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Thread: Apache warns Web server admins of DoS attack tool

  1. #1
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    Default Apache warns Web server admins of DoS attack tool

    Anyone who runs an Apache webserver should urgently look into securing their server(s) ASAP to avoid any DDOS attacks, as per this email from Apache:

    From: dirkx @ apache.org (Dirk-Willem van Gulik)
    > Subject: Advisory: Range header DoS vulnerability Apache HTTPD 1.3/2.x \(CVE-2011-3192\)
    > Date: August 24, 2011 9:16:39 AM PDT
    > To: announce@httpd.apache.org
    >
    > -----BEGIN PGP SIGNED MESSAGE-----
    > Hash: SHA1
    >
    > Apache HTTPD Security ADVISORY
    > ==============================
    >
    > Title: Range header DoS vulnerability Apache HTTPD 1.3/2.x
    >
    > CVE: CVE-2011-3192:
    > Date: 20110824 1600Z
    > Product: Apache HTTPD Web Server
    > Versions: Apache 1.3 all versions, Apache 2 all versions
    >
    > Description:
    > ============
    >
    > A denial of service vulnerability has been found in the way the multiple
    > overlapping ranges are handled by the Apache HTTPD server:
    >
    > Full Disclosure: Apache Killer
    >
    > 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:
    > ============
    >
    > However there are several immediate options to mitigate this issue until
    > a full fix is available:
    >
    > 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 the 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...questfieldsize
    >
    > 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_...l.gmail.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.
    SoftDux - Windows & Linux Website Hosting, Virtual Private Servers, Domain Registrations.

  2. #2
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    Default

    If you're using CentOS / Red Hat and probably any other Red Hat derivative like Fedora Core or Scientific Linux then you can apply the following fix:

    create a file in /etc/httpd/conf.d/ like for example "/etc/httpd/conf.d/CVE-2011-3192.conf" and add the following to the file:
    Code:
      # Drop the 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

    I'm not sure if this structure will work on Debian / Ubuntu / OpenSuse / etc systems, but on Red Hat type Linux systems Apache will read any file in the /etc/httpd/conf.d/ folder with a .conf extension.
    SoftDux - Windows & Linux Website Hosting, Virtual Private Servers, Domain Registrations.

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