En nuestra condición de partners de Parallels, hemos recibido un aviso de vulnerabilidad en Apache a través de la cual se pueden llevar a cabo ataques de denegación de servicio DDOS y afecta a instalaciones de Apache 1.3 a 2.0.

Para saber si su sistema Apache es vulnerable a dicho error lo más sencillo será ejecutar un sencillo comando como será, curl -I -H "Range: bytes=0-1,0-2" -s www.example.com/robots.txt | grep Partial Y si obtenemos un mensaje de "206 Partial Content in output" quiere decir que su sistema Apache es vulnerable. Podéis obtener más info sobre la vulnerabilidad en http://www.securityfocus.com/bid/49303/info . Parallels recomienda comprobar la existencia de últimos parches para Apache, igualmente ellos están desarrollando un micro-patch dedicado a la resolución de esta incidencia a través de su software Plesk.  Por lo que os recordamos la importancia de tener nuestros sistemas lo más actualizados posible para evitar estos agujeros de seguridad que alguien pudiera utilizar con un objetivo malicioso sobre nuestro sistema. Os dejamos copia del mensaje enviado por Parallels y las opciones que se pueden aplicar para mitigar el problema mientras aparece una solución factible por parte de los desarrolladores. "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. Parallels Plesk Panel Team"