“Application proxies and filters are also becoming increasingly
common. The reasons for deploying them are numerous, from simply
speeding up access to certain services (for example by using a Web
proxy) to filtering and access control and buffering internal
servers against the Internet. Most application proxy servers can be
attacked at the network level, but this will generally not result
in a compromise of the server or movement of packets past the proxy
server.”
“It is much more common (and productive in most cases) to attack
the proxy software itself, as it tends toward large complicated
packages of software. Because application proxies work at the
application layer, the data packets have been reassembled and
turned into their respective data formats. The proxy then handles
the data in some manner, from simply passing it on, to possibly
relabeling the request.”
“If a service is interactive, such as WWW, it can strip incoming
data of dangerous items, such as Web cookies or browser type
requests. Application proxies and filters allow you to reassemble
the data and inspect it. Scanning for viruses is one common
requirement, and generally cannot be achieved by a network layer
proxy or firewall.”