[DRE-maint] Bug#700436: Denial of Service and Unsafe Object Creation Vulnerability in JSON [CVE-2013-0269]

[Ondřej Surý]

Package: ruby-json
Version: 1.7.3-2,1.6.1-1~bpo60+1
Severity: critical
Tags: security patch

https://groups.google.com/forum/?fromgroups=#!topic/rubyonrails-security/4_YvCpLzL58

Denial of Service and Unsafe Object Creation Vulnerability in JSON 

There is a denial of service and unsafe object creation vulnerability in the json gem. This vulnerability has been assigned the CVE identifier CVE-2013-0269. 

Versions Affected:  All. This includes JSON that ships with Ruby 1.9.X-pXXX. 
Not affected:       NONE 
Fixed Versions:     1.7.7, 1.6.8, 1.5.5 

Impact 
------ 
When parsing certain JSON documents, the JSON gem can be coerced in to creating Ruby symbols in a target system.  Since Ruby symbols are not garbage collected, this can result in a denial of service attack. 

The same technique can be used to create objects in a target system that act like internal objects.  These "act alike" objects can be used to bypass certain security mechanisms and can be used as a spring board for SQL injection attacks in Ruby on Rails. 

Impacted code looks like this: 

    JSON.parse(user_input) 

Where the `user_input` variable will have a JSON document like this: 

    {"json_class":"foo"} 

The JSON gem will attempt to look up the constant "foo".  Looking up this constant will create a symbol. 

In JSON version 1.7.x, objects with arbitrary attributes can be created using JSON documents like this: 

    {"json_class":"JSON::GenericObject","foo":"bar"} 

This document will result in an instance of JSON::GenericObject, with the attribute "foo" that has the value "bar".  Instantiating these objects will result in arbitrary symbol creation and in some cases can be used to bypass security measures. 

PLEASE NOTE: this behavior *does not change* when using `JSON.load`.  `JSON.load` should *never* be given input from unknown sources.  If you are processing JSON from an unknown source, *always* use `JSON.parse`. 

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

Releases 
-------- 
The FIXED releases are available at the normal locations. 

Workarounds 
----------- 
For users that cannot upgrade, please use the attached patches.  If you cannot use the attached patches, change your code from this: 

    JSON.parse(json) 

To this: 

    JSON.parse(json, :create_additions => false) 

If you cannot change the usage of `JSON.parse` (for example you're using a gem which depends on `JSON.parse` like multi_json), then apply this monkey patch: 

    module JSON 
      class << self 
        alias :old_parse :parse 
        def parse(json, args = {}) 
          args[:create_additions] = false 
          old_parse(json, args) 
        end 
      end 
    end 

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

* 1-7-VULN.patch - Patch for the 1.7 series 
* 1-6-VULN.patch - Patch for the 1.6 series 
* 1-5-VULN.patch - Patch for the 1.5 series 

Credits 
------- 
A huge thanks goes to the following people for responsibly disclosing this issue and working with the Rails team to get it fixed: 

* Thomas Hollstegge of Zweitag (www.zweitag.de) 
* Ben Murphy 


-- System Information:
Debian Release: 7.0
  APT prefers testing
  APT policy: (500, 'testing')
Architecture: amd64 (x86_64)

Kernel: Linux 3.2.0-3-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash