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Apache HTTPd Range Header Denial Of Service Update 2

Apache HTTPd Range Header Denial Of Service Update 2
Posted Aug 26, 2011
Site apache.org

A denial of service vulnerability has been found in the way the multiple overlapping ranges are handled by the Apache HTTPd server. Both the 1.3 and 2.x releases are affected. An attack tool is circulating in the wild. Active use of this tool 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.

tags | advisory, denial of service
advisories | CVE-2011-3192
MD5 | b5c007318a9b3360fafa0d7847ad85e3

Apache HTTPd Range Header Denial Of Service Update 2

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Apache HTTPD Security ADVISORY
==============================
UPDATE 2

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

CVE: CVE-2011-3192
Last Change: 20110826 1030Z
Date: 20110824 1600Z
Product: Apache HTTPD Web Server
Versions: Apache 1.3 all versions, Apache 2 all versions

Changes since last update
=========================
In addition to the 'Range' header - the 'Range-Request' header is equally
affected. Furthermore various vendor updates, improved regexes (speed and
accommodating a different and new attack pattern).

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 tool 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 24 hours.

Background and the 2007 report
==============================

There are two aspects to this vulnerability. One is new, is Apache specific;
and resolved with this server side fix. The other issue is fundamentally a
protocol design issue dating back to 2007:

http://seclists.org/bugtraq/2007/Jan/83

The contemporary interpretation of the HTTP protocol (currently) requires a
server to return multiple (overlapping) ranges; in the order requested. This
means that one can request a very large range (e.g. from byte 0- to the end)
100's of times in a single request.

Being able to do so is an issue for (probably all) webservers and currently
subject of an IETF discussion to change the protocol:

http://trac.tools.ietf.org/wg/httpbis/trac/ticket/311

This advisory details a problem with how Apache httpd and its so called
internal 'bucket brigades' deal with serving such "valid" request. The
problem is that currently such requests internally explode into 100's of
large fetches, all of which are kept in memory in an inefficient way. This
is being addressed in two ways. By making things more efficient. And by
weeding out or simplifying requests deemed too unwieldy.

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

There are several immediate options to mitigate this issue until a full fix
is available. Below examples handle both the 'Range' and the legacy
'Request-Range' with various levels of care.

Note that 'Request-Range' is a legacy name dating back to Netscape Navigator
2-3 and MSIE 3. Depending on your user community - it is likely that you
can use option '3' safely for this older 'Request-Range'.

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 2.2)

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

# We always drop Request-Range; as this is a legacy
# dating back to MSIE3 and Netscape 2 and 3.
RequestHeader unset Request-Range

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

Above may not work for all configurations. In particular situations
mod_cache and (language) modules may act before the 'unset'
is executed upon during the 'fixup' phase.

Option 2: (Pre 2.2 and 1.3)

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

# We always drop Request-Range; as this is a legacy
# dating back to MSIE3 and Netscape 2 and 3.
RequestHeader unset Request-Range

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.

Furthermore to ignore the Netscape Navigator 2-3 and MSIE 3 specific
legacy header - add:

RequestHeader unset Request-Range

Unlike the commonly used 'Range' header - dropping the 'Request-Range'
is not likely to affect many clients.

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
http://svn.apache.org/viewvc?view=revision&sortby=date&revision=1161534

OS and Vendor specific information
==================================

Red Hat: Option 1 cannot be used on Red Hat Enterprise Linux 4.
https://bugzilla.redhat.com/show_bug.cgi?id=732928

NetWare: Pre compiled binaries available.

mod_security: Has updated their rule set; see
http://blog.spiderlabs.com/2011/08/mitigation-of-apache-range-header-dos-attack.html


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 - note 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 24 hours. Note that, while popular, Apache 1.3 is deprecated.

- -- end of advisory - update 2
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