The STP protocol, as enabled in Linux 2.4.x, does not provide sufficient security by design, which allows attackers to modify the bridge topology.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A380
http://www.redhat.com/support/errata/RHSA-2003-239.html
http://www.redhat.com/support/errata/RHSA-2003-238.html