Hacking and defeating Google's reCAPTCHA with a 99% accuracy

http://www.dc949.org/projects/stiltwalker/

by ahmadss • 14 years ago

224 80 14 years ago

"Note: In the hours before our presentation/release, Google pushed a new version of reCAPTCHA which fully nerfs our attack."

drharris• 14 years ago

After so much work, gotta love the footnote here: "Note: In the hours before our presentation/release, Google pushed a new version of reCAPTCHA which fully nerfs our attack."

bmj1• 14 years ago

Yes - I almost missed that - that should be highlighted more clearly not buried in a footnote - as it undermines the impact of the research...

eli2• 14 years ago

The research is just as solid as it ever was, it just minimizes the immediate, real-world effects.

barik2• 14 years ago

This isn't a huge problem for security people. Indeed, at most academic security conferences, by the time you present your work it has usually already been broken and/or countered.

To goal of good research (and one that differentiates researchers from criminals) is to present a proof of concept and to advance the state of security. The fact that security is a perpetual arms race is incidental.

Well, at least that's what security researchers tell themselves anyway to avoid going mad. :)

JackC• 14 years ago

What makes it funny is that the bulk of the article isn't "here's how we did it and what we learned," but "here's what you need to do to get our code running on Ubuntu." Then you get to the footnote: "PS: It's pointless to get our code running on Ubuntu."

I spent the whole article wondering why they were so interested not only in presenting a proof of concept, but in getting as many people as possible actively breaking captchas. Then I got to the end and switched to wondering whether the whole thing is an elaborate prank.

djKianoosh• 14 years ago

Still, the important takeaway for webapp developers is never to rely on just one form of protection. Add timestamp checks, honeypots, etc, in addition to captcha and use them appropriately for your application.

drone• 14 years ago

Alas, I'm running a handful of websites, and nearly nothing works, outside of stopping the most egregious of automated bots. Every new protection technique seems to be broken fairly fast. The best results so far have been combining basic WAF techniques (like placing poisoned form fields re-named as regular form fields and randomizing the names of standard form fields, one-time hash values in required fields, etc.) with some form of captcha.

However, every day five or six make it through. Observing their patterns and timing, and the fact that they make zero mistakes while interacting very slowly with the site, I can only presume that humans are directly involved as well, either as mechanical turks or simply manually posting the spam.

notably, they follow normal, guided flows through the site - not like robots at all, but hit pages that would obviously be interesting to a human (say, have a certain type of image content linking them versus another), while avoiding the least popular pages - never hit urls that are hidden via CSS, etc.

dutchbrit5• 14 years ago

I actually tried hacking reCaptha via audio and the Google Speech to text API a few days ago. It didn't work unfortunately, it really frustrates me at times when I have to refresh reCaptcha 10 times to actually be able to read the damn thing!!

robotmay2• 14 years ago

What problems did you run into when trying to use the speech API? It's quite an interesting avenue for bypassing these in the future as speech recognition gets better. Also highly entertaining when you use Google's own service against them.

dutchbrit5• 14 years ago

Exactly my thought, with using their own service! The thing was, is that the background noise made the API not recognise the actual words (I think - since you can't really "debug"), but maybe if you reduce the noise, it might be possible - I wanted to give that a go, but haven't had a chance yet. I just don't know of any ways to automatically reduce noise.

dutchbrit5• 14 years ago

Ps. Here's how you send the audio to the API - the mp3 that you can download from reCaptcha needs converting to flac first, which can be achieved with ffmpeg.

http://pastebin.com/LQ30iWKD

monsterix• 14 years ago

Now here is some serious hacker's news! Was getting nagged by 'what NewEgg can/can't do' posts off late.

omonra• 14 years ago

This may be very interesting to crack, but who is responsible for Google making their CAPTCHA almost impossible for human to decipher now? I seriously have to click 5 times before even seeing anything resembling letters I can parse

duiker101• 14 years ago

I didn't go in depth of the article/method but from what I've read it takes advantages of the audio function not the images.

verroq3• 14 years ago

You only have the type one word correct, and they make sure one word is readable so with some practice you can easily recognise which word is the key-word.

ohgodthecat3• 14 years ago

He isn't speaking of reCAPTCHA but google's nearly impossible to read captchas that take way too much time to decipher.

If you'd like to see it you can usually get it by putting in a bad password to a gmail account too many times (though I don't know if that has other consequences).

Edit: Here is an image example http://www.techian.com/wp-content/uploads/captcha.png

conradfr• 14 years ago

After reading that previously on HN, I tried and sadly had to type both word.

btilly• 14 years ago

With practice and decent eyes. Once your eyes go downhill, it is amazing how much changes.

