BID 14752 is not only an XSS vulnerability, the real problem is a directory
transversal flaw and affects Guppy versions less than 4.5.6a.

PoC (works for versions <4.5.4):
http://localhost/printfaq.php?lng=en&pg=/../../../../../../../etc/passwd%00

Explanation of the problem:

The code in printfaq.php <4.5.4 reads:

if ($pg!="") {
include(DBBASE.$pg.INCEXT);

If you set $pg to "<script>alert(XSS></script>" you receive an error that
PHP can't include the file and the javascript gets executed. This assumes
register_globals and display_errors are enabled. You can also set $pg to:
"/../../../../../../../etc/passwd%00" and read the password file
provided register_globals is enabled and magic_quotes is disabled.

In the printfaq.php patch in versions 4.5.4 and 4.5.5 the code reads:
$pg = strip_tags($pg);

if ($pg!="" && file_exists(DBBASE.$pg.INCEXT)) {
include(DBBASE.$pg.INCEXT);

strip_tags removes HTML and PHP tags from the string but does not affect
directory transversal sequences. $pg is also filtered with the following
eregi():
if
(@eregi("//|/\*|#|:/|\.\./|document.cookie|/script",$_SERVER["QUERY_STRING"])) 
{
   die("Requ<EA>te non autoris<E9>e - Request not allowed");
}

This only filters GET requests you can still pass in a directory
transversal request through $pg via a POST or cookie.

Note that the flaw is only vulnerable in versions 4.5.4 and 4.5.5 under
EasyGuppy (Guppy for MS Windows) due to the fact that DBBASE is prepended to
$pg and DBBASE is set to "data/doc". FreeGuppy does not have any directories
under data/ (only data/doc*inc files).

Solution:
The Guppy maintainer was contacted and released guppy v4.5.6a that fixes this flaw.