The Realm implementations in Apache Tomcat versions 9.0.0.M1 to 9.0.0.M9, 8.5.0 to 8.5.4, 8.0.0.RC1 to 8.0.36, 7.0.0 to 7.0.70 and 6.0.0 to 6.0.45 did not process the supplied password if the supplied user name did not exist. This made a timing attack possible to determine valid user names. Note that the default configuration includes the LockOutRealm which makes exploitation of this vulnerability harder.
https://www.oracle.com/security-alerts/cpuoct2021.html
https://www.oracle.com//security-alerts/cpujul2021.html
https://usn.ubuntu.com/4557-1/
https://security.netapp.com/advisory/ntap-20180605-0001/
https://access.redhat.com/errata/RHSA-2017:2247
https://access.redhat.com/errata/RHSA-2017:0456
https://access.redhat.com/errata/RHSA-2017:0455