Breaking CAPTCHA with automated humans

http://www.troyhunt.com/2012/01/breaking-captcha-with-automated-humans.html

by troyhunt • 15 years ago

51 46 15 years ago

samarudge• 15 years ago

Now to write a script that repeatedly submits CAPTCHAs with the text "terrible job", "boring", "pointless life".

foxylad• 15 years ago

Or in case they are working against their will, "Return HELP if you're being held prisoner in a captcha factory", "My location is..." and "Please tell Amnesty International".

itmag• 15 years ago

captcha factory

Reason #237 why we are living in the future: there are captcha factories.

asomiv2• 15 years ago

This is actually a pretty good idea. The captcha should include some kind of system for reporting slavery or other kinds of work-related abuse, whereby the reporter will be rewarded if the report results in the slavedriver being successfully prosecuted. As the number of employees become bigger, the likelihood that at least one of the employees will rat out his employer increases exponentially.

user249• 15 years ago

$0.001 per 27 seconds gives an hourly wage of 13.85 cents.

This system has to be using enforced labour, no-one will work for that little, even in the poorest countries. And that 13 cents figure is assuming that 100% of revenue goes to the individual worker.

Taking the best case from the reported average of 10-15 seconds, that means an operator can solve 360 captchas per hour; an hourly revenue per worker of 36 cents.

I have a suspicion that this makes use of a similar 'economy' as the one about Chinese prisoners being forced to farm gold in WoW[1]

If you have just ten 'workers' on 12 hour shifts, that's $43 per day, $301 per week, $15,652 per year. The numbers start to stack up fairly well in bulk - assuming all 1.5 million Chinese prisoners[2] were able to sustain 12 hour shifts 365 days a year and maintain an average of 10 seconds per solve (and that there was a constant supply of paying customers), that's over 2 billion dollars annual revenue. Obviously that number's way off, but probably not by an order magnitude.

[1] http://www.pcworld.com/article/228716/chinese_prisoners_alle...

[2] As of 2006: http://www.kcl.ac.uk/depsta/law/research/icps/downloads/worl... [PDF]

biot• 15 years ago

I'd be surprised if it takes more than 5 seconds for a human to solve each. Keep in mind that 27 seconds was the time in the queue from submit to solve. 24 seconds of that might have been spent in the backlog and 3 seconds being viewed and solved by a human.

Additionally, it supposes that all images being solved are the kind that a human must do. Tons of CAPTCHA images being output by naive generators can be solved automatically. I'd be surprised if every image submitted isn't going through an automated solver first with only the < 95% confidence ones being sent to a human.

Actually, you could do a hybrid model where every image is auto-solved and a human can either click to confirm that the auto-solver got it right or manually type a new response. That should get your time per solve down to a second or two each.

user249• 15 years ago

> I'd be surprised if every image submitted isn't going through an automated solver first with only the < 95% confidence ones being sent to a human.

Yes, that's how I'd do it too, but I don't know if they're that sophisticated yet (they'll get there though).

Even with 1 second per solve, that's only $3.60 per hour. A massive improvement over 13c, but still certainly not worth anyone in my country's time.

JoachimSchipper• 15 years ago

Or they have a mostly-automated system that occasionally has to fall back on human labour, or even just human verification.

lusr• 15 years ago

Here's a crazy conspiracy theory for fun: Google et al are well aware that CAPTCHAs don't achieve anything. So they build all these CAPTCHA "breaking" sites that actually just return the correct result from their internal databases after a random delay, thereby earning themselves a pretty penny from spammers that were going to abuse their systems anyway.

user249• 15 years ago

> Google et al are well aware that CAPTCHAs don't achieve anything

????

> So they build all these CAPTCHA "breaking" sites that actually just return the correct result from their internal databases

Oh, haha, I see ;)

user249• 15 years ago

I can no longer edit my post, but here's an interesting source:

"Spammers Pay Others to Answer Security Tests"[1]

choice quotes:

"spammers are paying people in India, Bangladesh, China and other developing countries to tackle ... captchas"

"attractive enough to lure young people in developing countries where even 50 cents an hour is considered a decent wage. Unskilled male farm workers earn about $2 a day in many parts of India. "

"students typically work two and a half to three hours a day from their homes and make at least $6 every 15 days"

"Another operator in Bangladesh who goes by the screen name Workcaptcha on Freelancer.com boasts on his profile page that his firm has 30 computers, up from just five a year ago. Three shifts of workers allow the operation to hum 24 hours a day, seven days a week"

