A unknown flaw in the code for processing byte range headers allows versions 2.2.x of the Apache Web Server to be crippled from a single PC. A suitable “Apache Killer” Perl script that impressively demonstrates the problem. This has been assigned CVE-2011-3192 as its CVE identifier.
How does killapache DDOS tool work?
killapache sends GET requests with multiple “byte ranges” that will claim large portions of the system’s memory space. A “byte range” statement allows a browser to only load certain parts of a document, for example bytes 500 to 1000. It is normally used while downloading large files. This method is used by programs such as download clients to resume downloads that have been interrupted; it is designed to reduce bandwidth requirements. However, it appears that stating multiple unsorted components in the header can cause an Apache server to malfunction.
There is no patch yet released for this vulnerability on apache, but a few work arounds have been found. These have been posted by The Apache Software Foundation and can be used until a stable fix is released.
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/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.
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
As it is a plain perl script it can be modified and run on any operating system just right modules need to be installed.
Download killapache:
Killapache (killapache.pl) -
http://lists.grok.org.uk/pipermail/full-disclosure/attachments/20110820/848b4dca/attachment.obj
UPDATE: A NMAP script will check for a denial of service vulnerability that has been found in the way the multiple overlapping/simple ranges are handled.
http-vuln-cve2011-3192.nse – http://nmap.org/nsedoc/scripts/http-vuln-cve2011-3192.html
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