On 26/08/11 12:35, Dirk-Willem van Gulik wrote:
> 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]
^^
Better use this:
RewriteEngine on
RewriteCond %{HTTP:range} !(^bytes=[^,]+(,[^,]+){0,4}$|^$) [NC,OR]
RewriteCond %{HTTP:request-range} !(^bytes=[^,]+(,[^,]+){0,4}$|^$) [NC]
RewriteRule .* - [F]
Because if you don't specify the [OR] apache will combine the rules
making an AND (and you don't want this!).
Also use NC=(nocase) to prevent the attacker upper casing "bytes="
(don't know if it will work.. but just to prevent)
>
> # 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@xxxxxxxxxxxxxx%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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