CERT Advisory CA-2000-20: Mulitple Denial-of-Service Problems in ISC BINDNov 15, 2000, 20:59 (1 Talkback[s])
(Other stories by Aleph One)
Date: Wed, 15 Nov 2000 10:36:36 -0800
CERT Advisory CA-2000-20 Mulitple Denial-of-Service Problems in ISC BIND
Original release date: November 13, 2000 Source: CERT/CC
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 8.2.2-P6Overview
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 see
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 vulnerability.
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
Compaq Computer Corporation
SOURCE: Compaq Computer Corporation
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 further information.
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 bug" vulnerability.
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
Email: firstname.lastname@example.org Phone: +1 412-268-7090 (24-hour hotline) Fax: +1 412-268-6989 Postal address: CERT Coordination Center Software Engineering Institute Carnegie Mellon University Pittsburgh PA 15213-3890 U.S.A.CERT personnel answer the hotline 08:00-20:00 EST(GMT-5) / EDT(GMT-4) Monday through Friday; they are on call for emergencies during other hours, on U.S. holidays, and on weekends.
We strongly urge you to encrypt sensitive information sent by email. Our public PGP key is available from
If you prefer to use DES, please call the CERT hotline for more information.
Getting security information
CERT publications and other security information are available from our web site
To subscribe to the CERT mailing list for advisories and bulletins, send email to email@example.com. Please include in the body of your message
* "CERT" and "CERT Coordination Center" are registered in the U.S. Patent and Trademark Office.
NO WARRANTY Any material furnished by Carnegie Mellon University and the Software Engineering Institute is furnished on an "as is" basis. Carnegie Mellon University makes no warranties of any kind, either expressed or implied as to any matter including, but not limited to, warranty of fitness for a particular purpose or merchantability, exclusivity or results obtained from use of the material. Carnegie Mellon University does not make any warranty of any kind with respect to freedom from patent, trademark, or copyright infringement.
Conditions for use, disclaimers, and sponsorship information
Copyright 2000 Carnegie Mellon University.