[1] http://www.nytimes.com/2010/04/26/technology/26captcha.html?...

user249• 15 years ago

I found workcaptcha's profile[1], and I found them on GetAFreelancer.com [GAF] quotes page[2]:

"This company help many poor company. I am received big position for gaf help so pls maintain gaf term & condition. I am really Love gaf & must going lifetime relation. Gaf best in this world. Trusty, Reliable & more. Thanks" - workcaptcha, Bhai Bhai Data Entry Service

[1]http://www.freelancer.com/users/feedback_674411.html

[2]http://www.freelancer.com/quotes/

pax• 15 years ago

Of course, their (kolotibablo.com) registration page also features a captcha, spam seems to be annoying them too :)

samps• 15 years ago

Leaving aside the ethical issues, has anyone considered building a "white-hat" CAPTCHA cracker tool using one of these services? Personally, I hate solving CAPTCHAs to sign up for stuff, and I wish there were a browser extension button I could hit that would do it for me. I'd gladly pay a few cents and wait several seconds (browsing in another tab) if it meant I didn't have to try to decipher some of the crazier CAPTCHAs out there.

user249• 15 years ago

But who'd be doing the solve? If you have a machine to do it then you have a product (but only until the captcha improves, after all that's the whole point). If it's humans, you're either pricing yourself out of the market or using slave labour.

praptak• 15 years ago

There is captchatrader, which is sort of like that - you can "store" captchas by solving a lot of them at once. This earns you credit which you can use later to have your captchas solved for you.

See this: http://www.ghacks.net/2011/08/19/captcha-trader-automatic-fr...

Of course any such service can be used by spammers, so there is no real "white-hat"/"black-hat" distinction.

pavel_lishin3• 15 years ago

Seems like there would be no difference between the white-hat and the black-hat version except possibly in the mission statement on the "about us" page.

ghshephard• 15 years ago

Ideal approach to running one of these services is to front a site that actually uses Captchas, ideally in significant volume, and then just _pass through_ the captchas being sent by others needing them to be solved.

Say you run a site that has 1000 users/minute authenticating, that means you actually have 1000/captchas/minute being solved - may as well sell those users (unwittingly performed) time.

samwillis• 15 years ago

I have heard of people doing exactly this with pornography websites...

dspillett2• 15 years ago

I've never seen a real example of this (having asked many times when people have mentioned it), it is one of those ideas that on first though everyone things must be real.

The problem with that scheme is that you need to create or curate enough content to make it work and somehow make the people who might want that content aware that you can give it to them, and that is a lot more hassle and expensive than paying some impoverished 3rd world farmer to parse the captchas.

mikeknoop• 15 years ago

> But what must have been even worse – and I need to take some blame here – is that while testing I kept submitting the same CAPTCHA over and over and over again. I can picture the poor operator sitting there thinking “WTF is this guy doing already?!”

Perhaps this is why the author saw such a high 94% success rate?

TheSeb4• 15 years ago

I typically have a 94%+ success rate (using Death By Captcha)- and I do thousands of unique captcha submissions per day. His results are accurate.

troyhunt3• 15 years ago

Nope, that was only when testing the CAPTCHA cracker console before I'd scripted the ability to pull a live one from the registration page. The bulk cracking was done with fresh loads and the logs show all unique CAPTCHAs.

TheSeb4• 15 years ago

One thing that the author has clearly misunderstood is the following:

"I must admit, I do feel a bit sorry for the folks sitting there endlessly solving a never ending stream of CAPTCHAs; frankly, just one drives me a bit nuts! But what must have been even worse – and I need to take some blame here – is that while testing I kept submitting the same CAPTCHA over and over and over again. I can picture the poor operator sitting there thinking “WTF is this guy doing already?!” Then again, maybe they made some quick bucks because recognising the same pattern time and again becomes more efficient."

For the majority of the captcha solving services out there the same captcha is just solved ONCE. When a request is submitted to the API the image data is typically read from the image file and encoded in Base64 or in a similar format. The Base64 string representing the image is then sent to the captcha API.

The API will use the Base64 string as a key and check if it exists in a database/key-value store. If it exists, return the value for the Base64 string, if it doesn't exist, send it to one of the available workers. When the captcha is solved by a worker it is saved in the database. If someone would send in the same captcha next time (thus resulting in an identical Base64-encoded string representation of the image), the solved captcha text would just be sent back without any human interference.

