Deep Copy Params Hash When working with Rack

August 7, 2010

I’m not sure why this happened. I think I remember something about Rack::Request being a singleton or something. You should deep copy anything you’re going to modify from your rack env in middleware.

One of my projects for my internship is to create a data mining tool that logs requests to a Riak datastore. For me this involves a pretty simple Rack middleware pipeline. The other day I got it in my head that I should probably not be logging passwords in the raw. I threw together a really simple, configurable password filter for my middleware stack that changes parameters to [FILTERED] just like rails does. I spent the better part of the morning trying to figure out why my selenium-driven tests kept losing my session.

It turns out that parsing the request with as the culpret. I don’t remember exactly how that is implemented but it must use a singleton or something. It turns out that if you assign your request’s params to something else and then start modifying that value it will modify the request params before they get to your app. Yeah, that’s bad. My middleware looks something like

class PasswordFilter < Middleware::Generic
  def call(env)
    req =
    env['request_logger.log_entry'][:params] = req.params # set up the env key for the rest of the pipeline
    @hide_params.each {|param| set_hash_path(env['request_logger.log_entry'][:params], param)}

Evidently my modification of my copy of params affected the request so poor selenium was doing its job and Rack was submitting every login with the password [FILTERED]. Much to my surprise, #dup and #clone did nothing to solve the problem. I ultimately had to use Marshal.load(Marshal.dump(the_hash)) to get this working. Hopefully I can prevent someone from losing an hour or two to such a stupid issue.