DoS vulnerability in REXML

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             AUSCERT External Security Bulletin Redistribution

                    ESB-2008.0827 -- [Win][UNIX/Linux]
                        DoS vulnerability in REXML
                             1 September 2008

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        AusCERT Security Bulletin Summary
        ---------------------------------

Product:              Ruby
Publisher:            Ruby
Operating System:     Windows
                      UNIX variants (UNIX, Linux, OSX)
Impact:               Denial of Service
Access:               Remote/Unauthenticated
CVE Names:            CVE-2008-3790

Original Bulletin:    
  http://www.ruby-lang.org/en/news/2008/08/23/dos-vulnerability-in-rexml/

Revision History:     September 1 2008: Added CVE Reference
                        August 26 2008: Initial Release

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DoS vulnerability in REXML

There is a DoS vulnerability in the REXML library used by Rails to parse
incoming XML requests. A so-called "XML entity explosion" attack technique
can be used for remotely bringing down (disabling) any application which
parses user-provided XML. Most Rails applications will be vulnerable to
this attack.

Impact

An attacker can cause a denial of service by causing REXML to parse a
document containing recursively nested entities such as:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE member [
  <!ENTITY a "&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;">
  <!ENTITY b "&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;">
  <!ENTITY c "&d;&d;&d;&d;&d;&d;&d;&d;&d;&d;">
  <!ENTITY d "&e;&e;&e;&e;&e;&e;&e;&e;&e;&e;">
  <!ENTITY e "&f;&f;&f;&f;&f;&f;&f;&f;&f;&f;">
  <!ENTITY f "&g;&g;&g;&g;&g;&g;&g;&g;&g;&g;">
  <!ENTITY g "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx">
]>
<member>
&a;
</member>

Vulnerable versions

1.8 series

    * 1.8.6-p287 and all prior versions
    * 1.8.7-p72 and all prior versions

1.9 series

    * all versions

Solution

Please download the following monkey patch to fix this problem.

    * http://www.ruby-lang.org/security/20080823rexml/rexml-expansion-fix.rb

Then fix your application to load rexml-expansion-fix.rb before using REXML.

require "rexml-expansion-fix"
...
doc = REXML::Document.new(str)
...

If you have a Rails application, copy rexml-expansion-fix.rb into a
directory on the load path (such as RAILS_ROOT/lib/), and put the
following line into config/environment.rb.

require "rexml-expansion-fix"

If your application is Rails 2.1 or later, you can simply copy
rexml-expansion-fix.rb to RAILS_ROOT/config/initializers and it will be
required automatically.

By default, XML entity expansion limit is 10000. You can change it by
changing REXML::Document.entity_expansion_limit. e.g.

REXML::Document.entity_expansion_limit = 1000

This fix will be made available as a gem and used by future versions of
rails, but users should take corrective action immediately.

Credit

Credit to Luka Treiber and Mitja Kolsek of ACROS Security for disclosing
the problem to Ruby and Rails Security Teams.

Credit to Michael Koziarski of Rails Core Team for creating the monkey
patch to fix the vulnerability.

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