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Are "CAPTCHA" losing their value for security? New bypass method completely avoids the problem of reading them.

By Andrew Pollack on 01/11/2008 at 01:01 PM EST

You might be a hacker.

If you've ever been asked to read an image of distorted letters and numbers, then type them into a field on a web form, you're using a Captcha.

picture of john stewart on my pc in a slingbox windowThe thing is, even though the programs that make the images are keeping ahead of the programs that read them, it is quickly loosing its effectiveness. Hackers are now using YOU to do the work.

Suppose a comment spammer wants to automate adding stuff to your blog, but you've got captcha codes set up. These spammers are resourceful people. Here's a method I heard about the other day.

1. Identify the part of the target page that shows the captcha image.

2. Code a script to read the page, and grab that image location - maybe even copy down the image.

3. Set up a PORN, Free Music, or other such site that also uses captcha for access. However, instead of having your own captch generator program, each time someone loads a page that asks for a user to interpret and enter the captcha digits, serve them the image from the page you're targeting.

4. The user, to get access to your porn or music site dutifully interprets the captcha image you shown him, and types it in. He submits the page, and now you have your human interpreted captcha result to use against your target page.

This method can easily and quickly be set up so that every single time you get some user to go through YOUR page using captcha, you're also using that same person to give you the data you need to create another bogus yahoo mail account, post a bogus blog comment, or otherwise mess with people.

Brilliant. Evil, but brilliant. Its a little insight into how the mind of a hacker works.


There are  - loading -  comments....

re: Are "CAPTCHA" losing their value for security? New bypass method completely avoids the problem of reading them.By Bruce Perry on 01/13/2008 at 12:03 PM EST
This could be largely defeated by having a random name for the captcha image.
Also, there's a good posting over on the "Coding Horror" web site about how
even a very simple captcha works great for most web sites.

High-value targets (like ticketmaster) do need to be thinking about this issue.
re: Are "CAPTCHA" losing their value for security? New bypass method completely avoids the problem of reading them.By Kerr on 01/14/2008 at 05:08 AM EST
Yeah, I heard about this a couple of years ago. Pretty sneaky, but a fairly
standard attack strategy these days.

As always it's an arms race.
re: Are "CAPTCHA" losing their value for security? New bypass method completely avoids the problem of reading them.By mdmadph on 01/14/2008 at 10:25 AM EST
Aye, it's a neat crack for CAPTCHA's, and I can see how this might be useful
against high-profile blogs -- of course, high-profile blogs need to be using
some sort of moderation or URL-checking, anyway.

there's never going to be enough people filling out these fake captcha forms
for spammers to ever be much of a problem. There's always going to be more
blogs than people cracking them, and for this to work, it needs to be the other
way around.
that's missing the point.By Andrew Pollack on 01/14/2008 at 10:59 AM EST
The people entering the fake ones don't know that's what they're doing.

The hacker sets up a porn site, or an unsubscribe on a spam list, or something
like that. Users doing what users do, see the captcha as part of their normal
process. They never know it came from somewhere else.


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