKCE validation in client applications to limit the value of parameter manipulation
• Disable unused Keycloak clients and rotate client secrets after configuration hardening

# Example: tighten a Keycloak client redirect URI using kcadm.sh
# Replace wildcard patterns with exact URIs
./kcadm.sh update clients/<client-uuid> \
  -r <realm> \
  -s 'redirectUris=["https://app.example.com/oidc/callback"]' \
  -s 'rootUrl=https://app.example.com' \
  -s 'attributes."pkce.code.challenge.method"=S256'