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From Dirk-WIllem van Gulik <>
Subject Re: CVE-2011-3192: Range header DoS vulnerability in Apache 1.3 and Apache 2 (DRAFT-3)
Date Wed, 24 Aug 2011 13:07:52 GMT
*	Folks - do we also need to add Request-Range ?

*	Updated with Rudigers comments., Eric, Florians

*	Consensus that the deflate stuff needs to go out reflected.

*	More Comments please. Esp. on the quality and realisticness of the mitigtions.

*	Is this the right list (and order) of the mitigations - or should ReWrite be first ?

*	Timeline mentioning fine (we've never done that before) -- or best avoided ?

My plan is to wait for the US to fully wake up - and then call for a few quick +1's to get
this out - ideally before 1600 zulu.



Title:	    CVE-2011-3192: Range header DoS vulnerability in Apache 1.3 and Apache 2
Date:       20110824 1600Z
# Last Updated:  20110824 1600Z
Product:   Apache Web Server
Versions:  Apache 1.3 all versions, Apache 2 all versions


A denial of service vulnerability has been found in the way the multiple overlapping ranges
are handled by apache (  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 leads to very significant
memory and CPU usage. 

The default apache installation is vulnerable.

There is currently no patch/new version of apache which fixes this vulnerability. This advisory
will be updated when a long term fix is available. A fix is expected in the next 96 hours.


However are several immediate options to mitigate this issue until that time:

1)	Use mod_headers to 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.

2)	Use mod_rewrite to limit the number of ranges:

	RewriteCond %{HTTP:range} !^bytes=[^,]+(,[^,]+){0,4}$
	RewriteRule .* - [F]

3)	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 sizable
	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.


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

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

Apache HTTPD users are advised to investigate wether they are vulnerable (e.g. allow use of
the Range header )and 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.


This advisory will be updated when a fix/patch or new release is available. A patch or new
apache release for Apache 2.0 and 2.2 is expected in the next 96 hours. Note that, while popular,
Apache 1.3 is deprecated. 

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