CVE-2015-7576.yml

---
gem: actionpack
framework: rails
cve: 2015-7576
date: 2016-01-25
url: "https://groups.google.com/forum/#!topic/rubyonrails-security/ANv0HDHEC3k"

title: Timing attack vulnerability in basic authentication in Action Controller.

description: |
    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.

patched_versions:
  - "~> 5.0.0.beta1.1"
  - ">= 4.2.5.1"
  - "~> 4.1.14.1"
  - "~> 3.2.22.1"