Knocking the IDOR

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Hello to you all.

Sorry for the no-new-posts-November, FogMarks has been very busy experiencing new fields and worlds. But now – we’re on baby!

Today’s case-study is on an old case (and by old I mean 3 months old), but due to recent developments in an active research of a very known company’s very known product, I would like to present and explain the huge importance of an Anti-IDOR mechanism. Don’t afraid, we’re not biting.


Basically, an IDOR (Insecure Direct Object Reference) allows attacker to mess around with an object that does not belong to him. This could be the private credentials of users, like the email address, private object that the attacker should not have access to, like a private event, or public information that should simply, and rationally – not be changed by a 3rd person, like a title of a user (don’t worry – case-study about the Mozilla vulnerability is on its way).

When an attacker is able to mess around with an object that does not belongs to him, the consequences might be devastating. I am not talking just about critical information disclosure that could lead the business to the ground, I am talking about messing around with objects that could lead the attacker to execute code on the server. Don’t be so shocked – it is very much possible.

From IDOR to RCE

I’m not going to disclosed the name of the company or software that this serious vulnerability was found on. I am not even going to say that this is a huge company with a QA and security response team that could fill an entire mall.
But I am going to tell you how an IDOR became an RCE on the server, without violent graphic content of course. For Christ’s sake, children might be reading these lines!

Ideally speaking,
An IDOR is being prevented using an Anti-IDOR Mechanism (AIM). Us at FogMarks have developed one a few years ago, and, know-on-wood, none of our customers ever dealt with an IDOR problem. Don’t worry, we’re not going to offer you to buy it. This mechanism was created only for two large customers who shared the same code base. Create your own mechanism with the info below, jeez!
But seriously, AIM’s main goal is to AIM the usage of a certain object only to the user who created it, or have access to it.

This is being done by holding a database table especially for sensitive objects that could be targeted from the web clients.
When an object is being inserted to the table, the mechanism generates to it a special 32 chars long identifier. This identifier is only being used by the server, and it is calld SUID (Server Used ID). In addition, the mechanism issues a 15 chars long integer identifier for the client side that is called, of course, CUID (Client Used ID). The CUID integer is being made from part of the 32 chars long SUID and part of the object permanent details (like name-if name cannot be changed afterwards) using a special algorithm.

In the users’ permissions table there is also a row of list of nodes that contains the SUID of objects that the user has access to it.

When the user issues a request from the client side (from the JS – a simple HTTP request (POST/GET/OPTIONS/DELETE/PUT…), the CUID is being matched with the SUID – the algorithm tries to generate the SUID from the supplied CUID. If it succeed, it then tries to match the generated SUID the SUIDs list in the users’ permission table. If it match, the requesting user gets one time, limited access to the object. This one time access is being enabled for x minutes and for one static IP, until the next process of matching CUID to SUID.

All this process, of course, is being managed by only one mechanism – The AIM. AIM handles request in a queue form, so when dealing with multiple hundreds of requests – AIM might not be the perfect solution (due to possible object changes by 2 different users).

In conclusion, in order to keep your platform cure from IDORs, requests to access sensitive objects should be managed only by one mechanism. You don’t have to do the exact logic like we did and to compile two different identifiers to the same object, but if you’ll like to prevent IDORs from the first moment (simply spoofing the ID), our proposed solution is for the best.

Here are some more examples of IDORs found by FogMarks in some very popular companies (and were patched, of course):