I talked to one guy running one of these services a while ago and they set it up the following way:

1. Requests are sent using JSON to their API.

2. The Base64-encoded string representation of the image is checked against Redis to see if it has already been solved. If it has, return it to the client requesting it. Award the person who originally solved it 50% of the fee that the client is paying for it (thus enabling these captcha solvers to earn some kind of passive income).

3. If it hasn't been solved, run it through OCR-software. Is the result reliable enough? Save it in redis and return it to the client.

4. Is the OCR result not reliable enough? Send it off to a human worker. The worker solves the captcha, the captcha text is saved in Redis as a value associated with the key (the original Base64-representation of the image). Award the worker 75% of the fee the client paid. Return the solved captcha text to the client.

5. Deal with invalid captchas (remove entry from Redis, subtract from due commission to worker etc.)

6. Rinse and repeat. Scale with more servers + more workers.

Pretty cool stuff :D

shalmanese• 15 years ago

If that were the case, why did he get a 94% success rate with the same image?

TheSeb4• 15 years ago

Did I say that all captcha solving services work the way I described? I said "most of them". Haven't tried the service he was using but I've used quite a few other providers.

user249• 15 years ago

But the captcha is randomly distorted afresh each time, so each captcha image will never have been seen before. You'll never get an exact hash match unless you're looking at an extremely naive implementation.

TheSeb4• 15 years ago

Depends on the implementation, just as you suggested. There are actually quite a few sites that use really simple captchas that don't randomly destort letter placements, lines, colors etc.

troyhunt3• 15 years ago

During my testing, I literally had a saved bitmap on my local machine which I reissued many, many times. Obviously this meant it was an exact match on each submission.

troyhunt3• 15 years ago

Interesting insight, thanks for that. I suppose the success of that strategy depends on both the frequency with which CAPTCHAs are reused and the practicality of storing the encoded previous CAPTCHAs (considering the potential volume).

In my testing, reissuing the same CAPTCHA I had saved locally over and over again didn't make a difference to the duration in which it was solved. Now it could be that they throttle responses for pre-solved CAPTCHAs, but then of course they're also throttling their revenue stream. In this case, I'm more inclined to conclude that Antigate don't have a CAPTCHA reuse mechanism in place.

uptown2• 15 years ago

Is it juts me, or has CAPTCHA gotten considerably more difficult to read lately? It takes me at least 3-4 reloads before I get something I can both read and type with the keyboard in front of me.

ryandvm• 15 years ago

Are you a robot?

uptown2• 15 years ago

A crazy cab driver called me an unfeeling robot one day, so I suppose it's possible.

drucken2• 15 years ago

Sometimes there is no need to reload. If a CAPTCHA has multiple words, it is likely a reCAPTCHA and therefore at least one of the words will always be easily readable for any normal human. Any half-decent approximation for the other word should be accepted.

dzhiurgis2• 15 years ago

Back in 2004, when RuneScape introduced captcha to combat SCAR and AutoRune automatic trainers, someone figured the very same system. I remember solving hundreds of captchas and never getting any credits, which were usually traded for CC's, shell and paypal accounts. owww, RuneScape + cheats = worst drugs ever.

stuaxo• 15 years ago

I'm sure that on file sharing sites when you solve captchas this is exactly what is happening ...

zuppy• 15 years ago

Some years ago I read about one of the best ways to solve captchas: you need to set up a porn site and require visitors to solve them for you, in order to "view" the content. They have no idea that they're doing. Free labour :)

lelele• 15 years ago

Next submission: "Breaking CAPTCHA breakers with automated humans". That is, having humans filter comments submitted through a CAPTCHA. I'd bet they would work even faster than CAPTCHA breakers: typing CAPTCHAs is time-consuming (at least for me, and often I can't read what I should type).

Further next submission: "CAPTCHA-breakers-breakers team up with CAPTCHA-breakers to blackmail sites."

drucken2• 15 years ago

The interesting thing about these human-based attempts to defeat individual CAPTCHA-"protected" services is that if they are actually reCAPTCHA or variants, they perform a strong socially useful function!

This is because reCAPTCHA solving is linked to the digitization of paper documents.

There was recently a TEDTalk by the inventor of the CAPTCHA about this topic. It is very informative and entertaining. I highly recommend it: http://www.youtube.com/watch?v=-Ht4qiDRZE8

Breaking CAPTCHA with automated humans

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