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Posted Nov 25, 2002
Authored by Vagner Sacramento

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.

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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);

- -----------------------------------------------------------------------

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, 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:

6. Actions Recommended

Some applicable recommendations regarding the security of DNS servers

. 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


7. References

[1] Internet Software Consortium;
[2] Securing na Internet Name Server; Cricket Liu;
[3] DNS and BIND, 4th Edition; Paul Albitz & Cricket Liu; May 2001
[4] Securing an Internet Name Server; CERT Coordination Center; Allen
Householder, Brian King, Ken Silva

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

- -------------------------------------------------------------------------

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

- --------------------------------------------------------------------------

This document can be found at:


- --------------------------------------------------------------------------



+55 (19) 3787-3300
+55 (19) 3787-3301

- ------------ Output from pgp ------------
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>"

Version: GnuPG v1.0.6 (GNU/Linux)
Comment: For info see http://www.gnupg.org


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