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Google reCAPTCHA API Key

Google reCAPTCHA is a service that helps protect websites from spam and abuse by using advanced risk analysis techniques to differentiate between humans and bots. The reCAPTCHA API key is used to authenticate requests to the reCAPTCHA service, allowing websites to verify user interactions. Exposure of this key can lead to unauthorized use of the reCAPTCHA service, potentially resulting in increased costs and degraded service performance due to abuse.

How Does It Look​

reCAPTCHA API keys can appear in various contexts, such as:

  • Environment variables:

    export RECAPTCHA_API_KEY="6Lc_aXkTAAAAABx1234567890abcdefgHIJKLMN"

  • Configuration files (JSON, YAML, .env):

    {
      "recaptcha": {
        "apiKey": "6Lc_aXkTAAAAABx1234567890abcdefgHIJKLMN"
      }
    }

  • Code snippets:

    const recaptchaKey = "6Lc_aXkTAAAAABx1234567890abcdefgHIJKLMN";

  • Connection strings (if applicable): Not typically applicable for reCAPTCHA API keys.

Severity​

  • 🟡 Medium

The severity is considered medium because while the reCAPTCHA API key does not provide direct access to sensitive data or systems, its exposure can lead to unauthorized use of the service. This can result in increased operational costs and potential service degradation due to abuse, especially if the key is used in high-volume automated attacks.

What Can an Attacker Do?​

With immediate access to a reCAPTCHA API key, an attacker can:

  • Bypass CAPTCHA challenges: Use the key to programmatically solve CAPTCHA challenges, reducing the effectiveness of the CAPTCHA protection (if the key is not restricted by domain).
  • Increase service costs: Generate excessive requests to the reCAPTCHA service, leading to increased billing (if the account is billed based on usage).
  • Degrade service performance: Cause legitimate users to experience delays or failures in CAPTCHA verification due to abuse of the service.

While the reCAPTCHA API key does not provide direct access to sensitive data, its misuse can lead to increased costs and potential service disruptions. Additionally, if the key is not restricted by domain, it can be used in attacks against other websites, leading to broader security implications.

Real-World Impact​

Exposure of a reCAPTCHA API key poses several business risks:

The primary impact is financial and operational, as unauthorized use can lead to increased costs and service degradation.

Potential consequences include:

  • Data Exposure: Not directly applicable, as reCAPTCHA keys do not provide data access.
  • Financial Loss: Increased billing due to unauthorized use of the reCAPTCHA service (if usage is billed).
  • Operational Disruption: Service degradation or failure due to abuse of the CAPTCHA service (if the key is used in high-volume attacks).
  • Reputational Damage: Loss of trust if users experience frequent CAPTCHA failures or delays.

In worst-case scenarios, widespread abuse of the reCAPTCHA service can lead to significant financial costs and damage to the brand's reputation due to perceived security weaknesses.

Prerequisites for Exploitation​

To exploit an exposed reCAPTCHA API key, an attacker needs:

  • Network access: Ability to send requests to the reCAPTCHA service.
  • Additional context: Knowledge of the website or application using the key (if domain restrictions are not in place).
  • Rate limits: Awareness of any rate limits or restrictions imposed by the reCAPTCHA service.

How to Verify If It's Active​

To verify if a reCAPTCHA API key is active, use the following command:

curl -X POST "https://www.google.com/recaptcha/api/siteverify" -d "secret=[API_KEY]&response=[USER_RESPONSE]"

Valid credential response: A successful verification will return a JSON object with "success": true.

Invalid/expired credential response: An invalid or expired key will return a JSON object with "success": false and an error message.

Detection Patterns​

Common Variable Names:

  • RECAPTCHA_API_KEY
  • GOOGLE_RECAPTCHA_KEY
  • CAPTCHA_KEY
  • RECAPTCHA_SECRET
  • CAPTCHA_SECRET
  • GOOGLE_CAPTCHA_KEY

File Locations:

  • .env
  • config.json
  • settings.yaml
  • appsettings.json
  • config.js

Regex Pattern:

6[0-9a-zA-Z_-]{39}

Remediation Steps​

  1. Revoke immediately - Go to Google Cloud Console > reCAPTCHA > Settings and delete the compromised key.
  2. Audit access logs - Review Google Cloud audit logs for unusual activity or excessive requests during the exposure window.
  3. Assess blast radius - Identify all systems, applications, and environments that used the exposed key.
  4. Rotate credential - Generate a new reCAPTCHA API key in Google Cloud Console with domain restrictions.
  5. Update dependent systems - Deploy the new key to all applications and update CI/CD pipelines securely.
  6. Harden access controls - Enable domain restrictions in Google Cloud Console to limit key usage to authorized domains.
  7. Implement secrets management - Migrate keys to a secrets manager (HashiCorp Vault, AWS Secrets Manager) to prevent hardcoding.
  8. Add detection controls - Set up pre-commit hooks and repository scanning to catch key leaks before they reach production.

Credential exposures often go undetected for extended periods, increasing the window for exploitation. As a long-term strategy, plan to establish an internal process or engage an external vendor for continuous external exposure monitoring. This helps identify leaked secrets across public repositories, paste sites, dark web forums, and other external sources before attackers can leverage them. Proactive detection and rapid response are essential to minimizing the impact of credential leaks.

References​