TL;DR: This is about how I got Account Takeover (ATO) vulnerabilities on two big e-commerce companies and a bypass after the first fix for one of the issues with a nice exfiltration technique. These two companies have private bug bounty programs so I’m not allowed to reveal their names. 🤐


I think the ATO is underestimate by the developers when coding and even more by the testers when performing their penetration tests. I’m saying that because most of ATO that I found was actually a logical flaw which didn’t really require much technical skills. Basically it was just thinking a little bit out of the box. 📦

Out Of Scope

I found this ATO in a private bug bounty program so I’m not allowed to reveal any information regarding the company so let’s call this company as Ecom Fake.

Everything started when I found a subdomain takeover under the There was a CNAME pointing to and the root domain was available to buy.

This is a common mistake where DNS administrators publish internal names on their external DNS which is supposed to be valid only in the internal network.

I don’t like to report only the subdomain takeover vulnerability since it could have a lot different impact and in this program they made clear subdomain takeover as out of scope. Because of that I decided to do a quick check on the application to use the subdomain takeover in a chain for another vulnerability with a nice impact.

In less than five minutes I was able to find out their cookies domain attribute were too permissive which means any logged user on (main application) would send their cookies to the subddomain under my control if the user access it.

I didn’t create a PoC but it was quite easy for an attacker include some links under or any other subdomain under and get the session cookie from all the users who access the page with the malicious links.

The program initially closed my report with the following statement.

Thanks for finding the subdomain takeover, unfortunately, it is out of scope for this project. I will close this ticket as resolved so you don’t lose a point.

I replied back with the following comment.

The subdomain takeover is in the chain but it’s not the vulnerability that I’m reporting here.
I’m reporting the massive account takeover vulnerability which is possible only because of the cookie domain attribute is too permissive.

There was a long discussion which doesn’t worth to put here and in the end they fixed the subdomain takeover and close as resolved but not eligible for a bounty.

To avoid the same issue with other bug hunters I’ve suggested them to make the policy clear by including a text saying that any issue combined with subdomain takeover would be invalidated but they didn’t update the policy.

Regarding the permissive domain cookie issue they’ve informed me this is a “known generic issue” and they won’t fix it. 🤷‍♂️

Trying Again

It took some time but I found an security issue which I’ve already seen in other applications before. The “Forgot your password?” feature had a logical flaw allowing an attacker to takeover an user account. There were only two requirements for an successful attack:

  1. The attacker needs to know the victim email and name.
  2. The victim needs to be tricked to validated his own OTP.

The step by step process wasn’t so intuitive so I sent a video and also described the steps below.

  1. Set any proxy tool like Burp in your browser.
  2. Access and perform the “Forgot your password?” using the victim email.
  3. When the application ask for the OTP copy he latest URL the browser address bar.
  4. Send an e-mail to the victim with the URL from the previous step. A real attacker would create a phishing email like “We detect a malicious activity in your account. Please reset your password clicking here.”
  5. Keep refreshing the page multiple times from step 3 until get a different response. As soon as the victim provide the OTP code you will be actually loading the next step.
  6. Provide the victim name and after that the new password before the victim. Note this can be easily automated with a python script in order to change the password before the victim since you can detect when the user has already provided the OTP by checking the response.
  7. Login into the victim account with the new password.

On step 5 even if the user resend the OTP it’d work anyway. The key was trick the user to load the URL from step 3 and provide the OTP to reset his own password.

For the second time but now directly from Hackerone triagger I got the bad news below.

I believe the best course of action for this report is for you to self-close this report.

He also included some text explaining why I should self-close the report. One of them was this one below.

The policy page explicitly states that this report is out-of-scope as it requires social engineering/phishing a user to exploit.

I almost closed my report because it seems every time that I open a report the Hackerone triagger has the goal to disqualify it. Even though I tried to argument with the comment below.

The problem here is not the phishing attack but the security issue in how the forgot password feature was designed.
From my perspective the forgot password feature should never allow be started from one browser and get part of it process in a different browser session. My suggestion for the solution in this case is when the user access the site for the very first time he should get an anonymous session and any ID for any process must be tied to this session which would prevent this attack.
Even that take look the instructions from Ecom Fake regarding phishing attacks. In the video and text they are asking to the user o check if the domain is a real domain from Ecom Fake and in this attack it is from Ecom Fake. Why an user shouldn’t trust in a real URL from Ecom Fake? Actually it should trust and that’s why the security issue is on the “Forgot your password?” feature in this case.

