source: http://www.securityfocus.com/bid/8695/info

It has been reported that Null HTTPd is prone to a cross-site scripting vulnerability when displaying error pages that may allow an attacker to execute HTML or script code in a user's browser. The issue was previously reported and fixed (BID 5603), however an attacker may send a long HTTP request to the software, which may overwrite memory containing the fix for cross-site scripting issue. As a result, an attacker may bypass the check for cross-site scripting and construct a link containing malicious HTML and script code to be rendered in a user's browser upon visiting that link.

Successful exploitation of this issue may allow an attacker to steal cookie-based authentication credentials. Other attacks are also possible.

Null HTTPd version 0.5.1 and prior are reported to be prone to this issue. 

http://www.example.com/ [1799 bytes --needed to avoid XXS] [243 bytes --XXS code]


Answer from NULLhttpd:

----
HTTP/1.0 200 OK
Cache-Control: no-store
Connection: Close
Content-Length: 472
Date: Tue, 23 Sep 2003 11:39:30 GMT
Expires: Tue, 23 Sep 2003 11:39:30 GMT
Last-Modified:
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaano-cache
Pragma: no-cache
Server: Null httpd 0.5.1
Content-Type: text/html

<script>alert('hello');</script>xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxx<HTML><HEAD><TITLE>400 Bad Request</TITLE></HEAD>
<BODY BGCOLOR=#F0F0F0 TEXT=#000000 LINK=#0000FF ALINK=#0000FF VLINK=#0000FF>
<H1>400 Bad Request</H1>
Can't Parse Request.
<HR>
<ADDRESS>Null httpd 0.5.1</ADDRESS>
</BODY></HTML>
----