Tamper proof session cookies and session storage

As a follow-up to my previous article regarding User sessions, what data should be stored where?, I wanted to discuss how to store the session, and how to generate cookies that are tamper proof.

What are we trying to accomplish?

Ultimately we want to be able to have X amount pieces of data that are tied to a particular user. Unfortunately due the fact that HTTP is a stateless protocol we have to use cookies. Cookies are small little pieces of data that are transmitted from the server to the client (generally done once), and then upon the user coming back to the website they are transmitted from the client to the server. This allows us to uniquely track a single user across connections to our website.

If the website allows a user to authenticate and the fact that they are authenticated is stored in the session, we also want to make sure that we can aggressively expire a session, if this is possible depends on our session storage.

Session storage

There are a multitude of ways to store the session data, but it ultimately boils down to server-side or client-side. Server-side can be done in Cassandra, Memcache, Redis or even in a SQL database.

Server-side storage

The main one that has been used for years is to use server side storage. Storing a small file on the servers hard drive that contains the data, and the client is sent a cookie that contains a unique identifier that is linked to the on-disk storage.

For example:

1 => /tmp/session_1
2 => /tmp/session_2
...
N => /tmp/session_N

Easily expire sessions

The upside to server-side storage is that it is possible for us to very easily expire a session, simply remove the associated file/data that is stored and the users session has now become invalid.

Client-side storage

The other method that has recently started being used more to make it easier to scale the server side is to store session data encoded in base64 in the cookie itself. In this case there is no unique session ID, and no data is stored server side.

Expiration is more difficult

The downside to using client-side storage is that there is no way, short of the expiration on the cookie itself for the website to expire a session. There are work-arounds, but they all require storing state server-side. A hybrid approach for example is possible, store a unique ID along with the session data, and store that unique ID server side, but none of the extra data. Remove the unique ID server side and if we receive a session that contains a unique ID we don't recognise, we simply clear the session.

Expiration, why do I care?

Being able to easily expire a users sessions allows for extra security measures. For example in Google Mail it is possible to sign out all other locations, this forces those other locations to re-authenticate before gaining access to your account.

This is a good security measure to have, so that if a users cookie is stolen, or their credentials are compromised upon changing their password all their sessions are invalidated and an attacker using an old cookie/session ID can't continue to wreak havoc on the users account.

Cookie format

If we are just storing a session ID, or the full session the cookie should be hardened so that it can not be tampered with by a client. Even if you are protecting the cookie using SSL, we still don't want to allow a malicious user to modify the cookie to change the session ID or the session itself.

Signing your cookie

The single best way to make sure your cookie has not been tampered with is to cryptographically sign your cookie, and upon receiving the cookie from the client verifying that the signature matches what you are expecting. This is especially important if you are using client-side storage, because you don't want someone to be able to change the user ID from 950 to 1 and suddenly impersonate a different user.

Use an HMAC

HMAC (Hash-based message authentication code) is an cryptographic construct that uses a hashing algorithm (SHA-1, SHA-256, SHA-3) to create a MAC (message authentication code) with a secret key. It is very easy given the secret key and the original data to create the MAC, but it is very difficult if not impossible to take the original data, and MAC and get the secret key.

This allows us to do the following:

data = "Hello World"
mac = HMAC(data, sha256, "SEEKRIT")

Our mac would now be equal to:

e655f98cb9b3c02f45576f7906d64b0b7f8731f25a5319c42ca666917aca45a4

If we now create our cookie as follows:

cookie = mac + " " + data

It would look as follows:

cookie = e655f98cb9b3c02f45576f7906d64b0b7f8731f25a5319c42ca666917aca45a4 Hello World

We can then send that to the client that requested the page. Once the client visits the next page, their browser will send that same cookie back to use. If we split the mac from the data, we can then do the following operation:

cookie = e655f98cb9b3c02f45576f7906d64b0b7f8731f25a5319c42ca666917aca45a4 Hello World
data = "Hello World"
mac = e655f98cb9b3c02f45576f7906d64b0b7f8731f25a5319c42ca666917aca45a4

mac_verify = HMAC(data, sha256, "SEEKRIT")

mac_verify == mac

If and only if mac_verify and mac are the same can we be sure that the cookie has not been tampered with.

This requires that the client is NEVER aware of what we are using as our secret key. In the above exmaples that is "SEEKRIT". In your web application you will be required to make this a configuration variable, and you will have to take care not to commit that configuration variable to a git repository and upload it to github (for example).

Do not use a bare hash algorithm

Using a bare hash algorithm allows for length extension attacks if used incorrectly, this would allow an attacker to concatenate extra data to the end of our existing data, modify the "MAC" and the server would accept it.

This construct is thus very dangerous:

data = "Hello World"
key = "SEEKRIT"
mac = SHA1(key + data)

The following construct is still not recommended, but is not nearly as dangerous:

mac = SHA1(data + key)

Due to the key being last, this is not vulnerable to a length extension attack, however please don't do this, instead stick to using an HMAC instead.