Most software developers are under 40 and therefore have little to no appreciation for what happens to people's eyes after they are past 40.

dkersten• 14 years ago

For me, the key word is usually the unreadable one. I've never had success typing only the readable one.

fibertbh• 14 years ago

Agreed. They are unbearable. But name a better alternative!

Anything that I can think of like adding to the CAPTCHA such as a half-line of letter above and below or adding noise would probably make cracking them programmatically even easier.

joelthelion2• 14 years ago

This simply means computers are getting really good at this game. And that Google, with all its power, hasn't found a better alternative to Captchas yet.

I find that pretty worrying for the future of the internet. An internet without working captchhas will probably be full of bots and spam.

masonlee• 14 years ago

Is this an argument that future web services may depend more heavily on identity/reputation services?

Jach2• 14 years ago

Naive Bayes spam classifiers are fast enough that I think they could be used as drop-in replacements, and they'd be well-trained too considering Google's existing success with spam in email. And Naive Bayes isn't the only solution, there are plenty of other heuristics. Even something as dumb as "first post from this user/IP/'identity' and full of links?" would catch a lot of common spam.

s_henry_paulson• 14 years ago

Here's the Ars Technica article which does much better job explaining the system:

http://arstechnica.com/security/2012/05/google-recaptcha-bro...

ahmadss• 14 years ago

agreed, thanks for posting the ars link.

the youtube video in the OP does a great job of explaining and the thinking behind the attack, and even though it's an hour long, it's worth the watch.

seanp2k22• 14 years ago

Video tutorials are awful if they're the only option. I read very quickly, and videos are typically paced at the lowest "average" comprehension level, so they're a painfully inefficient way to relay technical information.

Plain text wins again, at least for one-way technical communication.

danso• 14 years ago

In systems that are less secured than Google, the audio catchpa seems trivial to break...I think I've seen one on court sites that read a combination of numbers from 1 to 9 with some variance in the vocal speed. I'm not an audio engineer but that seems fairly trivial to crack (though maybe their visual catchpa would be easier...I dunno, not an expert in OCR either).

It's a good lesson in a form of social engineering. Sites have to provide this alternative access for the visually impaired...yet I bet the resources/creativity put behind it is not at the same level as the kind put into the catchpa used by 99% of the userbase. Furthermore, the most important client -- your boss -- is likely to not be blind him/herself, which eliminates that extra critical layer of oversight.

throwaway1979• 14 years ago

Google's captcha system is horrid. I've mentioned this to people on the accessibility team but to no avail. They used to have a wheel chair icon next to the bloody scrambled text. I taught a computer class to seniors and it was painful watching them deal with the account sign up process (also, I thought it was insulting asking a mobile senior to click on the wheel chair icon ... to the designer ... FU!). Clicking on the wheel chair would give audio that barely made any sense to me. The whole process was stupid.

Like many others, I can barely get through their captcha service. I'm actually happy people circumvented it. Maybe someone will think it through this time around.

tmh88j• 14 years ago

I also thought that was insulting with the wheel chair icon. A person in a wheel chair has problems walking, not (necessarily) their vision. How about "Help" in text?...less confusion and possible anger

excuse-me• 14 years ago

Since the point of the audio version is not to be hit with lawsuits under the ADA - perhaps it should just be a little icon of a lawyer?

ceejayoz• 14 years ago

Apparently the wheel chair is actually an ISO standard.

http://en.wikipedia.org/wiki/International_Symbol_of_Access

TazeTSchnitzel• 14 years ago

OK. Show me a CAPTCHA that is easy for humans to read, and very difficult for computers to read.

rytis• 14 years ago

It shouldn't be about being able to "read". Ideally it should be something that only human could "solve". May be stupid example, but I can't come up with a better one (and if I could, I would be a lot wealthier by now ;) ): show a picture where a forest in landscape is violet and ask a user to identify what's odd on that picture. Another example, show a face with two noses. Ask them to identify the odd item. Things like that. Easy for human, impossible for computers. The problem is that these tasks need to be generated by humans, as I cannot think of any (irreversable) way to do this automatically. May be mechanical turk to the rescue?

entropy_• 14 years ago

Well, chances are it'll get even harder now, not easier since they'll need to add further complexity to differentiate humans from bots.

aidenn0• 14 years ago

http://en.wikipedia.org/wiki/International_Symbol_of_Access

saraid216• 14 years ago

> Like many others, I can barely get through their captcha service. I'm actually happy people circumvented it. Maybe someone will think it through this time around.

