Remote DoS Vulnerabilty in Apache

Apache Killer a new exploit that uses a serious Apache vulnerability discovered over 54 months ago, the bug allows hacker to conduct a denial of service attack and turn any web server down.

Under certain conditions Apache internally is inefficient at handling such request which ‘explode’ into many 100′s of internal requests for large byte ranges. By sending to the servers with Apache 1.3 and 2 GET-complex queries that contain overlapping ranges of data, an attacker can consume all system memory and make server out of service. On Wednesday morning, Apache developers said they hope to release a patch within the next 96 hours.

Currently there are several immediate options to mitigate this issue until a full fix:
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)

[php]# 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 [/php]

Option 2: (Also for Apache 1.3)

[php]# Reject request when more than 5 ranges in the Range: header.
# CVE-2011-3192
#
RewriteEngine on
RewriteCond %{HTTP:range} !(^bytes=[^,]+(,[^,]+){0,4}$|^$)
RewriteRule .* – [F][/php]

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

About 235 million websites use Apache, making it the most widely used webserver with about 66 percent of the entire internet, according to figures released last month by Netcraft. IIS ranked second with more than 60 million sites, or about 17 percent.

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