How a Single Parameter Led to Two ATO Cases

Fahad Faisal
3 min readMay 18, 2024


Hey guys, Today, I’m excited to share an intriguing story about how a single parameter in an application led to two account takeover cases and earned me a four-digit bounty. The program is a private one on HackerOne, and I was among the first few hackers to get invited. I’ll explain two bugs I found here. Both account takeover (ATO) cases are one-click exploits, meaning the victim only needed to perform a single interaction.

Weakness in the Password Reset Functionality

I was testing the password reset functionality of this application and discovered an interesting parameter called CallbackUrl from the password reset request.

POST /auth/resetpassword HTTP/2
User-Agent: Mozilla/5.0
Accept: application/json
Content-Type: application/json


When a user requests a password reset link, the server sends a request to the specified email with a password reset link that contains a token. However, I discovered that if an attacker changes the request parameter CallbackUrl to a domain that they control, the password reset link will be sent to the user's email. This means that the attacker can obtain the password reset token when the user clicks the link and use it to take over the user's account.

I reported the issues, and they marked the report as high severity and rewarded me with a $$$$ digit bounty.

This discovery sparked my curiosity to dig deeper into the platform. I searched the application JavaScript for corresponding parameters and found that the same parameter is used in the application’s 0Auth flow.

Account Takeover via Misconfigured OpenID

During my testing on the 0Auth mechanism of the application, I got another security flaw that could potentially lead to One click account takeover. This vulnerability stems from a misconfiguration in the OpenID implementation, specifically in the callbackUrl parameter used in the authentication flow with various identity providers.

I discovered that an attacker could exploit this misconfiguration by manipulating the callbackUrl parameter by redirecting it to an attacker-controlled domain, malicious actors could intercept the authentication process and potentially gain unauthorized access to user accounts.

For example:

  • Google: Initiating the authentication process with the respective provider results in the authentication response being redirected to the attacker's domain.

Exploiting this vulnerability allows attackers to intercept authentication responses, potentially obtaining access tokens. The authentication response is redirected to the attacker-controlled domain specified in the modified callbackUrl parameter.

After reporting the issue, the team responded by explaining that they considered the reported vulnerability as part of the previous callbackUrl fix. They emphasized that their policy only allows for one bounty payment per bug and considered duplicates unacceptable. Despite their explanation, I reiterated my findings, emphasizing that the vulnerability persisted specifically in the callbackUrl of the Auth0 endpoint. I acknowledged their efforts in resolving the previous bug but urged them to consider this as a separate issue due to its potential risk of account takeover.

Following internal discussions, the team ultimately recognized the severity of the vulnerability and awarded me another $$$$ digit bounty along with an exclusive swag pack as a token of appreciation for bringing it to their attention.

Thanks for taking the time to read through the write-up, and stay tuned for more insights and discoveries in the future



Fahad Faisal

A bug bounty hunter by day and a cybersecurity superhero by night (or whenever my WiFi connection is strong enough). Armed with my laptop and a sense of humor