Avoiding MongoDB hash-injection attacks

Conrad Irwin — January 2014

MongoDB is a popular document store. Its query API neatly sidesteps SQL-injection attacks by not being SQL. Unfortunately when using a framework like Rails, it’s easy to build an app that’s vulnerable to a hash-injection attack.

Hash-injection is much less severe than SQL-injection, because an attacker is more limited in the kind of changes they can make, but it can lead to authentication-bypass, denial of service, or timing attacks.

I reported this to MongoDB as SECURITY-90, and they recommend fixing this by patching your application as described below. Alternatively you can install my mongoid-rails gem which automatically fixes this for you.

The Problem with Rails

The underlying problem is that Rails query parameters are not strongly typed. This was first noticed in early versions of Rails when people were writing code that blindly passed parameters to the update function of a model.


A devious attacker could add ?user[is_admin]=1 to the URL or the form, and make themselves an admin. This problem eventually led to the adoption of strong parameters in rails 4 which protects you against this kind of attack.

The Problem with MongoDB

The rails problem is exacerbated by the MongoDB API. MongoDB lets you either query by equality with a String or using a query operator nested in a Hash:

User.where(email: params[:email])

User.where(email: {"$regex" => params[:search]})

Unfortunately, Rails doesn’t guarantee that params[:email] is a String. This means that a devious attacker can again change the structure of the URL to get an unexpected result.

They can turn a look-up by String into a look-up by regex by passing a parameter like ?email[$regex]=.*@google.com.

The consequences

This is much less severe than an SQL injection attack would be, but it still has several dangerous consequences.

  1. Authentication Bypass. If you’re verifying API tokens using a query like User.where(api_token: params[:api_token]), an attacker could use this to access the API without having an API token by passing in a regex.
  2. Denial of Service. If you have a large table with an index on _id and you do a query like BlogPost.find(params[:id]), an attacker can craft a query that forces MongoDB to do a full table scan.
  3. Data leakage. An attacker who can force you to do a full table scan on your users table can use a regex search to find out which domains are signed up to your site by seeing how fast the query returns. This is an example of a timing attack

The solution

If you’re using Rails 3 with strong parameters and Mongoid then the quick fix is to use my mongoid-rails gem. This protects you from hash-injection attacks both when querying and when updating.

For everyone else, I’m afraid the only way to fix this is to manually call .to_s on the parameters. You should find most instances of the problem if you grep for where.*params, find.*params, creates.*params and update.*params.

# These examples are safe, because params[:event_id] and
# params[:auth_token] are coerced to strings.
Account.where(auth_token: params[:auth_token].to_s).first