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From di...@apache.org (Dirk-Willem van Gulik)
Subject Advisory: Range header DoS vulnerability Apache HTTPD 1.3/2.x \(CVE-2011-3192\)
Date Wed, 24 Aug 2011 16:16:39 GMT
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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#limitrequestfieldsize

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.mbox/%3cCAAPSnn2PO-d-C4nQt_TES2RRWiZr7urefhTKPWBC1b+K1Dqc7g@mail.gmail.com%3e

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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