In May 2006 a reporter found a bug in Apache where an invalid Expect header sent
to the server (Apache 1.3.3 onwards) would be returned to the user in an error
message, unescaped.  This could allow a cross-site scripting attack only if a
victim can tricked into connecting to a site and sending such a carefully
crafted Expect header.  Whist browsers do not provide this functionality, it was
recently discovered that Flash allows you to make a connection with arbitrary
headers.  The attack mechanism is therefore:

1. User is tricked into visiting a malicious web site with a flash-enabled browser
2. Malicious web site uses a flash movie to make a connection to the target site
with custom Expect header
3. This results in cross-site scripting (attacker could steal your cookies from
the third party site, or inject content etc)

On http 2.0 versions the cross-site scripting does not happen immediately, step 
is delayed until the Apache server times out.  On a default installation this
could be 2-5 minutes.  Therefore the attack is less likely to succeed.

Patch:
http://svn.apache.org/viewcvs?rev=394965&view=rev [1.3]
http://svn.apache.org/viewcvs?rev=395172&view=rev [2.0]