Incomplete Argument Checks

Written by Pete Corey on Aug 31, 2015.

I’ve been shouting for months about the importance of checking your method and publication arguments. But what do I mean when I say that? Is checking your arguments as simple as throwing in a check(argument, Match.Any); statement and moving on with your life? Absolutely not!

Incomplete Checks

I frequently see people running incomplete checks against their method and publication arguments. Check out this quick example:

Meteor.methods({
  processEvent: function(event) {
    check(event, Object);

    // Do processing...
    
    Events.remove(event._id);
  }
});

The processEvent takes in an event object, does some processing on it, and then removes the event object from the Events collection. This is all fine and good. We’re even checking the event argument!

Unfortunately, we’re not checking event thoroughly enough. What would happen if a user were to run this code in their browser console?

Meteor.call("processEvent", {_id: {$ne: ""}});

{_id: {$ne: ""}} is, in fact, an object, so it slips past the check statement. Unexpectedly though, the _id within event is an object as well. After processing the event object, the processEvent method would go on to remove all events in the Events collection. Behold the dangers of incomplete checks!

A More Thorough Check

The solution to this issue is to more thoroughly check the event argument. If we’re expecting event to be an object, we want to make a type assertion (and sometimes even a value assertion) over each field in that object. Take a look:

Meteor.methods({
  processEvent: function(event) {
    check(event, {
      _id: String,
      // Check more fields here...
      // name: String,
      // data: {
      //   value: Number
      // }
    });

    // Do processing...
    
    Events.remove(event._id);
  }
});

By checking that the _id of the event object is a string, we can avoid potential NoSQL injections that could wreak havoc within our application.

Final Thoughts

Incomplete checks can take many forms, be it check(..., Object);, check(..., Match.Any);, check(..., Match.Where(...));, etc… Regardless of the form they come in, incomplete checks are all little flaws stitched together with good intentions.

Checking your arguments is vitally important for a huge number of reasons, but it’s important that you follow through completely with your good intentions. Stopping with incomplete checks can leave you with a false sense of security and a vulnerable application.

Always (thoroughly) check your arguments!

Hijacking Meteor Accounts by Sniffing DDP

Written by Pete Corey on Aug 23, 2015.

You’re in your neighborhood Starbucks scarfing down an Everything With Cheese Bagel while browsing the web, and you decide to visit your favorite Meteor application. You go to the website, type in your authentication credentials, and hit “Log In”, paying no mind that the application is running over http, not https.

Unbeknownst to you, sitting in a dimly lit corner closest to the restrooms, someone is sniffing the public wifi. As you hit “Log In”, they see your authentication credentials fly across the wire.

In its raw form, a login attempt over DDP using the account-password package looks like this:

["{\"msg\":\"method\",\"method\":\"login\",\"params\":[{\"user\":{\"email\":\"joe@schmoe.com\"},\"password\":{\"digest\":\"5e884898da28047151d0e56f8dc6292773603d0d6aabbdd62a11ef721d1542d8\",\"algorithm\":\"sha-256\"}}],\"id\":\"9\"}"]

The attacker sees the website you’re connected to, your email address and a hash of the password you provided. Now that he has all of this information, hijacking your account is as simple as navigating to the Meteor application and running the following in his browser console:

Accounts.callLoginMethod({
    methodArguments: [{
      user: {email: "joe@schmoe.com"},
      password: {digest: "5e884898da28047151d0e56f8dc6292773603d0d6aabbdd62a11ef721d1542d8", algorithm: "sha-256"}
    }]
});

And just like that, someone was able to catch your login credentials as they flew unencrypted across the network, and use them to take control of your account.

Smells Like Hash

But how can this be? You know that when you call Meteor.loginWithPassword on the client, the password you provide is hashed before it’s sent to the server. How can an attacker log in without access to the actual password string?

The accounts-password package hashes the provided password before sending it across the network in an attempt to prevent attackers from being able to see the raw password. The server then compares the user-provided hash with the hash it keeps the users collection. If the two hashes match, the server assumes that the user provided the correct password, and logs them into the application.

This means that the hash is effectively being treated as a password. If you send the right hash, you will be logged into the associated account, regardless of whether you know what the actual password is. Because this hash, or pseudo-password, is being sent in plain text over the network, anyone who intercepts it can easily replay the message and send their own login request.

SSL To The Rescue

People often ask me if they should be using SSL/TLS (https) with their Meteor applications. My answer is always a resounding, “Yes!” At its core, DDP is a plain text protocol that offers no protection against inspection, tampering or replay over the network. This means that all of your users’ potentially private data is being broadcast to the slice of the world between the client and the server.

So how does SSL save the day? By adding and correctly configuring SSL and navigating to your Meteor application over https, you’re creating a secure connection between the client and the server. All network communications are completely encrypted and protected from replay attacks.

Using SSL is an easy way to ensure that private data stays private, even when it’s being shipped back and forth between the client and the server.

The Ecstasy of Testing

Written by Pete Corey on Aug 18, 2015.

describe("The ecstasy of testing", function() {

You dive in, equipped with nothing more than a creeping dissatisfaction and a passing test suite.

As the darkness of uncertainty begins to envelop you, the shining green light of your passing suite fills you with unbounded confidence. Fearlessly, you make your first refactor. Vast swaths of code fall away before you, subjected to the annals of git history. Intricate towers of loosely coupled components assemble themselves upon the foundations of your assumptions and beliefs.

In your gut, you feel that it’s ready. You hit save. Your waiting tests eagerly devour your creation, but something is wrong. The suite is failing. Exceptions smoulder red with roars of unmet assertions. You realize that you’ve missed an entire set of edge cases. Your assumptions were unfounded, and were it not for the meticulous diligence of your test suite, devastating. Cries of what could have been are drowned out by a torrent of mechanical clicks and clacks as your fingers deftly recover from your oversight.

Once again you set the suite into motion. Between the hushed sounds of your bated breath you can nearly hear the whirring cogs of your test harness as it churns through vast permutations of possibilities and potentialities. Finally, the gears come to rest.

The screen glows green in front of you.

Your tests are passing.

It’s finished.

});