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Nasty Apache denial of service vulnerability

From:  dirkx-AT-apache.org (Dirk-Willem van Gulik)
To:  announce-AT-httpd.apache.org
Subject:  Advisory: Range header DoS vulnerability Apache HTTPD 1.3/2.x \(CVE-2011-3192\)
Date:  Wed, 24 Aug 2011 16:16:39 +0000 (UTC)
Message-ID:  <20110824161640.122D387DD@minotaur.apache.org>
Archive-link:  Article

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          Apache HTTPD Security ADVISORY
          ==============================

Title:    Range header DoS vulnerability Apache HTTPD 1.3/2.x

CVE:      CVE-2011-3192: 
Date:     20110824 1600Z
Product:  Apache HTTPD Web Server
Versions: Apache 1.3 all versions, Apache 2 all versions

Description:
============

A denial of service vulnerability has been found in the way the multiple 
overlapping ranges are handled by the Apache HTTPD server:

     http://seclists.org/fulldisclosure/2011/Aug/175 

An attack tool is circulating in the wild. Active use of this tools has 
been observed.

The attack can be done remotely and with a modest number of requests can 
cause very significant memory and CPU usage on the server. 

The default Apache HTTPD installation is vulnerable.

There is currently no patch/new version of Apache HTTPD which fixes this 
vulnerability. This advisory will be updated when a long term fix 
is available. 

A full fix is expected in the next 48 hours. 

Mitigation:
============

However there are several immediate options to mitigate this issue until 
a full fix is available:

1) Use SetEnvIf or mod_rewrite to detect a large number of ranges and then
   either ignore the Range: header or reject the request.

   Option 1: (Apache 2.0 and 2.2)

          # Drop the Range header when more than 5 ranges.
          # CVE-2011-3192
          SetEnvIf Range (,.*?){5,} bad-range=1
          RequestHeader unset Range env=bad-range

          # optional logging.
          CustomLog logs/range-CVE-2011-3192.log common env=bad-range

   Option 2: (Also for Apache 1.3)

          # Reject request when more than 5 ranges in the Range: header.
          # CVE-2011-3192
          #
          RewriteEngine on
          RewriteCond %{HTTP:range} !(^bytes=[^,]+(,[^,]+){0,4}$|^$)
          RewriteRule .* - [F]

   The number 5 is arbitrary. Several 10's should not be an issue and may be
   required for sites which for example serve PDFs to very high end eReaders
   or use things such complex http based video streaming.

2) Limit the size of the request field to a few hundred bytes. Note that while 
   this keeps the offending Range header short - it may break other headers; 
   such as sizeable cookies or security fields. 

          LimitRequestFieldSize 200

   Note that as the attack evolves in the field you are likely to have
   to further limit this and/or impose other LimitRequestFields limits.

   See: http://httpd.apache.org/docs/2.2/mod/core.html#limitreque...

3) Use mod_headers to completely dis-allow the use of Range headers:

          RequestHeader unset Range 

   Note that this may break certain clients - such as those used for
   e-Readers and progressive/http-streaming video.

4) Deploy a Range header count module as a temporary stopgap measure:

     http://people.apache.org/~dirkx/mod_rangecnt.c

   Precompiled binaries for some platforms are available at:

	http://people.apache.org/~dirkx/BINARIES.txt

5) Apply any of the current patches under discussion - such as:


http://mail-archives.apache.org/mod_mbox/httpd-dev/201108...

Actions:
========

Apache HTTPD users who are concerned about a DoS attack against their server 
should consider implementing any of the above mitigations immediately. 

When using a third party attack tool to verify vulnerability - know that most 
of the versions in the wild currently check for the presence of mod_deflate; 
and will (mis)report that your server is not vulnerable if this module is not 
present. This vulnerability is not dependent on presence or absence of 
that module.

Planning:
=========

This advisory will be updated when new information, a patch or a new release 
is available. A patch or new apache release for Apache 2.0 and 2.2 is expected 
in the next 48 hours. Note that, while popular, Apache 1.3 is deprecated.


