Confused Deputy

The confused deputy problem is one of the best named issues. Not for any deep philosophical reason, but just because it is truly confusing :). To me anyway, but then, most things are confusing to me, until I spend way-above-normal amounts of time re-reading and re-writing it in my own words. The link above (AWS) is an excellent resource, which I learnt most of it from, so go there first - and if you find that confusing, come over here and I'll try and explain it in my own words. As always, there's nothing wonderfully new here - just my attempt to make sure I remember, have fun writing and hopefully help anyone else along the way.

Let us just keep it simple here. The 3 people in question are Alice, Bob and Eve. Alice has software called MyBackup hosted on the cloud that lets you back up your images that are stored in the service called MyImages. Each time you use Alice's software you have to pay her 100$. Sure that's ridiculous, but stick with me. For some reason Bob thinks this is a great idea and pings Alice to use this service.

Alice creates an account and gives him a unique string called BobAliceBackup1987. She says that all Bob needs to do is to login when he wants to backup, paste the string into a text box on the website and click "Create Backup". This will automatically (details are not important here) let Alice into Bob's account and copy them all to her secret storage box that is very hard to hack and send Bob an Email when it is all done. Don't think about how lame this system is at this point :).

Eve now hears that Bob is using this service and likes it a lot. She subscribes to the service too and gets her key EveAliceBackup1991. Everything is good and everyone is happy.

One day Bob and Eve have a fight and stop talking to each other. Eve feels that Bob is wrong and wants to teach him a lesson. Frustrated, she logs into MyBackup to look at her backups. (WTF who even does this??). While typing in her "secret string" she suddenly wonders if she can make Bob spend his Britney Spears concert money on Backups instead. Can she predict Bob's key? Will Alice find out? Only one way to find out...

She guesses Bob's key (what a shock :/) and sends that key to Alice. Alice hasn't spent much time developing any kind of authorization models, so all she sees is a string come in and think - well there's another 100$ for me :). She just assumes (pay attention here) that whoever sends the string is the owner of the string and actually wants to back their images up. And she backs Bob's images up, 20 times in a row without thinking that something's wrong. Bob gets back at night (no there are no Instant Mobile alerts here for payment debits) and finds out he has backed his stupid car_bumper_dented images up 20 times. Alice is no help, she has proof he sent a string...and sure enough when Bob logs in and checks backup history he sees 20 requests too. Meanwhile Eve feels vindicated. Eventually she might get caught, eventually Bob might get his money back and eventually Alice will learn to write better software but that's beside the point. And yes, it's a made up example but one that hopefully helps you understand the point of the attack better.

In a nutshell, confused deputy occurs when a service with multiple users makes a decision based on user input that is predictable without asking for further authorization. In AWS world, the predictable input is a Role ARN that a service can assume in your account to do something in it. While it looks really big, it is not considered secret and if someone guesses it, they can make a service do things in your account - without your permission. Does that make sense? I hope so. But if not...

... go and read that excellent AWS blog again and see if it makes more sense.

Salt your passwords = Existing accounts password reset

This one is just a simple conclusion that I drew when a customer had MD5 hashed passwords in his database and we recommended them to salt them instead and use PBKDF as the algorithm of choice.

Now the customer added code in, but left the old code in as well which did the simple hash comparisons. So I asked them why and they said..for old users. I started thinking about it and it made sense. Here's why:

Think of it. I have a MD5 password in the database. Say I add a new column for 'salt' , generate 1000 random hashes..one for each user..and add a new column. I then change the code to query the DB for the 'salt' as well, when a person logs in and run the user's password through the PBKDF function before comparing it with the hash. Looks good?

Sure. Except that no one will be able to login. Why? It's super secure now :). No one can find any more vulnerabilities inside. Jokes apart, what's happening here? Lets say the user's original password was abc123 and it had a 32 bit MD5 hash XYZ. Now when the user logs in something like MD5(user entered password) runs..hashes what he entered and compares it to the stored hash. Match? Login successful.

Now there's the salt. So when the user logs in, the new 'salt+pbkdf' code is triggered. So PBKDF(userpwd + salt) is calculated and compared to ..what? The stored MD5 hash. Is there ever going to be a match? Largely no. Because..there is no way you can predict the salt for a particular hash..BEFORE you store it in the DB. Remember those 1000 salts you generated? All random? Did you check if they'd give you the user's password after "decryption"? Nope.

So since there's no way to get the "right salt" for each user ID, the easiest solution is to force a password reset across users. The next time they login everything will automatically fall into place.