If the response looks legitimate, we then make a request to the web-service with the secret-key, the captcha response, and the client's IP address: public class CaptchaService implements ICaptchaService CaptchaSettings RestOperations restTemplate “Google will return the correct answer in over 97% of all cases.The captcha response obtained should be sanitized first. “The idea of the attack is very simple: You grab the mp3 file of the audio reCAPTCHA and you submit it to Google’s own Speech to Text API.” reads the post published by the researcher. Tschacher published a video PoC of the attack that demonstrates how a bot can solve the Google audio reCAPTCHA using the speech-to-text API. The expert pointed out that Google introduced reCAPTCHA v3 in 2018, but reCAPTCHA v2 is still used as a fall-back mechanism. The Germany-based researcher Nikolai Tschacher was able to introduce some changes to the unCaptcha2 attack to bypass the reCAPTCHA once again.

Google one again implemented some changes to make the attack technique ineffective, but once again a researcher modified the attack technique to use it again. The researchers behind unCaptcha2 shared their findings with Google that also agreed on the release of a proof-of-concept (PoC) code.

Since January 2019, unCaptcha was updated to bypass Google’s security service once again, the attack technique was dubbed unCaptcha2. The unCaptcha system uses a screen clicker to mimic human movement on a page. Google experts introduced a couple of features to improve reCAPTCHA, they enhanced the browser automation detection and used spoken phrases instead of spoken digits. The unCAPTCHA parses the response and types the answer, then it clicks submit and checks if the response to the challenge was correct.

The system receives the audio challenge, downloads it, and submits it to Speech To Text.

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