Thomas De Maesschalck blog

Spammers abuse CAPTCHA technique to fool spam filters

Like most of you know spam is a big problem on the Internet. There are various forms of spam and various techniques to stop spam. A lot of websites and blogs with comment and user registration forms use so-called CAPTCHAs to eliminate most spam.

CAPTCHAs are simple tests to determine whether the user is a human or an automated spam bot. Computers have a really hard time solving these CAPTCHAs and now some clever spammers have found that they can use this technique to their advantage.

Last week Neil Cook from anti-spam firm Cloudmark said that a new trend in the spam world are e-mails with distorted text images in PDF files.

He added: “We’ve been seeing a lot of PDF stock spams for the last 10 days or so, and there was another spike last night. Images are particularly easy for humans to pick up, but particularly hard for computers.

“These ones are distorted too – it’s the same technique that websites use to keep spammers off by making visitors type in distorted text during registration, and now the spammers are using it on us.”

I get lots of spam mails (hundreds a day) and one of the things I’m seeing a lot lately are image-based spam mails like this one for all kinds of pills and also for software like AutoCAD:
Spam mail

The most anything thing about it is that my spam filter doesn’t catch these mails yet

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Thomas De Maesschalck

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2 responses to “Spammers abuse CAPTCHA technique to fool spam filters”

  1. Tyler Ingram Avatar
    Tyler Ingram

    Spam is a pain isn’t it? I go through roughly 2500 spam messages a day to weed out the actual legitimate ones that get caught by our spam filters.

    If it weren’t for the outlook filters I have running, I would have to go through over 7000 messages a day!

  2. Thomas De Maesschalck Avatar
    Thomas De Maesschalck

    7000 a day, and I thought I received a lot of spam 😮

    I go through roughly 2500 spam messages a day to weed out the actual legitimate ones that get caught by our spam filters.

    Indeed, sometimes the spam filter catches a bit too much. A couple of months ago my filter had caught a mail from a company that wanted to advertise on my website and without knowing it I accidentally deleted the mail. Fortunately they mailed me back about two weeks later to ask if I had received their last mail.

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