About
Most Internet forums allows the poster to attach a signature to their post. The signature typically contains an image. What some people have realized is that there is no rule saying that the image must be static.
Instead some crafty people have written programs that dynamically generate an image on the fly each time someone views the signature.
The classical viewer info sign
The most typical type is an image that states information about the viewer, such as their IP address, their ISP and their webbrowser.
Exploitable php code
This code writes out the viewer IP address on a white background.
<php
$img=imagecreate(200,100);
$white=imagecolorallocate($img,255,255,255);
$black=imagecolorallocate($img,0,0,0);
imagestring($img,0,0,0,$SERVER[’REMOTEADDR’],$black);
header(‘Content-Type: image/png’);
imagepng($img);
Random image
Some signatures just use a random image out of a premade set.
Exploitable php code
This code redirects to a random image file
<php
$nr=rand(1,5);
header(‘Location: img’.$nr.‘.png’,false,303);
View counter
The most advanced type remembers each time it has been viewed and does something with that. The most obvious use is to just count each time the signature has been viewed.
Special statistics
It is rather simple to create an image that lists special statistics about an user, such as their performance in a game or what media they are viewing.