Timing attack vulnerability in basic authentication in Action Controller.

There is a timing attack vulnerability in the basic authentication support
in Action Controller. This vulnerability has been assigned the CVE
identifier CVE-2015-7576.

Versions Affected: All.
Not affected: None.
Fixed Versions: 5.0.0.beta1.1, 4.2.5.1, 4.1.14.1, 3.2.22.1

Impact

Due to the way that Action Controller compares user names and passwords in
basic authentication authorization code, it is possible for an attacker to
analyze the time taken by a response and intuit the password.

For example, this string comparison:

“foo” == “bar”

is possibly faster than this comparison:

“foo” == “fo1”

Attackers can use this information to attempt to guess the username and
password used in the basic authentication system.

You can tell you application is vulnerable to this attack by looking for
http_basic_authenticate_with method calls in your application.

All users running an affected release should either upgrade or use one of
the workarounds immediately.

Releases

The FIXED releases are available at the normal locations.

Workarounds

If you can’t upgrade, please use the following monkey patch in an initializer
that is loaded before your application:

$ cat config/initializers/basic_auth_fix.rb
module ActiveSupport
  module SecurityUtils
    def secure_compare(a, b)
      return false unless a.bytesize == b.bytesize

      l = a.unpack "C#{a.bytesize}"

      res = 0
      b.each_byte { |byte| res |= byte ^ l.shift }
      res == 0
    end
    module_function :secure_compare

    def variable_size_secure_compare(a, b)
      secure_compare(::Digest::SHA256.hexdigest(a), ::Digest::SHA256.hexdigest(b))
    end
    module_function :variable_size_secure_compare
  end
end

module ActionController
  class Base
    def self.http_basic_authenticate_with(options = {})
      before_action(options.except(:name, :password, :realm)) do
        authenticate_or_request_with_http_basic(options[:realm] || "Application") do |name, password|
          # This comparison uses & so that it doesn't short circuit and
          # uses `variable_size_secure_compare` so that length information
          # isn't leaked.
          ActiveSupport::SecurityUtils.variable_size_secure_compare(name, options[:name]) &
            ActiveSupport::SecurityUtils.variable_size_secure_compare(password, options[:password])
        end
      end
    end
  end
end

Patches

To aid users who aren’t able to upgrade immediately we have provided patches for
the two supported release series. They are in git-am format and consist of a
single changeset.

• 4-1-basic_auth.patch - Patch for 4.1 series
• 4-2-basic_auth.patch - Patch for 4.2 series
• 5-0-basic_auth.patch - Patch for 5.0 series

Please note that only the 4.1.x and 4.2.x series are supported at present. Users
of earlier unsupported releases are advised to upgrade as soon as possible as we
cannot guarantee the continued availability of security fixes for unsupported
releases.

Credits

Thank you to Daniel Waterworth for reporting the problem and working with us to
fix it.