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(Log in to post comments)

Nasty Apache denial of service vulnerability

Posted Aug 24, 2011 21:38 UTC (Wed) by jonabbey (guest, #2736) [Link]

This seems remarkably bad. I'm surprised I've not heard more buzz about this on the social networks.

Nasty Apache denial of service vulnerability

Posted Aug 24, 2011 23:21 UTC (Wed) by jonabbey (guest, #2736) [Link]

Note that the killapache.pl script linked from the fulldisclosure forum will report that a server is 'Not Vulnerable' if the '/' resource is provided by PHP, as PHP does not support the Range header.

If such a server provides any image files, though, a URL for an image file can be substituted in the killapache script, whereupon the Range DoS attack will function just fine.

Nasty Apache denial of service vulnerability

Posted Aug 25, 2011 1:20 UTC (Thu) by sjlyall (subscriber, #4151) [Link]

Amusingly the ad I'm getting next to this story is for "Neoload Web Stress Tool"

Nasty Apache denial of service vulnerability

Posted Aug 25, 2011 2:02 UTC (Thu) by Hausvib6 (guest, #70606) [Link]

This vulnerability can be a nice addition to LOIC. Fewer drones, more damages, though it's far easier to mitigate...

Nasty Apache denial of service vulnerability

Posted Aug 25, 2011 6:19 UTC (Thu) by imgx64 (guest, #78590) [Link]

>Versions: Apache 1.3 all versions

That's not entirely true. It doesn't affect OpenBSD's fork according to http://marc.info/?l=openbsd-misc&m=131424693000610&...

Nasty Apache denial of service vulnerability

Posted Aug 25, 2011 7:53 UTC (Thu) by etrusco (subscriber, #4227) [Link]

If the fix was intentional and not communicated upstream, I "can't say nothing" for OpenBSD.

Nasty Apache denial of service vulnerability

Posted Aug 25, 2011 11:02 UTC (Thu) by tialaramex (subscriber, #21167) [Link]

Let's not get ahead of ourselves, the Apache advisory itself says that some of the POC / test scripts don't actually work on a typical out-of-box install, not because it isn't vulnerable, but because they're making bad assumptions. Real bad guys could fix this, but obviously the Apache team isn't going to spell out how.

If we look at Red Hat's security numbers we see that a significant number of POCs fail out-of-box against RHEL, but a knowledgeable hacker could fix them because RHEL was actually vulnerable. This means you're safer than you might appear to be against script kiddies (who won't know how) but could get a false sense of security if your adversaries are sophisticated. The same probably applies here to Apache on OpenBSD.

Nasty Apache denial of service vulnerability

Posted Aug 25, 2011 13:00 UTC (Thu) by SEJeff (subscriber, #51588) [Link]

Great fix for gnome.org servers from bkor for your apache config:
# Drop the Range header when more than 5 ranges.
# CVE-2011-3192
SetEnvIf Range (,.*?){5,} bad-range=1
RequestHeader unset Range env=bad-range

Allows legit range requests to work and kills it after > 5.

Nasty Apache denial of service vulnerability

Posted Aug 25, 2011 13:54 UTC (Thu) by ovitters (subscriber, #27950) [Link]

I just copied that from this announcement. Yay for Puppet :P

Nasty Apache denial of service vulnerability

Posted Aug 25, 2011 18:23 UTC (Thu) by SEJeff (subscriber, #51588) [Link]

Yup just trying to point it out for the TL;DNR people who are much like myself :)

Nasty Apache denial of service vulnerability

Posted Aug 25, 2011 18:18 UTC (Thu) by rickmoen (subscriber, #6943) [Link]

Don't forget to do
a2enmod headers
...or Apache httpd may choke on invalid configuration lines and refuse to start (if the 'headers' module isn't enabled). Rick Moen rick@linuxmafia.com

Nasty Apache denial of service vulnerability

Posted Aug 28, 2011 21:59 UTC (Sun) by happynut (subscriber, #4117) [Link]

The vulnerability was updated. Request-range is vulnerable too:


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