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Apache HTTP Server Security Advisory
====================================

Title:       mod_proxy reverse proxy exposure

CVE:         CVE-2011-3368
Date:        20111005
Product:     Apache HTTP Server
Versions:    httpd 1.3 all versions, httpd 2.x all versions

Description:
============

An exposure was reported affecting the use of Apache HTTP Server in
reverse proxy mode.  We would like to thank Context Information
Security Ltd for reporting this issue to us.

When using the RewriteRule or ProxyPassMatch directives to configure a
reverse proxy using a pattern match, it is possible to inadvertently
expose internal servers to remote users who send carefully crafted
requests.  The server did not validate that the input to the pattern
match was a valid path string, so a pattern could expand to an
unintended target URL.

For future releases of the Apache HTTP Server, the software will
validate the request URI, correcting this specific vulnerability.  The
documentation has been updated to reflect the more general risks with
pattern matching in a reverse proxy configuration.

Details:
========

A configuration like one of the following examples:

  RewriteRule (.*).(jpg|gif|png)    http://images.example.com$1.$2 [P]
  ProxyPassMatch (.*).(jpg|gif|png) http://images.example.com$1.$2

could result in an exposure of internal servers.  A request of the form:

  GET @other.example.com/something.png HTTP/1.1

would get translated to a target of:

  http://[email protected]/something.png

This will cause the proxy to connect to the hostname
"other.example.com", as the "images.example.com@" segment would be
treated as user credentials when parsing the URL.  This would allow a
remote attacker the ability to proxy to hosts other than those
expected, which could be a security exposure in some circumstances.

The request-URI string in this example,
"@other.example.com/something.png", is not valid according to the HTTP
specification, since it neither an absolute URI
("http://example.com/path") nor an absolute path ("/path").  For
future releases, the server has been patched to reject such requests,
instead returning a "400 Bad Request" error.

Actions:
========

Apache HTTPD users should examine their configuration files to determine 
if they have used an insecure configuration for reverse proxying.  
Affected users can update their configuration, or apply the patch from:

   http://www.apache.org/dist/httpd/patches/apply_to_2.2.21/

For example, the above RewriteRule could be changed to:

  RewriteRule /(.*).(jpg|gif|png)    http://images.example.com/$1.$2 [P]

to ensure the pattern only matches against paths with a leading "/".

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   The Apache Software Foundation and the Apache HTTP Server Project are
   pleased to announce the release of version 2.2.21 of the Apache HTTP
   Server ("Apache").  This version of Apache is principally a security
   and bug fix release:

     * SECURITY: CVE-2011-3348 (cve.mitre.org)
       mod_proxy_ajp when combined with mod_proxy_balancer: Prevents
       unrecognized HTTP methods from marking ajp: balancer members
       in an error state, avoiding denial of service.

     * SECURITY: CVE-2011-3192 (cve.mitre.org)
       core: Further fixes to the handling of byte-range requests to use
       less memory, to avoid denial of service. This patch includes fixes
       to the patch introduced in release 2.2.20 for protocol compliance,
       as well as the MaxRanges directive.

   Note the further advisories on the state of CVE-2011-3192 will no longer
   be broadcast, but will be kept up to date at;

     http://httpd.apache.org/security/CVE-2011-3192.txt

   We consider this release to be the best version of Apache available, and
   encourage users of all prior versions to upgrade.

   Apache HTTP Server 2.2.21 is available for download from:

     http://httpd.apache.org/download.cgi

   Please see the CHANGES_2.2 file, linked from the download page, for a
   full list of changes.  A condensed list, CHANGES_2.2.21 provides the
   complete list of changes since 2.2.19.  A summary of all of the security
   vulnerabilities addressed in this and earlier releases is available:

     http://httpd.apache.org/security/vulnerabilities_22.html

   This release includes the Apache Portable Runtime (APR) version 1.4.5
   and APR Utility Library (APR-util) version 1.3.12, bundled with the tar
   and zip distributions.  The APR libraries libapr and libaprutil (and
   on Win32, libapriconv version 1.2.1) must all be updated to ensure
   binary compatibility and address many known security and platform bugs.

   Apache 2.2 offers numerous enhancements, improvements, and performance
   boosts over the 2.0 codebase.  For an overview of new features
   introduced since 2.0 please see:

     http://httpd.apache.org/docs/2.2/new_features_2_2.html

   This release builds on and extends the Apache 2.0 API.  Modules written
   for Apache 2.0 will need to be recompiled in order to run with Apache
   2.2, and require minimal or no source code changes.

     http://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x/VERSIONING

   When upgrading or installing this version of Apache, please bear in mind
   that if you intend to use Apache with one of the threaded MPMs (other
   than the Prefork MPM), you must ensure that any modules you will be
   using (and the libraries they depend on) are thread-safe.

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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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