The host is installed with Apache Tomcat 5.5.0 through 5.5.27 or 6.0.0 through 6.0.18 and is prone to security bypass vulnerability. A flaw is present in the application, which fails handle a error checking in some authentication classes. Successful exploitation allows remote attackers to enumerate valid usernames via requests to /j_security_check with malformed URL encoding of passwords, related to improper error checking in the (1) MemoryRealm, (2) DataSourceRealm, and (3) JDBCRealm authentication realms, as demonstrated by a % (percent) value.