Rack/Ruby on Rails: ArgumentError: invalid byte sequence in UTF-8

# Quick test - just copy-paste this: ?%28t%B3odei%29 into your app url - if app crashes, you should read stuff below ;)

If you're here, than probably you've encountered this weird issue:

ArgumentError: invalid byte sequence in UTF-8

you might even have a backtrace like this:

rack-1.5.2/lib/rack/utils.rb:104→ normalize_params
rack-1.5.2/lib/rack/utils.rb:96→ block in parse_nested_query
rack-1.5.2/lib/rack/utils.rb:93→ each
rack-1.5.2/lib/rack/utils.rb:93→ parse_nested_query
rack-1.5.2/lib/rack/request.rb:373→ parse_query
actionpack-4.0.4/lib/action_dispatch/http/request.rb:321→ parse_query
rack-1.5.2/lib/rack/request.rb:188→ GET
actionpack-4.0.4/lib/action_dispatch/http/request.rb:274→ GET
actionpack-4.0.4/lib/action_dispatch/http/parameters.rb:16→ parameters
actionpack-4.0.4/lib/action_dispatch/http/filter_parameters.rb:37→ filtered_parameters
...
activesupport-4.0.4/lib/active_support/cache/strategy/local_cache.rb:83→ call
rack-1.5.2/lib/rack/sendfile.rb:112→ call
railties-4.0.4/lib/rails/engine.rb:511→ call
railties-4.0.4/lib/rails/application.rb:97→ call
railties-4.0.4/lib/rails/railtie/configurable.rb:30→ method_missing
puma-2.7.1/lib/puma/configuration.rb:68→ call
puma-2.7.1/lib/puma/server.rb:486→ handle_request
puma-2.7.1/lib/puma/server.rb:357→ process_client
puma-2.7.1/lib/puma/server.rb:250→ block in run
puma-2.7.1/lib/puma/thread_pool.rb:92→ call
puma-2.7.1/lib/puma/thread_pool.rb:92→ block in spawn_thread

First of all, this issue is not super-important. It's not a security issue as well. It's just an invalid byte sequence in your request url. Either way it would be good to fix it, even for a sole purpose of getting rid of this from our bug tracker.

But before we do anything with this, how can we determine, that our URL is an invalid UTF-8? We can use URI decode method for that:

# With an invalid byte sequence
url = 'http://senpuu.net/?techniki,Sawarabi_no_Mai_%28taniec_m%B3odej_paproci%29'
URI.decode(url).force_encoding('UTF-8').valid_encoding? #=> false

# and with a valid one
url = 'http://www.senpuu.net/aktualnosci'
URI.decode(url).force_encoding('UTF-8').valid_encoding? #=> true

So, how can we handle this? Well we need to catch it in middleware before anything else wants to process it. I think, that in such cases we should just raise 400 error - bad request, since this is not something that we expect. Middleware like this can be really simple:

class Utf8Sanitizer
  SANITIZE_ENV_KEYS = %w(
    HTTP_REFERER
    PATH_INFO
    REQUEST_URI
    REQUEST_PATH
    QUERY_STRING
  )

  def initialize(app)
    @app = app
  end

  def call(env)
    SANITIZE_ENV_KEYS.each do |key|
      string = env[key].to_s
      valid = URI.decode(string).force_encoding('UTF-8').valid_encoding?
      # Don't accept requests with invalid byte sequence
      return [ 400, { }, [ 'Bad request' ] ] unless valid
    end

    @app.call(env)
  end
end

and after that you just put into your config/application.rb this:

  config.middleware.use Utf8Sanitizer

and you're resistant to this issue.

## Update

It seems that there's a gem called utf8-cleaner that sanitizes non-utf8 strings. It has one issue - instead of rising 400 error it just removes invalid bytes but still - it's way better that nothing. If you just want to get rid of this problem, put this into your gemfile:

gem 'utf8-cleaner'

## Update 2
I've got a response from Rack guys and it seems that it's more like a Rails issue than a Rack one:
Raggi stated here that:

It is a web servers responsibility to translate IO to valid binary representations for the application layer. This isn't the whole picture though, in this case, the webserver has done that - the webserver does not know the encoding of the URI...

It is the responsibility of the IETF to define the validity of URI data in various encodings (not done), and so it is not entirely valid for web servers to make no assumptions for this field for the above...

Rack itself uses a binary regular expression here, which expects binary input strings. This is our response to the above subtleties. In normal operation (say, Webrick + Rack), this error is not raised...

The reason that this error is raised in your application is:

You have middleware in your stack that is forcing this string to UTF-8, even when it is not valid UTF-8. The code that is doing this is bugged.

Observe:

s = "a=\xff"
# => "a=\xFF"
s.force_encoding("binary")
# => "a=\xFF"
s.valid_encoding?
# => true
Rack::Utils.parse_nested_query(s)
# => {"a"=>"\xFF"}
s.force_encoding("utf-8")
# => "a=\xFF"
s.valid_encoding?
# => false
Rack::Utils.parse_nested_query(s)
ArgumentError: invalid byte sequence in UTF-8
        from /usr/local/google/home/raggi/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/rack-1.5.2/lib/rack/utils.rb:93:in `split'
        from /usr/local/google/home/raggi/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/rack-1.5.2/lib/rack/utils.rb:93:in `parse_nested_query'
        from (irb):21
        from /usr/local/google/home/raggi/.rbenv/versions/2.0.0-p247/bin/irb:12:in `<main>'

This is a rails bug. Calls to force_encoding should always assert that their output is valid.