CVE-2026-18209 in Keycloak
Summary
by MITRE • 07/31/2026
A flaw was found in the keycloak-services component of Keycloak, which handles OpenID Connect (OIDC) authentication flows. The issue occurs because the security check designed to prevent HTTP parameter pollution only inspects the query portion of a redirect URL and ignores the fragment portion. When a client is configured with a wildcard redirect URI, an attacker can use this to inject duplicate security parameters into the login response. If a client application is not configured correctly, it might trust the attacker's injected data instead of the real security information from Keycloak, leading to session fixation or account confusion.
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Analysis
by VulDB Data Team • 07/31/2026
This vulnerability resides within the keycloak-services component of the Keycloak identity and access management platform, specifically affecting OpenID Connect authentication flows that are fundamental to modern single sign-on implementations. The flaw represents a critical security oversight in the parameter validation mechanism that is designed to prevent HTTP parameter pollution attacks. The vulnerability stems from an incomplete security check implementation that only examines the query string portion of redirect URLs while completely disregarding the fragment identifier portion, creating a significant attack surface that adversaries can exploit.
The technical exploitation occurs when clients are configured with wildcard redirect URIs, which in turn creates an opportunity for attackers to manipulate the authentication flow by injecting duplicate security parameters into the login response. This manipulation specifically targets the fragment portion of URLs where security parameters such as state tokens or nonce values can be duplicated or overridden. The fundamental flaw lies in the assumption that all security parameters are contained within the query string, while fragment parameters remain unvalidated and potentially exploitable.
The operational impact of this vulnerability is severe and can lead to session fixation attacks where an attacker can hijack legitimate user sessions by injecting malicious state parameters into the authentication response. Additionally, the vulnerability creates conditions for account confusion scenarios where client applications may inadvertently trust attacker-controlled data over legitimate security information provided by Keycloak. This can result in unauthorized access to user accounts, session takeover, and potential privilege escalation within the affected systems that rely on Keycloak for authentication.
This vulnerability aligns with CWE-1237 which specifically addresses HTTP Parameter Pollution attacks and represents a failure in input validation and sanitization of redirect URI parameters. The attack vector can be mapped to ATT&CK technique T1566.002 which covers phishing with malicious links, as attackers can craft deceptive URLs that appear legitimate but contain malicious fragment parameters. Organizations using Keycloak for authentication should implement immediate mitigations including disabling wildcard redirect URIs, enforcing strict redirect URI validation, and ensuring proper client application configuration to reject potentially malicious parameters from the fragment portion of URLs. The vulnerability underscores the importance of comprehensive input validation across all URL components and demonstrates the critical need for security controls that consider the complete HTTP request surface rather than focusing on isolated parameter sections.
The flaw highlights a common pattern in authentication system design where security controls are implemented with incomplete understanding of HTTP protocol specifications. This type of vulnerability is particularly dangerous because it can be exploited without requiring authentication credentials, and the impact can affect multiple user sessions simultaneously. The issue demonstrates how seemingly minor oversights in security implementation can create significant risks within identity management systems that are critical to enterprise security infrastructure. Organizations should conduct comprehensive security assessments of their authentication flows and ensure that all redirect URI validation mechanisms properly handle both query string and fragment parameters to prevent similar vulnerabilities from being exploited in other components or systems.