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Image spammer tricks

Posted by Michael Bloch in online world (Thursday March 22, 2007 )

The pump and dump stock spam scams that are reliant on images to relay their pitch continue with seeming increasing ferocity. How do they continue to push this garbage through even some of the most tweaked filters that employ OCR (optical character recognition)? A security firm has published some tricks of the trade.

A white paper from SecureComputing states image spam has increased by 200% in recent months and gives some insight as to the lengths that image spammers will go to in order to evade filters:

1. Sliced images:
Some image spam doesn’t consist of a singly image, but multiple images stitched together when they are rendered. These messages remain undetected by anti-spam scanners because the image message slicing is randomized throughout the campaign, therefore minimizing an easily detectable signature.

2. Random pixel modification
Small blocks of pixels will be varied in a spam campaign that don’t detract from the message, but make the image seem unique to many anti-spam filters.

3. Color/text modification
By simply regularly changing colors and fonts in an image, it completely changes the pixel locations and identitifiers, making it difficult for anti-spam filters to recognize a signature.

4. Multi-frame animated images
This method is very (evil) clever. An image will consist of multiple frames, each frame containing what appears to be garbage and with no clear message. If these frames are animated at a very high rate, the message then appears in a readable format to the viewer.

While all this randomizing and multiple messages may seem labor-intensive, all of the above is made easier for the spammer through the use of purpose made tools that automate the process.

While none of these methods will work forever, SecureComputing predicts that the next wave of image spam may consist of photographs with the spam message written in freehand and overlayed on the photo. Current Optical Character Recognition (OCR) in anti-spam solutions will have problems with this as it requires known fonts in order to be effective.

Fascinating stuff, yet repulsive and frustrating at the same time.


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