Skip to main content

Why OCRing spam images is useless

Nick FitzGerald forwards me another animated GIF spam that takes the animation plus transparency trick I outlined in the blog post A spam image that slowly builds to reveal its message to a new level. And it shows why spammers will work around OCR as fast as they can.

Here's what you see in the spam image:

Looks simple enough until you take a look at the GIF file that actually generated what you see. It's animated and it has three frames:

The first image is the GIF's background and is displayed for 10ms then the second image is layered on top with a transparent background so that the two images merge together and the image the spammer wants you to see appears. That image remains on screen for 100,000 ms (or 1 minute 40 seconds). After that the image is completely blanked out by the third frame.

My favourite touch is that it's not the entire image that's transparent, not even the white background, but just those pixels necessary to make the black pixels underneath show through. If you look carefully above you can see that some of pixels appear yellow (which is the background color of this site) indicating where the transparency is.

That is darn clever.


Original Syn said…
It's a snake eat tale situation, sooner or later anti-spam companies will come up with a way to OCR these images, this could simply be defeated by merging the frames and then runnings the software. And when these tactics no longer work spammers will find a more novel way of absuing some technology.
Anonymous said…
Simple solution: block emails with gif attachments.

I can't remember the last time I received a valid gif attachment.
Ron & Devy said…
There may be a simple solution that doesn't involve blocking GIF entirely. If the GIF file itself can be inspected for multiple images then you should be able to send to quarantine or OCR based on that check. I would think that there must be something in the header and/or some delimiter between the images.
Polecat said…
don't block gif's, block GIF 89a's (a for animated). It's in the header.
Nick FitzGerald said…
original syn "...sooner or later anti-spam companies will come up with a way to OCR these images..."

Actually, there has been an OCR plugin for SpamAssassin since early this year.

"...this could simply be defeated by merging the frames and then runnings the software."

It's a bit more complex than that. Read John's other blog and TSC entries about image[-only] and animated GIF spam, paying attention to the initial emergence of animated GIFs with "noise frames" and such. You may also get a feel for what is involved by reading the "history" comments at the top of
Nick FitzGerald said…
mkaatman said "Simple solution: block emails with gif attachments."

Not only simple, but simplistic. This may work for you, but in general will have a fantastic false-positive rate. You may not care, but for very good reason anti-spam developers are desperately sensitive to their FP rates.
Nick FitzGerald said…
ron said "If the GIF file itself can be inspected for multiple images..."

It can be...

...then you should be able to send to quarantine or OCR based on that check."

...but Incredimail users are prone to including animated GIFs, as are various webmail (and other) users who like to include those (small) animated sprite/avatar/etc images in their signatures. Again, it's not many folk in total, but enough to prevent serious anti-spam developers from adopting such a simple rule because of what it will do to their FP rates.
Paul McNamara said…
Interesting wrinkle. John had a few things to add in an interview he had with me, including the fact that he and Nick FitzGerald saw this one coming:

And Nick tells me that if it's fresh spam you want, it pays to be located in New Zealand:

Popular posts from this blog

Making an old USB printer support Apple AirPrint using a Raspberry Pi

There are longer tutorials on how to connect a USB printer to a Raspberry Pi and make it accessible via AirPrint but here's the minimal one that's just a list of commands and simple instructions. 1. Install Raspbian on a SD card 2. Mount SD card on some machine and navigate to / . Add a file called ssh and set up wpa_supplicant.conf for WiFi access. Now you have headless and don't need a keyboard or monitor. 3. Boot. Login. sudo raspi-config . Change password. 4. Connect printer via USB cable 5. Then execute the following sequence of commands to set up CUPS and make it accessible on the network. sudo apt-get update sudo apt-get full-upgrade sudo apt-get install cups sudo usermod -a -G lpadmin pi sudo cupsctl --remote-any sudo systemctl restart cups 6. Visit http://raspberrypi:631/admin and add the local printer. Make sure "sharing" is enabled on the printer. 7. Then make sure AirPrint is set up sudo apt-get install avahi-daemon sudo reboot Printer should work

How to write a successful blog post

First, a quick clarification of 'successful'. In this instance, I mean a blog post that receives a large number of page views. For my, little blog the most successful post ever got almost 57,000 page views. Not a lot by some other standards, but I was pretty happy about it. Looking at the top 10 blog posts (by page views) on my site, I've tried to distill some wisdom about what made them successful. Your blog posting mileage may vary. 1. Avoid using the passive voice The Microsoft Word grammar checker has probably been telling you this for years, but the passive voice excludes the people involved in your blog post. And that includes you, the author, and the reader. By using personal pronouns like I, you and we, you will include the reader in your blog post. When I first started this blog I avoid using "I" because I thought I was being narcissistic. But we all like to read about other people, people help anchor a story in reality. Without people your bl

Your last name contains invalid characters

My last name is "Graham-Cumming". But here's a typical form response when I enter it: Does the web site have any idea how rude it is to claim that my last name contains invalid characters? Clearly not. What they actually meant is: our web site will not accept that hyphen in your last name. But do they say that? No, of course not. They decide to shove in my face the claim that there's something wrong with my name. There's nothing wrong with my name, just as there's nothing wrong with someone whose first name is Jean-Marie, or someone whose last name is O'Reilly. What is wrong is that way this is being handled. If the system can't cope with non-letters and spaces it needs to say that. How about the following error message: Our system is unable to process last names that contain non-letters, please replace them with spaces. Don't blame me for having a last name that your system doesn't like, whose fault is that? Saying "Your