Researchers have managed to crack Google's audio captcha with an average of 85 percent accuracy, allowing bots to automatically create accounts on websites and place spam messages. To distinguish robots from people, captcha's often need to solve puzzles and distorted texts. The captcha of Google also allows users to resolve an audio captcha.
The audio captcha consists of multiple digits read in different speeds, accents and pitches with background noise. Researchers at the University of Maryland devised an attack targeting Google's audio captcha. To crack the audio captcha, the researchers developed " unCaptcha ", software that downloads the captcha audio file and then splits the parts with voice.
The split audio fragment of each digit is then sent to six free online audio transcription services, including Google's. Based on the different results, which figure was read in the audio clip. The results are then entered 'organic' by the software in the captcha window. On average, the software knows how to solve the captcha with 85 percent accuracy. After the researchers published their research ( pdf ), Google has taken various measures that limit the effectiveness of unCaptcha.
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