Monday, October 16, 2006

A spam image that slowly builds to reveal its message

Nick FitzGerald sent me a stunning example of lateral thinking on the part of a spammer. The spammer has taken a standard stock pump-and-dump spam image and split it horizontally into strips.



Each of the 17 horizontal strips cuts fairly randomly through the text making OCR on each strip not very useful. The spammer has then mounted each strip in its correct position on a transparent background and put each strip into an animated GIF. Here, for example, are a couple of strips:




The end result is that only once the entire image animation has completed is the complete spam visible making this a challenge for spam filters. And the spammer has thrown in a couple of frames at the end of the image, that get displayed after such a long delay (8 minutes) that they essentially never get shown. But those final frames are there just to throw off a spam filter trying to find the actual image.

Here's what gets displayed:



and here's the final image in the animation:



Very clever! (I'm calling this 'Strip Mining')

6 comments:

Justin Mason said...

So each strip is "left in place" in later frames of the animation? ie. the subsequent frames simply start drawing halfway down the image, and just don't overwrite the previous frames?

Wow, I never knew the GIF standard allowed that -- and I wrote a GIF viewer for DEC VT320 terminals back in 1990 ;)

Manni said...

Damn. I've seen a couple of these today. And what did I do? Made a mental note to find out why my client was so slow in downloading those gifs. Duh.

Manni said...

I forgot to mention where I saw those gifs: They were all in my spam folder.

flyingdutchman-wa said...

Manni: how'd they get in your spam folder? I've been getting these cid emails for a few weeks now and have been not able to figure out how I can filter them to go to my junk folder....

Nick FitzGerald said...

Justin Mason said "So each strip is "left in place" in later frames of the animation? ie. the subsequent frames simply start drawing halfway down the image, and just don't overwrite the previous frames?"

We've seen both, I think.

"Wow, I never knew the GIF standard allowed that..."

I think this was a feature added in the "89" spec and was not part of the earlier (1987??) GIF spec. I'm sure Google will give you an answer in a few seconds...

DML said...

"...the subsequent frames simply start drawing halfway down the image..."

Wrong...read the original post:

"The spammer has then mounted each strip in its correct position on a transparent background."

Hence, each image is a full image, with some visible areas, where each portion of the spam message resides, and some transparent areas, which allow us to view the layers beneath.