Graylog before 3.3.3 lacks SSL Certificate Validation for LDAP servers. It allows use of an external user/group database stored in LDAP. The connection configuration allows the usage of unencrypted, SSL- or TLS-secured connections. Unfortunately, the Graylog client code (in all versions that support LDAP) does not implement proper certificate validation (regardless of whether the "Allow self-signed certificates" option is used). Therefore, any attacker with the ability to intercept network traffic between a Graylog server and an LDAP server is able to redirect traffic to a different LDAP server (unnoticed by the Graylog server due to the lack of certificate validation), effectively bypassing Graylog's authentication mechanism.