The problem can be corrected by upgrading the affected package to
version 2.5.5-6ubuntu0.9 (for Ubuntu 4.10), or 2.5.8-3ubuntu1.2 (for
Ubuntu 5.04). In general, a standard system upgrade is sufficient to
effect the necessary changes.
It was discovered that Squid did not verify the validity of DNS server
responses. When Squid is started, it opens a DNS client UDP port whose
number is randomly assigned by the operating system. Unless your
network firewall is configured to accept DNS responses only from known
good nameservers, this vulnerability allowed users within the local
network to inject arbitrary DNS responses into Squid ("DNS spoofing").
This could be used to present different web pages to users from those
they actually requested.