Multiple vulnerabilities has been discovered and corrected in tomcat
5.5.x:

The implementation of HTTP DIGEST authentication in tomcat was
discovered to have several weaknesses (CVE-2011-1184).

Apache Tomcat, when the MemoryUserDatabase is used, creates log entries
containing passwords upon encountering errors in JMX user creation,
which allows local users to obtain sensitive information by reading
a log file (CVE-2011-2204).

Apache Tomcat, when sendfile is enabled for the HTTP APR or HTTP
NIO connector, does not validate certain request attributes, which
allows local users to bypass intended file access restrictions or
cause a denial of service (infinite loop or JVM crash) by leveraging
an untrusted web application (CVE-2011-2526).

Certain AJP protocol connector implementations in Apache Tomcat allow
remote attackers to spoof AJP requests, bypass authentication, and
obtain sensitive information by causing the connector to interpret
a request body as a new request (CVE-2011-3190).

The updated packages have been patched to correct these issues.