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@ Copyright CAIS - Brazilian Research Network CSIRT
  Security Incidents Response Center (CAIS/RNP)

Subject         : Vulnerability in the sending requests control of BIND
                  versions 4 and 8 allows DNS spoofing
Date            : November 19th, 2002
Credits         : Vagner Sacramento, DIMAp-UFRN
Systems affected: 4.9.11 and priors (4.9.x); 8.2.7 and priors (8.2.x);
                  8.3.4 and priors (8.3.x);

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1. Abstract

   CAIS/RNP (the Brazilian Research Network CSIRT) and Vagner Sacramento
   from DIMAp/UFRN (Department of Computer Science and Applied
   Mathematics/Federal University of Rio Grande do Norte) made some
   experiments with several versions of the Internet Software Consortium's
   (ISC) Berkeley Internet Name Domain (BIND), demonstrating the
   possibility of successful DNS Spoofing attacks to versions 4 and 8.

   The BIND application is one implementation of the Domain Name System
   (DNS)  protocol, maintained by ISC. This application resolves Internet
   host names into IP addresses and IP addresses back into host names,
   receiving requests from DNS clients at port fifty-three (53).

   The identified vulnerability seriously affects the operation of
   Internet basic services because many of them depend on DNS to perform
   their functionalities.

   Most of name servers in the Internet are running BIND.  Recent
   information obtained from Bill Manning of the USC/ISI indicates that
   more than 60% of the current DNS servers still use vulnerable and old
   versions of BIND.

   The problem described on this advisory certifies BIND versions 4 and 8
   do not prevent sending of two or more resolution requests for the same
   domain name allowing DNS Spoofing attacks with significant probability
   of success.


2. Details

   BIND versions 4 and 8 use procedures that allow a remote DNS Spoofing
   attack against DNS servers.

   The attack goal is to anticipate a reply with false information to the
   target DNS server, making the server to store in its cache a false IP
   address for a certain domain name.

   To better understand the identified vulnerability, consider the
   following scenario. When n different DNS clients send simultaneous
   requests to a target DNS server (running BIND 4 or BIND 8) to resolve
   the same domain name, the target server will forward the requests
   received to others DNS servers, starting from root-servers and trying
   to get replies for each one of the requests.

   In this context, the identified vulnerability can be exploited if an
   attacker sends simultaneously n requests to the target DNS server using
   in each one a different IP source address and the same domain name. The
   target DNS server will send all the received requests to others DNS
   servers in order to resolve them. Since these requests will be
   processed independently, they will be assigned different identifiers
   (ID).  As a result, this server will be waiting for n replies with
   different IDs for the resolution of the same domain name. The attacker
   then sends several replies with different IDs to the target DNS server
   attempting to guess one of the expected replies ID, thus applying a DNS
   Spoofing attack.

   The success probability in the implementation of DNS Spoofing attack in
   BIND 4 and BIND 8 is calculated by the equation: n-request-sent/65535,
   where n-request-sent is the number of requests sent simultaneously to
   the target DNS server.


3. Impact

   Normal operation of many Internet services depends on the proper
   operation of DNS servers. thus, other services could be impacted if
   this vulnerability is succesfully exploited. An attacker can use DNS
   spoofing mechanisms to apply a denial of service attack (DoS) or
   masquerade as "trusted" entity.

   The attacker can inject false information into a DNS cache mapping a
   host name to an arbitrary IP address. Some direct consequences of DNS
   spoofing attacks are:

   . compromise of applications that depend on DNS service to resolve host
     names (such as smtp, http, ldap, ftp, ssh, etc), generating false
     information and consequently intercepting, analyzing, or
     intentionally corrupting sensitive data;

   . impersonation of websites since the attacker can define for example
     the address of the site www.mydomain.br as being the IP address
     1.2.3.4, redirecting the access to a fake web server instead of to
     the real one;

   . attacks based on the exploitation of trust relationship among
     security systems.


4. Systems Affected

   DNS Servers running the following versions of ISC BIND:

   . 4.9.11 and prior (4.9.x);
   . 8.2.7 and prior (8.2.x);
   . 8.3.4 and prior (8.3.x);


5. Solutions

   Upgrade to BIND 9.2.1, available at:
   http://www.isc.org/products/BIND/bind9.html


6. Actions Recommended

   Some applicable recommendations regarding the security of DNS servers
   are:

  . configure DNS server in order to allow the use of recursion only at
    stations which belong to its domain;

  . configure anti-spoofing rules on the firewall or border router;

  . considering the network topology, set up the DNS server into a DMZ
    (demilitarized zone).

  In addition, best practices for secure configuration of DNS server
  referenced on a recently published document by CERT/CC:  "Securing an
  Internet Name Server" should be considered. This document is available
  in:

	http://www.cert.org/archive/pdf/dns.pdf


7. References

[1] Internet Software Consortium;
    http://www.isc.org
[2] Securing na Internet Name Server; Cricket Liu;
    http://www.linuxsecurity.com/resource_files/
[3] DNS and BIND, 4th Edition; Paul Albitz & Cricket Liu; May 2001
    http://www.oreilly.com/catalog/dns4/chapter/ch11.html
[4] Securing an Internet Name Server; CERT Coordination Center; Allen
    Householder, Brian King, Ken Silva
    http://www.cert.org/archive/pdf/dns.pdf


8. Acknowledgements

   To Vagner Sacramento that discovered the vulnerability described in
   this advisory during the development of his master thesis in the
   DIMAp/UFRN under the orientation of Prof. Thais Vasconcelos Batista and
   Prof. Guido Lemos de Souza Filho.

   To Thiago Alves da Silva that provides support during the tests at
   CAIS.

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Thanks to CERT Coordination Center (CERT/CC) for testing and validating
the reported vulnerability.

	http://www.kb.cert.org/vuls/id/457875
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This document can be found at:

	http://www.rnp.br/cais/alertas/2002/cais-ALR-19112002a.html

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CAIS PGP key:

http://www.rnp.br/chaves/cais-pgp-key.asc


Contact:
	+55 (19) 3787-3300
        +55 (19) 3787-3301
        cais@cais.rnp.br


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Good signature made 2002-11-19 20:59 GMT by key:
  1024 bits, Key ID C5E14F15, Created 1997-11-27
   "Centro de Atendimento a Incidentes de Seguranca <cais@cais.rnp.br>"



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