Account Takeover via Access Token Leakage

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Hello guys! My name is Tuhin Bose (@tuhin1729). I am currently working as a Chief Information Security Officer and Infosec trainer at DSPH. In this write-up, I am going to share one of my interesting findings. So without wasting time, let’s start:


Basically the target was a marketing automation website where you can automate your marketing stuffs efficiently. Let's call it I have already found more than 10 bugs on the target and earned $$$$ from there.

Now while testing the profile update feature, I came across with this interesting request:

PUT /api/account/general-info/ HTTP/1.1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Firefox/91.0
Accept: application/json, text/plain, */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: application/json
accessToken: 795b74eXXXXXXXXXXcba9abd3beaa3ec40b5d3ed
Content-Length: 213
DNT: 1
Connection: close
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: cross-site{"company":"DSPH","domain":"","cellphone":"+91-83xxxxxx36","companySize":"2","businessSector":20,"logo":{"height":0,"base64":"","square":true,"width":0}}

They are using accessToken header for changing the profile details (For other authenticated actions, there is no such header). I quickly changed the value of accessToken header with my 2nd account and my 2nd account’s details were changed. I tried to add the accessToken header in other authenticated requests and

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