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From Dirk-Willem van Gulik <di...@webweaving.org>
Subject Re: VOTES please -- CVE-2011-3192: Range header DoS vulnerability in Apache 1.3 and Apache 2 (Final-6)
Date Wed, 24 Aug 2011 14:56:17 GMT
Various suggest on-list and off-list fixes applied. Thanks all.

A few more +1's would be nice :)

Dw.





Title:    CVE-2011-3192: Range header DoS vulnerability Apache HTTPD 1.3/2.x
          Apache HTTPD Security ADVISORY

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 that time. 

1) Use mod_rewrite to limit the number of ranges:

   Option 1:
          # drop 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:
          # Reject request when more than 5 ranges in the Range: header.
          # CVE-2011-3192
          #
          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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