Code Review : CAPTCHA Bypass
~Vinayak Jituri, Mar 28, 2025
What is CAPTCHA and Why is it Important?
CAPTCHA, short for Completely Automated Public Turing test to tell Computers and Humans Apart, is a security feature designed to differentiate between real users and automated bots. You’ve probably seen them in the form of distorted text, image selection tasks, or simple puzzles when logging into websites or filling out forms. These challenges help prevent bots from performing malicious actions like spamming, brute-force attacks, or creating fake accounts. Without CAPTCHA, websites would be vulnerable to automated threats that could compromise security, overload systems, and disrupt online services. It acts as a simple but effective barrier, ensuring that only humans can access certain functionalities while keeping digital spaces safer.
Step to reproduce
Step 1: Navigating to the Code Review page
Navigate to the Code Review : CAPTCHA Bypass page and we can see the user interface.
Step 2: captured the request and analysed the response.
I tried to add random captcha, but it showed invalid captcha
Step 3: After removing the captcha captured the request and analysed the response, it showed Invalid Credentials!
Step 4: Here in the password parameter I tried to add a random passwords and it got logged in without verifying the captcha which makes it vulnerable to the web application.
Conclusion
The current CAPTCHA setup isn’t doing its job properly, making it easy for attackers to bypass it using brute-force techniques. Since the CAPTCHA doesn’t refresh with each login attempt, bots can repeatedly try different passwords without facing any real obstacle. On top of that, there’s no rate limiting, meaning attackers can flood the system with login attempts without restriction. These gaps create serious security risks, making it easier for unauthorized access attempts to succeed.
Changes to be Made
- Fix the CAPTCHA Implementation: Right now, the CAPTCHA doesn’t change with each login attempt, making it ineffective. To improve security, a new CAPTCHA should be generated every time a user tries to log in.
- Add Rate Limiting: To prevent attackers from repeatedly trying different passwords, a limit should be set on the number of login attempts allowed from a single IP address within a short period. This will help block brute-force attacks and keep accounts safer.
Keeping CAPTCHA Secure
To make CAPTCHA stronger and more resistant to attacks, consider using more advanced options like reCAPTCHA v3, which works in the background without bothering real users.
Other methods like monitoring login patterns, detecting unusual behavior, and adding extra verification steps (such as multi-factor authentication) can also help keep bots out. Regularly reviewing and updating security measures will ensure that CAPTCHA remains an effective defense against evolving threats.