CVE-2026-33557: Apache Kafka: Missing JWT token validation in OAUTHBEARER authentication

Severity: important Affected versions: - Apache Kafka 4.1.0 through 4.1.1 Description: A possible security vulnerability has been identified in Apache Kafka. By default, the broker property `sasl.oauthbearer.jwt.validator.class` is set to `org.apache.kafka.common.security.oauthbearer.DefaultJwtValidator`. It accepts any JWT token without validating its signature, issuer, or audience. An attacker can generate a JWT token from any issuer with the `preferred_username` set to any user, and the broker will accept it. We advise the Kafka users using kafka v4.1.0 or v4.1.1 to set the config `sasl.oauthbearer.jwt.validator.class` to `org.apache.kafka.common.security.oauthbearer.BrokerJwtValidator` explicitly to avoid this vulnerability. Since Kafka v4.1.2 and v4.2.0 and later, the issue is fixed and will correctly validate the JWT token. Credit: Павел Романов <promanov1994@gmail.com> (finder) References: https://kafka.apache.org/cve-list https://kafka.apache.org/ https://www.cve.org/CVERecord?id=CVE-2026-33557