In the end the Ecom Fake defined this ATO was a low severity issue and gave me the charity of US$125.

I’ve decided not working with this bug bounty program anymore. It just doesn’t worth the stress. 🙅‍♂️

Winner Winner Chicken Dinner!

In this case the account recover feature through Google had a logical flaw allowing an attacker to start the process and by a phishing attack force the user to perform the required steps to finish the ATO attack. Let’s take a look in the attack step by step that I’ve sent to the bug bounty program.

1 . Let’s use the email as an example. Set up a browser with a proxy tool like Burp and access

2 . Start the recover account process by providing the email

3 . In the next screen enable Burp interception and click on account recover feature link.

4 . On Burp forward the first POST request and stop on the GET request to

5 . Copy the URL from the GET described above and replace the redirect_url URL parameter with “https%3a//”. In my case I’m using this open redirect vulnerability pointing to a subdomain under my domain so I’d be able to know when the victim is done with the required steps. This is not really required because it’d work anyway by providing any invalid subdomain. Don’t worry you are going to get in the next steps.

6 . Now it’s time to phishing by sending the URL created in the previous step to the victim with any interesting story.

7 . From another browser open the URL from the fake email above to simulate the victim access and try to recover the account using Google.

8 . If the user has already been logged on Google and previously approved the login on Exemplo this step is not required. If not the victim will need to login on Google.

9 . In this PoC the page will be redirected to with doesn’t have any application returning 500 HTTP error. A real attack would redirect the user back to and the victim would never know what happened.

10 . If I was doing remote I’d know the user has finished the required steps by checking the access logs on and after that it’s just a matter to disable the Burp intercept from step 4. Winner winner chicken dinner!

This ATO was almost the same from the previous one but it doesn’t required much user interaction. If the victim was already logged on Google/Gmail by just loading the malicious URL the ATO would work totally transparent.

The Exemplo company was very nice during the process and rewarded me with US$1400. 👍

Winner Winner Chicken Dinner Bypass!

Few weeks later I had some free time to play again with this ATO bypass and noticed they include an extra step to validate the user email provided by the making impossible to exploit using the open redirect that I was using before. Because of this extra step validation now it was required a specific URL ID parameter. Basically if I was able to get the URL ID parameter I’d be able to exploit similar from the last time. The big problem was how to get the URL ID parameter!

After some time checking other services provided by the same Exemplo company I saw an interesting service where I could host my own store under the same root domain. Let’s call this service as MyStore.

The MyStore had some nice features and integrations with “Google Analytics” services which gave me a really nice idea. Let’s check the attack step by step now with a ninja exfiltration technique that I’ve sent to the bug bounty program.

1 . In order to perform this attack first we need to create a fake account under (e.g.

2 . Now we need to activate the MyStore for the account created in the previous step and enable the Google Analytics feature.

3 . Take the victim email account (e.g., set up a browser with a proxy tool like Burp and access

4 . Start the recover account process by providing the victim email (e.g. and click on continue button.

5 . In the next screen enable request and response Burp interception and click on forgot password link.

6 . On Burp forward the first POST request and stop on the response which will be a redirection (302 Found). Copy the redirected URL from the Location response header.

7 . Paste the URL from the previous step in a text file in order to have a copy of it. Change the copied URL by replacing the redirect_url URL parameter with your MyStore URL created from the first steps. Note the entire redirect_url URL parameter could be URL encoded to hide the content from the victim.

8 . Go back to Burp, drop the response and any following requests to stop the account recover process.

9 . Now it’s time to phishing by sending the URL created in the previous step to the victim with any interesting story.

10 . From another browser instance open the URL from the fake email above to simulate the victim access and try to recover the account using Google.

11 . If the user has already been logged on Google and previously approved the login this step is not required. If not the victim will need to login on Google. In the end the victim will be redirected to the MyStore created in the first steps.

12 . As the attacker access your Google Analytics dashboard and go to “Realtime > Overview” on the left panel. Copy the URL parameters under “Top Active Pages” section.

13 . Now is the real trick. Remember the original URL which we took a copy and pasted in a text file. Take the redirect_url URL parameter, decode it first and append the parameters extracted from Google Analytics.

14 . Load the URL from the previous step under the same browser where you started the recover account process. Winner winner chicken dinner again!

Since it was a bypass and required more user interaction the Exemplo company rewarded me with US$700. 😁

If you have any question or want to share any interesting technique please send an email to or contact me on twitter @ricardo_iramar.

Alguém a procura de autoconhecimento constantemente.