Encrypting session data

When using client-side storage, it may be beneficial to encrypt the data to add an extra layer of security. Even if encrypting the data you need to continue using a MAC.

Using just encryption will not protect you against decrypting bad data because an attacker decided to provide invalid data. Signing the cookie data with a MAC makes sure that the attacker is not able to mess with the ciphertext.

What are web frameworks/languages doing by default?

I am most familiar with the Pylons Project's Pyramid Web Framework, the default session implementation that is provided by the project is named SignedCookieSessionFactory, as the name implies this uses a client-side cookie to store the session data, which is signed using a secret key that is provided upon instantiation of the factory.

Flask sessions also uses a signed cookie for client-side session storage.

Ruby on Rails uses a signed/encrypted cookie for client-side session storage by default.

PHP does not by default sign the session cookie, it does however use server-side storage for session data by default. However extra security can be added by installing PHP SuHoSin which adds session cookie encryption/signing.

User sessions, what data should be stored where?

This article has been updated. For the old version please check Archive.org

A couple of days ago on reddit.com's /r/netsec a poster by the name of Dan Weber posted what he believed to be an attack on PHP sessions: Hacking PHP sessions by running out of memory (reddit link). The way the "attack" works is as follows:

1. Create a new session
2. Assign some new data to the session, in this case a username which is used to signify that the user is logged in.
3. Check to verify that the user should be logged in
4. If the user should not be logged in, destroy the session.

The "attack" would be to run the PHP script out of memory on number 3, since once something is set on the session it is immediately stored, so even if the user is not supposed to be logged in, they are now logged in since their session says they are.

I wouldn't necessarily call this a PHP hack, this is really just bad practice in terms of programming, the logic should be reversed.

1. Create a new session
2. Verify the user should be logged in
3. If so, set the session data
4. If the user should not be logged in, don't set the session data

That would solve the problem at hand, and now there is no way for the user to trick the PHP script into believing she is logged in when that is not the case.

However as the discussion went on on Reddit, it became even more clear that there are no good resources on what you should store in the user session, and what you shouldn't store in the user session. Some of these things may seem like common knowledge, but sadly this is something every single new person to programming has to learn on their own.

Some assumptions

Let's get this out of the way, this is in no way limited to PHP, but it is the one I will be using as an example. This can all apply to Ruby (Ruby on Rails), Python (Pyramid) or many other frameworks.

The basic problem is that generally writing to the session is not an atomic transaction based on the page accessed, so the assumption made in this article is that when you write to the session it is instantly committed, and there is no way to roll it back upon failure. If there was, our first example listed wouldn't be able to occur since upon running out of memory at step 3, the session would have been rolled back and cleaned up.

What should you store in the users session?

You should only really store anything in the session that if it were made public it would do very little harm.

Really the list of items to be stored in a session are as follows:

1. The users unique id (The ID that allows you to retrieve the users information from storage)
2. Temporary state (i.e. Flash messages)
3. CSRF token

More importantly, don't store permission bits, or group memberships, or anything that is used to allow/deny access to particular resources. You want to store just enough information that upon a user accessing your site you are able to retrieve the users information from storage, and based upon that information from storage you then make decisions such as permissions/group memberships.

Why is this important?

One of the things that Dan Weber brought up in the Reddit post was storing the users permission level and group membership in the session. If your code is then relying on the session to always contain the right permission level, then there is no way to expire someones access to the data.

1. A user logs in
2. A user gets various permissions, and they are set in the session
3. The user has been fired from his job, and an administrator removes the users permissions
4. Since the user is still logged in, the users permission bits are still set, and he continues to have access to parts of the site/information he shouldn't have access to.

If instead on every page visit we simply pull out the users unique id and verify the permissions upon access, as soon as the permissions are revoked by the administrator the user no longer has access to the various resources.

Temporary state

There has to be an easy way to remember something from page visit to page visit that isn't considered detrimental if the information gets lost. One of those things is flash messages. Flash messages are generally used to provide the user indication that something has changed, they are shown once and then never again.

Storing these as session data makes sense. If the flash message gets set, great, if it doesn't get set, it doesn't matter. Flash messages are simply a notification tool, if the user misses them it isn't important.

What you shouldn't store in the session

Definitely don't store any kind of permission bits, groups a user is a part of or anything that would allow the user access that they normally would not be able to access.

On each page access check what permissions the user has. While it may mean a little more heavy lifting server side it provides extra security, and the means to enforce changes in permissions instantly.

Good secure programming practices

Keep secure programming practices in mind at all times, always consider how the information you are storing/processing is accessed/viewed/administered. More importantly think about the access controls that are in place, and how one could expire access to a particular resource without requiring a co-operative client.

The ordering of how variables are set, and when they are set are very important. $_SESSION['isadmin'] = True at the top of a PHP script, and then removing it by checking to see if the user is actually an administrator later on in the script is a bad idea.

Always order your code so that if a failure does occur there is no chance that a critical section of your code is executed by accident, or that information is stored in a half-verified state. This is especially important for access control.