Mongrel Temporary Fix For cgi.rb 99% CPU DoS Attack

This is important so please read this message very carefully.

There is a DoS for Ruby’s cgi.rb that is easily exploitable. The attack
involves sending a malformed multipart MIME body in an HTTP request.
The full explanation of the attack as well as how to fix it RIGHT NOW is
given below.

Most of the work was done by Jeremy K. and Jamis B… They did all
the work of building the hot fix gem you’ll install and getting the
right people to finally agree to get this out.

The original report is attached to this message so you can read it in

I’m putting this fix into the Mongrel pre-release process to give Matz
time to get an official release out. If he doesn’t within the next few
days then I’ll turn this into an official Mongrel release.


There has been an exploitable bug in the Ruby CGI library named cgi.rb
which allows:

Anyone on the Internet to…
Send a single HTTP request to…
Any Ruby program (NOT just Mongrel) using…
cgi.rb multipart parsing with…
A malformed MIME body that…
Causes the Ruby process to go into a 99% CPU infinite loop killing it.

I broke this down so that it’s as clear as possible, and so you don’t
miss that it’s for ANY program using cgi.rb mime parsing. Not just
Rails and Mongrel.

What happens is that the final MIME boundary is sometimes given as:


Rather than:


And this causes cgi.rb to go into an infinite loop waiting for more
input that isn’t coming. This is caused by any system that reads
directly from an input stream that returns “” rather than EOF.

The fix described below has a full exploit/tester script demonstrating
the defect. It also doesn’t matter if you have file uploads on your
site or not. I can point this script at your site on any URI and cause
a DoS on your site.


Currently, the following servers are affected:

  • Mongrel – Reads from a socket so gets “” rather than EOF.
  • Litespeed – Affected but has an internal timeout that nails the
  • CGI Standalone – Impacted since reading from a normal input stream.
  • Any other custom server using the above similar operations.

Looks like FastCGI’s FCGIInputStream, WEBrick and mod_ruby are not
vulnerable since they either read from a domain socket or don’t use
normal cgi.rb.


Everyone using Mongrel can get the fix immediately by installing the
latest pre-release version 0.3.14:

sudo gem install mongrel --source=

Win32 people and anyone who can’t upgrade that way can get the fix by
doing this:

  1. gem install cgi_multipart_eof_fix
  2. Edit your environment.rb to have: require ‘cgi_multipart_eof_fix’
  3. Restart your services.

People using other frameworks can get the fix by simply requiring
rubygems and this fix in some start-up location for your framework.


If you can’t do the hot fix gem install, then there is also a patch for
cgi.rb attached to this e-mail. You can apply the patch with the
following process:

  1. Find the original cgi.rb file in your install. Mine’s in
  2. cd /usr/lib/ruby/1.8
  3. sudo patch < ~/cgi_multipart_eof_fix.patch

You can look at the patch. It’s literally changing one line, so you can
edit by hand if you get really desperate.


Based on how the cgi.rb file is coded it’s most likely that there will
be more of these kinds of defects in the future. If you find a defect
like this, then please don’t flip out. Just report it to me or anyone
else, and I’ll cook up another one of these hot fix releases rather than
wait for an official fix. I promise immediate turn-around from now on
using a hot-fix gem if I can’t get an official fix within a few days.

Suggestions on how to do a more standardized hot-fix release process are
much appreciated.

Flame wars about screwing goats or the merits of full-disclosure are not


I’ll be in the Mongrel lingr room:

And on in #rubyonrails, #rails-security, and #ruby-lang
fielding questions and helping people. If I don’t answer right away
then wait a bit.

I’ll also answer help e-mails directly if you can’t access any of the

Zed A. Shaw

On 10/25/06, Zed A. Shaw [email protected] wrote:

You can look at the patch. It’s literally changing one line, so you
can edit by hand if you get really desperate.

Looks like either you forgot the attachment, or maybe the mailing list
software (or gateway bridge?) ate it. Can you please include the patch
in the body of a response? Thanks!

Jacob F.

On 10/25/06, Jacob F. [email protected] wrote:

  1. sudo patch < ~/cgi_multipart_eof_fix.patch

Here’s the patch, inline:
— /opt/local/lib/ruby/1.8/cgi.rb 2005-10-06 19:01:22.000000000
+++ cgi.rb 2006-09-22 16:38:08.000000000 -0600
@@ -1017,7 +1017,7 @@

  •      if c.nil?
  •      if c.nil? || c.empty?
           raise EOFError, "bad content body"

Wilson B. wrote:

Jacob F.

Here’s the patch, inline:

Zed also pointed me to Google groups

which has the patch attachment

James B. - Ruby Help & Documentation
Ruby Code & Style - The Journal By & For Rubyists - The Ruby Store for Ruby Stuff - Hacking in the Desert

On Thu, 26 Oct 2006 04:49:33 +0900
James B. [email protected] wrote:

Zed also pointed me to Google groups

Thanks folks. Yep, either of those will work.

Another point to make is this:

If you can’t upgrade Mongrel or your software just install the
cgi_multipart_eof_fix gem.

If you can’t even install the gem then use the patch. The gem includes
a README, and a test script you can run to validate your server.

If you want the latest and greatest Mongrel that will just install the
gem for you and have the least hassle then install pre-release Mongrel
(see OP).

Only warning with pre-release Mongrel is that because of this fix
mongrel_upload_progress users might run into problems. Let me know if
this is the case, but it’s been heavily tested by many people already.