Anytime you're ready, we're listening.

roguecoder• 14 years ago

Pretty sure I don't have 99% accuracy at solving reCAPTCHAs. Perhaps it's become a CAPITCHA, Completely Automated Public Inverse Turing test to tell Computers and Humans Apart...

apendleton3• 14 years ago

One exciting thing about this: the entire model of reCaptcha (at least the text ones; I assume the audio ones are similar) is to make people do useful work when solving captchas by having them complete tasks that they consider too hard for computers to do well (in the text reCaptcha case, OCR). If someone writes software that can defeat the captcha, it does mean the security model is broken, but it also means the state of OCR technology (or audio recognition or whatever) has been advanced, and the digitization of books that had previously required human intervention can now be accomplished by automated means. In other words, spammers are incidentally creating the tools to expand the scope of digital human knowledge. Win-win, really.

aptwebapps• 14 years ago

Are the audio versions sourced from recordings that need transcription as the visual versions are sourced from scanned documents? I assumed that was just to improve access.

If not, then it isn't useful work, really.

vhf2• 14 years ago

Looking at how the hacked it, we can safely deduce the audio version is only to improve access.

Useless for transcription, because it 'validates' phonetically. This is part of the attack : Whether audio is "wagon" or "van", the 'word' "wagn" validates. Same for "Spoon" and "Teaspoon", which both can be validate by entering "Tspoon". (Since Ts can be said "Ss" as in "Tsunami" (Sunami) and "T S" as in T-Rex.)

fuelfive• 14 years ago

Unfortunately, this attack does nothing to advance the state of the art in OCR (or audio recognition). It's basically the same story as every other CAPTCHA attack to date: take advantage of some accidental statistical regularity in the generation function. As soon as this kind of flaw is discovered, it only takes a few hours for the generation code to be patched in such a way that completely prevents this sort of attack from working.

fghh45sdfhr3• 14 years ago

So either the code is easy to patch, or we DO advance. Win/Win?

TeMPOraL• 14 years ago

> If someone writes software that can defeat the captcha, it does mean the security model is broken, but it also means the state of OCR technology (or audio recognition or whatever) has been advanced, and the digitization of books that had previously required human intervention can now be accomplished by automated means.

No it doesn't. reCaptcha only checks one of two words it displays (the other one being what OCRs can't handle themselves), so naturally you only need to crack one and input garbage as the other, thus actually making the world a worse place.

apendleton3• 14 years ago

Probably not substantially worse. You would need to be able to tell which was which, since they're not consistently ordered and the "good" one is deliberately obfuscated/smudged/whatever, and, since recaptcha depends on multiple users agreeing on the right answer, you and other attackers would need to be consistent in your garbage in order for it to make it into the canonical book transcript.

anonymoushn• 14 years ago

For ReCAPTCHA, you only have to type the known word almost-correctly. You can just type anything for the word they don't already know. Even if someone had tech that could pass visual ReCAPTCHA reliably, it wouldn't need to be capable of doing useful work.

verroq3• 14 years ago

Their testing and high success rates on the public Google reCaptcha test probably tipped off a Google employee internally.

Graphon2• 14 years ago

> Note: In the hours before our presentation/release, Google pushed a new version of reCAPTCHA which fully nerfs our attack.

I take it that "fully nerfs" means this defeat of recaptcha is no longer useful?

kedyr2• 14 years ago

captchas are very annoying. it is surprising that they have lasted this long. what would be the best alternatives?

TillE• 14 years ago

Obscurity. If you're not a significant target, you can get by with simpler methods.

Deciphering obfuscated text is one of very few verifiable tasks that humans can do which computers cannot.

Alternatively, you can look at other methods of identity verification. If you ask for $5 to create an account, you'll have few spam problems.

joelthelion2• 14 years ago

What are we going to do once all CAPTCHAS are completely broken?

neotek• 14 years ago

CAPTCHAs are ultimately already completely broken - they don't really slow anyone down except ordinary users, because spammers are using human labour to solve CAPTCHAs at the rate of one or two dollars per thousand solved.

wendsday2• 14 years ago

A company that relies on a bot that accesses others' resources to make money and at the same time relies on reCAPTCHA to frustrate other bots from accessing it own resources.

It may be easy to do today, but, going forward, how do we determine which bots are "good" and which ones are "bad"?

Clearly, simply being a "bot" does not imply "bad" intent. If it did then we should all be blocking search engine bots. Yet this is what reCAPTCHA does: it blocks not based on intent, but based on the characteristic of being a "bot".

akincisor• 14 years ago

It's easy. One bot reads a text file and only accesses the resources outlined in the file. The other maliciously tries to eat up resources without restraint. Good - Bad.

wendsday2• 14 years ago

Like I said, it's relatively easy today. But how about going forward?

There will be lots more bots and lots more usages. Things might not be so simple. For example, some sites might exclude all search engine bots except a chosen few despite the fact they all honor robots.txt and behave essentially the same.

(BTW, if by "text file" you mean robots.txt, isn't that an exclusion list? You seem to be saying it's an inclusion list.)

Hacking and defeating Google's reCAPTCHA with a 99% accuracy

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