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.