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Author Topic: Security issue: DoS attack with range requests  (Read 2649 times)

Gregg

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Security issue: DoS attack with range requests
« on: August 25, 2011, 07:19:12 AM »

Quote
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:
============


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 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/%[email protected].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.

I do not expect to see this new release in 48 hours, a patch maybe.

mod_rangecnt for Apache 2.2.x for anyone that might want it:

apache_2.2.x-VC9-x86-mod_rangecnt.zip ApacheHaus/ApacheLounge x86 binaries (or any x86 VC9 build)
apache_2.2.x-VC9-x64-mod_rangecnt.zip ApacheHaus x64 binaries (or any x64 VC9 build)

apache_2.2.x-mod_rangecnt.zip Apache.org x86 (or any x86 VC6 build)

For our 2.3.14 builds:

apache_2.3.14-VC9-x86-mod_rangecnt.zip
apache_2.3.14-VC9-x64-mod_rangecnt.zip
« Last Edit: August 25, 2011, 08:35:08 AM by Gregg »
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