||Home : Advisories : CERT Advisory CA-2000-20 Mulitple Denial-of-Service Problems in ISC BIND|
||CERT Advisory CA-2000-20 Mulitple Denial-of-Service Problems in ISC BIND
||13th November 2000
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CERT Advisory CA-2000-20 Mulitple Denial-of-Service Problems in ISC BIND
Original release date: November 13, 2000
A complete revision history is at the end of this file.
* Systems running Internet Software Consortium (ISC) BIND version
8.2 through 8.2.2-P6
* Systems running name servers derived from BIND version 8.2 through
The CERT Coordination Center has recently learned of two serious
denial-of-service vulnerabilities in the Internet Software
Consortium's (ISC) BIND software.
The first vulnerability is referred to by the ISC as the "zxfr bug"
and affects ISC BIND version 8.2.2, patch levels 1 through 6. The
second vulnerability, the "srv bug", affects ISC BIND versions 8.2
through 8.2.2-P6. Derivatives of the above code sets should also be
presumed vulnerable unless proven otherwise.
The Internet Software Consortium, the maintainer of BIND, the software
used to provide domain name resolution services, has recently posted
information about several denial-of-service vulnerabilities. If
exploited, any of these vulnerabilities could allow remote intruders
to cause site DNS services to be stopped.
For more information about these vulnerabilities and others, please
Two vulnerabilities in particular have been categorized by both the
ISC and the CERT/CC as being serious.
The "zxfr bug"
Using this vulnerability, attackers on sites which are permitted to
request zone transfers can force the named daemon running on
vulnerable DNS servers to crash, disrupting name resolution service
until the named daemon is restarted. The only preconditions for this
attack to succeed is that a compressed zone transfer (ZXFR) request be
made from a site allowed to make any zone transfer request (not just
ZXFR), and that a subsequent name service query of an authoritative
and non-cached record be made. The time between the attack and the
crash of named may vary from system to system.
This vulnerability has been discussed in public forums. The ISC has
confirmed that all platforms running version 8.2.2 of the BIND
software prior to patch level 7 are vulnerable to this attack.
The "srv bug"
This vulnerability can cause affected DNS servers running named to go
into an infinite loop, thus preventing further name requests to be
handled. This can happen if an SRV record (defined in RFC2782) is sent
to the vulnerable server.
Microsoft's Windows 2000 Active Directory service makes extensive use
of SRV records and is reportedly capable of triggering this bug in the
course of normal operations. This is not, however, a vulnerability in
Microsoft Active Directory. Any network client capable of sending SRV
records to vulnerable name server systems can exercise this
The CERT/CC has not received any direct reports of either of these
vulnerabilities being exploited to date.
Both vulnerabilities can be used by malicious users to break the DNS
services being offered at all exposed sites on the Internet. System
administrators are strongly recommended to upgrade their DNS software
with either ISC's current distribution or their vendor-supplied
software. See the Solution and Vendor Information sections of this
document for more details.
Domain name resolution services (DNS) can be disabled on affected
servers from arbitrary remote hosts.
Apply a patch from your vendor
The CERT/CC recommends that all users of ISC BIND upgrade to the
recently-released BIND 8.2.2-P7, which patches both of the
vulnerabilities discussed in this document. Sites running
vendor-specific distributions of domain name resolution software
should check the Vendor Information section below for more specific
information on how to upgrade to non-vulnerable software.
Restrict zone transfers to trusted hosts
If it is not possible to immediately upgrade systems affected by the
"zxfr bug", the ISC suggests not allowing zone transfers from
untrusted hosts. This action, however, will not mitigate against the
effects of an attack using the "srv bug".
Although it has been reported that not allowing recursive queries may
help mitigate against the "zxfr" vulnerability, ISC has indicated that
this is not the case.
Appendix A. Vendor Information
The Internet Software Consortium
For the latest information regarding these vulnerabilities, please
consult the ISC web site at:
Our advisory will be available [at]:
Updated packages will be available from
OpenLinux Desktop 2.3
OpenLinux eServer 2.3
OpenLinux eDesktop 2.4
Compaq Computer Corporation
SOURCE: Compaq Computer Corporation
Software Security Response Team USA
Compaq Tru64/UNIX Operating Systems Software are not vulnerable to
these reported problems.
Please see Conectiva Linux Security Announcement CLSA-2000:339 at:
Note: Conectiva Linux Security Announcement CLSA-2000:338, also
regarding this issue, had a packaging error in it. Users who
downloaded updates based on CLSA-2000:338 should see CLSA-2000:339 for
Please see Debian Security notice 20001112, bind at:
All versions of FreeBSD after 4.0-RELEASE (namely 4.1-RELEASE,
4.1.1-RELEASE and the forthcoming 4.2-RELEASE) are not vulnerable to
this bug since they include versions of BIND 8.2.3. FreeBSD
4.0-RELEASE and earlier are vulnerable to the reported problems since
they include an older version of BIND, and an update to a
non-vulnerable version is scheduled to be committed to FreeBSD
3.5.1-STABLE in the next few days.
HP is vulnerable to these problems and is working to correct them.
Please see "MDKSA-2000:067: bind" at:
Microsoft is currently investigating these issues.
NetBSD is believed to be vulnerable to these problems; in response,
NetBSD-current has been upgraded to 8.2.2-P7 and 8.2.2-P7 will be
present in the forthcoming NetBSD 1.5 release.
Please see "RHSA-2000:107-01: Updated bind packages fixing DoS
attack", soon to be available at:
Updated Slackware distributions for bind may be found at:
The CERT Coordination Center thanks Mark Andrews, David Conrad, and
Paul Vixie of the ISC for developing a solution and assisting in the
preparation of this advisory. We would also recognize the contribution
of Olaf Kirch in helping us understand the exact nature of the "zxfr
Author: This document was written by Jeffrey S. Havrilla and Jeffrey
P. Lanza. Feedback on this advisory is appreciated.
This document is available from:
CERT/CC Contact Information
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
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November 13, 2000: Initial release
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