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IDOR
When faced with an encoded ID, it might be possible to decode the encoded ID using common encoding schemes.
And if the application is using a hashed/ randomized ID, see if the ID is predictable. Sometimes applications use algorithms that produce insufficient entropy, and as such, the IDs can actually be predicted after careful analysis. In this case, try creating a few accounts to analyze how these IDs are created. You might be able to find a pattern that will allow you to predict IDs belonging to other users.
Additionally, it might be possible to leak random or hashed IDs via another API endpoint, on other public pages in the application (profile page of other users, etc), or in a URL via referer.
For example, once I found an API endpoint that allows users to retrieve detailed direct messages through a hashed conversation ID. The request kinda looks like this:
GET /api_v1/messages?conversation_id=SOME_RANDOM_ID
This seems okay at first glance since the conversation_id is a long, random, alphanumeric sequence. But I later found that you can actually find a list of conversations for each user just by using their user ID!
GET /api_v1/messages?user_id=ANOTHER_USERS_ID
This would return a list of conversation_ids belonging to that user. And the user_id is publicly available on each user’s profile page. Therefore, you can read any user’s messages by first obtaining their user_id on their profile page, then retrieving a list of conversation_ids belonging to that user, and finally loading the messages via the API endpoint /api_v1/messages!
If the object reference IDs seem unpredictable, see if there is something you can do to manipulate the creation or linking process of these object IDs.
If no IDs are used in the application generated request, try adding it to the request. Try appending id, user_id, message_id or other object reference params and see if it makes a difference to the application’s behavior.
For example, if this request displays all your direct messages:
GET /api_v1/messages
What about this one? Would it display another user’s messages instead?
GET /api_v1/messages?user_id=ANOTHER_USERS_ID
HPP vulnerabilities (supplying multiple values for the same parameter) can also lead to IDOR. Applications might not anticipate the user submitting multiple values for the same parameter and by doing so, you might be able to bypass the access control set forth on the endpoint.
Although this seems to be rare and I’ve never seen it happen before, theoretically, it would look like this. If this request fails:
GET /api_v1/messages?user_id=ANOTHER_USERS_ID
Try this:
GET /api_v1/messages?user_id=YOUR_USER_ID&user_id=ANOTHER_USERS_ID
Or this:
GET /api_v1/messages?user_id=ANOTHER_USERS_ID&user_id=YOUR_USER_ID
Or provide the parameters as a list:
GET /api_v1/messages?user_ids[]=YOUR_USER_ID&user_ids[]=ANOTHER_USERS_ID
Sometimes endpoints susceptible to IDOR don’t respond with the leaked information directly. They might lead the application to leak information elsewhere instead: in export files, emails and maybe even text alerts.
If one request method doesn’t work, there are plenty of others that you can try instead: GET, POST, PUT, DELETE, PATCH…
A common trick that works is substituting POST for PUT or vice versa: the same access controls might not have been implemented!
Sometimes, switching around the file type of the requested file may lead to the server processing authorization differently. For example, try adding .json to the end of the request URL and see what happens.
Always look for IDORs in critical functionalities first. Both write and read based IDORs can be of high impact.
In terms of state-changing (write) IDORs, password reset, password change, account recovery IDORs often have the highest business impact. (Say, as compared to a “change email subscription settings” IDOR.)
As for non-state-changing (read) IDORs, look for functionalities that handle the sensitive information in the application. For example, look for functionalities that handle direct messages, sensitive user information, and private content. Consider which functionalities on the application makes use of this information and look for IDORs accordingly.