CVE-2026-11800: Org.keycloak:keycloak-services: keycloak: authentication bypass via jwt algorithm confusion
A flaw was found in Keycloak. This JWT algorithm confusion vulnerability in the JWT Authorization Grant flow allows an attacker with valid client credentials to bypass signature verification. By forging an assertion, the attacker can create unauthorized access tokens. This enables the attacker to impersonate any federated user linked to the affected Identity Provider, leading to unauthorized access and potential privilege escalation.
Other sources
A JWT algorithm confusion vulnerability was found in Keycloak's JWT Authorization Grant flow . A flaw in the signature verification logic allows an attacker with valid client credentials to submit a forged assertion using alg=HS256 with the Identity Provider's public key bytes as the HMAC secret, bypassing asymmetric signature verification. This enables the attacker to mint access tokens and impersonate any federated user linked to the affected IdP. The vulnerability requires an IdP configured with jwtAuthorizationGrantEnabled=true, a hardcoded public key (useJwksUrl=false), and no pinned assertion signature algorithm (the default).
— Red Hat
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Configuration
Disable the JWT Authorization Grant flow by setting jwtAuthorizationGrantEnabled=false on the configured IdP, since the vulnerability requires jwtAuthorizationGrantEnabled=true.
Keycloak JWT Authorization Grant (IdP / Federation) jwtAuthorizationGrantEnabled = false - Configuration
If the IdP is configured with useJwksUrl=false (hardcoded public key bytes), update the IdP configuration so it is not using the vulnerable setup (the described attack scenario requires useJwksUrl=false).
Keycloak JWT Authorization Grant (IdP / JWKS configuration) useJwksUrl = false - Compensating control
Configure/pin the assertion signature algorithm instead of leaving the default (no pinned assertion signature algorithm); the vulnerability description notes that the attack requires 'no pinned assertion signature algorithm (the default)'.
Event History
Frequently Asked Questions
What is the severity of CVE-2026-11800?
The severity of CVE-2026-11800 is rated as high, with a CVSS score of 8.1.
How do I fix CVE-2026-11800?
To fix CVE-2026-11800, update your Keycloak installation to the latest version where the vulnerability has been patched.
What does CVE-2026-11800 exploit?
CVE-2026-11800 exploits a JWT algorithm confusion vulnerability in the JWT Authorization Grant flow.
What impact does CVE-2026-11800 have on security?
CVE-2026-11800 allows an attacker to bypass signature verification and create unauthorized access tokens.
Who is affected by CVE-2026-11800?
Organizations using Keycloak for authentication that employ JWTs are affected by CVE-2026-11800.