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Atenção usuários PLESK


Alexandre Duran

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Nota de segurança para usuários PLESK, acabei de receber:

Security Vulnerability Notice-

Apache HTTP Server CVE-2011-3192 Denial Of Service Vulnerability

Summary:

An exploit exists in the wild that permits possible DDoS attacks. Parallels will release a new patch soon but mitigtion work-arounds are available to help affected customers now. Please distribute to support techs immediately.

NOTE: This vulnerability is not unique to Plesk and affects all Apache 2.0 and 1.3 installations. Please be sure to check other systems running Apache and contact vendors for patches.

Symptoms

Apache HTTP Server CVE-2011-3192 Denial Of Service Vulnerability

CVE-2011-3192 http://www.securityfocus.com/bid/49303/info

How to check that your Apache is affected.

Run following command

curl -I -H "Range: bytes=0-1,0-2" -s www.example.com/robots.txt | grep Partial

If you see message 206 Partial Content in output - your Apache is vulnerable.

Resolution

Parallels has a goal of making patches available by tomorrow as the fix was just released today by Apache. These releases will be available by both micro-update and download methods. For info on enabling micro-updates, see: http://kb.parallels.com/9294.

Here 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/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

Additional information

Also we recommend to check permanently all available Apache updates in OS vendor repository and keep Apache updated with it.

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