Friday, August 26, 2011

Apache Web Server Vulnerable CVE-2011-3192

Apache Software Foundation announced their Apache Web Server (Httpd) vulnerable (CVE-2011-3192) to attack. According to source, the attack can be done remotely and with a modest number of requests can cause very significant memory and CPU usage on the server. Patch to fix vulnerability is expected to release by another 46 Hours.

Luckily, there have mitigation steps are in place to counter the attack while waiting for patch to fix the vulnerable Httpd.

Mitigation Steps:

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.


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:

   Precompiled binaries for some platforms are available at:

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

Another latest research from Netcraft about Web Server Survey, it indicates Apache Web Server still dominate compare to other Web Server (like ngix, Microsoft, Google). It also potentially leaving up 65.86% Apache Web Server vulnerable (CVE-2011-3192) to DoS attack.

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