Cracking the Code: Account Takeover via Password Reset Token Prediction

Introduction

In the ever-evolving world of cybersecurity, the hunt for vulnerabilities is a critical part of ensuring online safety. In this blog, we'll explore a security threat that exploits Password Reset Functionality, Tokens, and Links, potentially leading to an account takeover.

The Vulnerability: During my bug bounty research, I discovered a method that could allow an attacker to compromise all user accounts on a vulnerable website. This vulnerability arises from the predictability of password reset tokens included in the password reset links.

Steps to Execute the Attack

Before diving into the details, it's important to note that this attack requires knowledge of the victim's email address.

Now, let's look at how it's done:

1. Initial Password Reset Request: The attacker begins by sending a password reset request to their own email address, e.g., attackeremailid@gmail.com.

2. Targeting the Victim: Simultaneously, the attacker sends another password reset request, this time targeting the victim's email address, e.g., victimemailid@gmail.com, using a specific password reset request link, such as https://testsite.com/account/resetpassword.

3. Receiving the Token: The attacker receives a password reset link on their email address (attackeremailid@gmail.com). The link contains a token, like this: https://testsite.com/account/resetpassword?code=GH193733-5mq1-0a37-e051-43fefa0aaed6.

4. Token Analysis: To understand the vulnerability, I created two test accounts. It was observed that by sending the password reset request to the attacker's email address first, and then quickly sending another request to a victim's email address, the attacker can predict the password reset token. Specifically, characters 9 to 12 of the token (e.g., GH193733-5mq1-0a37-e051-43fefa0aaed6) change incrementally.

5. Predicting the Victim's Token: Armed with knowledge of their own password reset token (e.g., GH193733-5mq1-0a37-e051-43fefa0aaed6), the attacker can predict the victim's token, which might look like GH193733-6nr2-0a37-e051-43fefa0aaed6.

6. Compromising the Victim's Account: With the predicted token, the attacker crafts a password reset link, effectively taking control of the victim's account. The crafted URL looks like this: https://testsite.com/account/resetpassword?code=GH193733-6nr2-0a37-e051-43fefa0aaed6.

Impact

The vulnerability lies in the lack of randomness in the password reset tokens. This flaw allows an attacker to predict token values for known email addresses, thereby resetting passwords. This provides attackers with a simple path to gaining unauthorized access or causing a denial of service to users on the application.

Recommendations for Mitigation

To bolster security and prevent this type of attack, consider the following recommendations.

• Unique Tokens: Implement a system that generates unique, one-time tokens for each password reset request. These tokens should be unguessable and expire after a successful password reset.

• Token Expiry: Ensure that generated tokens become invalid once the password has been reset, limiting the window of opportunity for attackers.

• Input Validation: Treat user input as untrusted and re-validate it when sent to the server. This is crucial for safeguarding against various forms of attacks.

• Prevent Default Behavior: Make sure that the system does not default to updating all accounts if an identifier is not explicitly provided, preventing unauthorized access to multiple accounts.

Conclusion

The threat of account takeover through password reset token prediction is a serious concern that demands immediate attention. By implementing the recommended security measures, we can fortify our systems against such attacks and provide a safer online experience for users. It's essential to remain vigilant and proactive in the dynamic field of cybersecurity.