How a Broken CAPTCHA Allowed Account Abuse
Service: Web Application Pentest
Industry: Crypto Exchange
Region: Poland
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1. Background — When Human Verification Goes Unchecked
During a security audit of a web platform hosted at https://my.nonexistent.site, our team discovered a critical vulnerability in the CAPTCHA implementation, a mechanism commonly used to distinguish human users from bots.
The frontend featured Google ReCAPTCHA v2, typically used to prevent automated attacks on forms such as sign-ups and logins. However, the backend never actually verified the CAPTCHA token with Google’s verification service. In essence, this left the gate unlocked while the system displayed a “secure” lock to the user.
This case is a textbook example of what we call a “UI-only control” — a security measure that exists visually but lacks enforcement in the system logic. And when it comes to access control, such illusions are particularly dangerous.
2. Discovery — Reproducing the Bypass with Minimal Effort
We initiated the assessment by interacting with the registration and login processes and inspecting the traffic between the frontend and backend systems.
In both scenarios — account registration and user login — the process included a g-recaptcha-response value passed in the POST request. Here’s how we tested it:
Registration Test Flow:
1. Navigated to https://my.nonexistent.site/sign-up.
2. Filled out the registration form normally.
3. Intercepted the outgoing request in a proxy tool (like Burp Suite).
4. Replaced the actual CAPTCHA token with a blank string or dummy value:"g-recaptcha-response": "fake_value"5. Sent the request.
Result:
The server returned HTTP 201 Created — the account was created successfully without any validation of CAPTCHA.
Login Test Flow:
A similar procedure was followed during login. By modifying the CAPTCHA field to an invalid value, the server still responded with HTTP 200 OK, issuing valid JWT tokens.
This confirmed a complete bypass of bot protection — allowing attackers to automate both registration and authentication processes.
3. Technical Root Cause — Missing Server-Side Verification
The security model of Google ReCAPTCHA requires the client to submit a g-recaptcha-response, which is a short-lived token generated in the browser after successful human verification. Crucially, this token must then be verified by the server via an HTTPS POST request to Google’s verification endpoint:
POST https://www.google.com/recaptcha/api/siteverify
This backend request includes:
the token,
the site secret (known only to the server),
and the user’s IP address.
In this case, the backend skipped this verification altogether. This often happens due to:
- Incorrect assumptions that CAPTCHA logic on the frontend is sufficient.
- Developer oversight during integration.
- Misconfigured frameworks that do not enforce server-side validation by default.
- Staging or demo logic promoted to production.
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4. Real-World Risks — What Attackers Could Do
The lack of CAPTCHA validation opened the platform to several real and exploitable risks:
A. Automated Account Generation
Attackers could mass-register accounts using simple scripts, bypassing all bot protections. This can lead to:
Fake identity farms used for spamming, money laundering, or social engineering.
Fraudulent sign-ups to abuse referral bonuses or promotional credits.
B. Brute-Force and Credential Stuffing
With login CAPTCHA disabled, attackers could:
Launch high-speed login attempts using leaked credentials.
Avoid detection by distributing login attempts across IPs and user agents.
C. Business Impact
From a business standpoint, the consequences could include:
Reputational damage from spam or fraud linked to fake accounts.
Increased support load due to account misuse or compromise.
Financial loss if attackers gain access to protected accounts or systems.
D. Abuse Chain
Worse, CAPTCHA bypass can become a link in a larger attack chain. For example:
- CAPTCHA is often used to protect password reset and 2FA removal flows.
- If these areas are also unprotected, the platform becomes ripe for full account takeovers.
5. Broader Lessons — CAPTCHA is Not a Checkbox
This issue is not uncommon. In fact, we’ve encountered similar cases in fintech, healthcare, and e-commerce sectors. Common patterns include:
- Developers believe ReCAPTCHA is validated “somewhere upstream.”
- Backend frameworks like Laravel or Django are misconfigured to skip validation logic under certain conditions (e.g., testing mode).
- Token expiration and replay protection are not enforced, making even “valid” tokens reusable.
Key takeaway: CAPTCHA must be treated like any authentication mechanism — the server is the final gatekeeper, not the browser.
6. Recommendations — How to Do It Right
We issued the following recommendations to address the issue immediately and prevent similar mistakes:
Implement Proper Server-Side Validation
Ensure the backend verifies every g-recaptcha-response against the official Google endpoint using the site’s private key. Reject all requests where:
The token is missing, empty, or expired.
The response indicates failure (e.g., low score or risk).
Fail Securely
Design the system so that missing CAPTCHA validation always leads to rejection, not silent bypass.
Monitor and Rate-Limit
Add detection for suspicious activity:
Track IPs with high registration or login frequency.
Apply additional verification or blocking based on behavioral heuristics.
Extend Protection
Apply CAPTCHA enforcement to other sensitive operations:
Password resets
Account settings changes
Financial operations (e.g., withdrawals)
7. Conclusion — A Reminder That Security Is Layered
CAPTCHA alone won’t stop determined attackers, but when implemented properly, it adds critical friction that raises the cost of automation. In this case, a missing backend check removed that friction entirely — and exposed the platform to trivial abuse.
This incident serves as a cautionary tale for any company integrating third-party security controls: never assume the frontend enforces the rules. Always verify on the server.
Severity: High
CVSS Score: 7.3 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L)
Affected Modules: Registration and Login APIs
Exploit Complexity: Low
Exploit Potential: High-volume automated abuse with simple scripting
All identifiers, domains, and user data in this case study have been anonymized to preserve client confidentiality.
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