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Home » Hacking News » Cisco 30 05 2003: CSS 11000 11500 DNS Denial of Service

Cisco 30 05 2003: CSS 11000 11500 DNS Denial of Service

by Nikola Strahija on May 5th, 2003 A vulnerability has been found which affects Cisco Content Service Switches (CSS) series 11000 and 11500. Only switches with Global Server Load Balancing enabled are affected to this denial of service attack.

Hash: SHA1

Cisco Security Advisory: Cisco Content Service Switch 11000 Series DNS Negative
Cache of Information Denial-of-Service Vulnerability

Revision Numeral 1.0

For Public Release 2003 April 30 15:00 (GMT)

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The Cisco Content Service Switch (CSS) 11000 and 11500 series switches respond
to certain Domain Name Service (DNS) name server record requests with an error
code and no Start of Authority (SOA) records, which can be negatively cached by
some DNS name servers resulting in a potential denial-of-service attack for a
particular domain name hosted by a CSS. To be affected by this vulnerability,
CSS devices must be configured for Global Server Load Balancing. The CERT/CC
issued a vulnerability note on this issue (VU#714121). Cisco is providing
repaired software, and customers are urged to upgrade to repaired code.

This vulnerability in CSS is documented as Cisco Bug IDs CSCdz62499 and

This advisory will be available at

Affected Products

The CSS 11000 and 11500 series switches (formerly known as Arrowpoint) consist
of the CSS 11050, CSS 11150, CSS 11800 11501, 11503, and 11506 hardware
platforms. They run the Cisco WebNS software.

CSS 11000 and 11500 series switches running any WebNS software revision are
affected by this vulnerability only if they are configured for Global Server
Load Balancing (also known as DNS Load Balancing).

To determine if your CSS equipment is configured for Global Server Load
Balancing, please check the configuration for the dns-server command. If this
command is not present, the configuration is not vulnerable to this issue.

No other Cisco product is currently known to be affected by this vulnerability.


Commonly, the name service in use by the Internet, DNS, uses various record
types for queries between DNS servers and clients. The common record types are
Address records (A-records), Name Server records (NS records), Mail Exchange
(MX records), Start of Authority records (SOA records), and Canonical Name
records (CNAME records). Each record or query type has various rules and
formats associated with it, including details about what may be cached, what
may be trusted by other clients, etc.

Clients usually send queries to a local server, and that local server may send
further queries to other servers on behalf of that client in order to formulate
a response for the client. When the local server receives the responses, it
will cache the information for future use and will respond to the client.

The CSS 11000 and 11500 series switches have the ability to act as an
authoritative DNS name server and will only respond to DNS A-record requests.
If a CSS configured for DNS via the Global Server Load Balancing feature
receives a DNS request or query for an unsupported record type, the CSS will
respond with rcode 4 "not implemented" or rcode 3 "NXDOMAIN," depending on the
version of WebNS. When an NXDOMAIN response code is received, the querying
server will typically stop attempting to resolve any other record type for that
name. For example, an NXDOMAIN response to the AAAA query may stop the server
from sending an A query, though there may indeed be an A-record in existence.
Some resolvers that receive an NXDOMAIN response and support negative caching
will not query for A-records for the same name until the negatively cached
error response has expired, which can take an extended period of time.

When the DNS query received is for a legitimate host name but an unsupported
record type, these negative responses may be cached by various proxies or
caching nameservers and will lead to apparent temporary service outages when
other clients query the caching nameserver or proxy for the legitimate host
name. Though network services are physically unaffected, end users are
dependent upon name resolution, and the lack of correct DNS information can
result in effective service outages.

Cisco Bug ID CSCdz62499 was the first fix, which changed the response from
rcode 3 to rcode 4. This result code is also negatively cached, so the complete
fix has been correctly addressed with Cisco Bug ID CSCea36989.

The CSS will now return an RFC 2308-compliant NODATA type 3 response, which is
an authoritative answer with rcode=NOERROR, answer=0, and no SOA. This response
should cause the specific client to query for another A-record instead of
continuing to query for the unsupported record type or using the negatively
cached error message or NXDOMAIN answer.


Exploitation of this vulnerability would result in a sporadic or partial denial
of service, affecting only the users of the DNS services that cache the
negative response information in response to an unsupported query type from
that same userbase. The administrators of the affected CSS and associated
resources may not be aware of any exploitation, since there are no locally
apparent symptoms. Only certain user groups would be affected, which may cause
significant difficulty in troubleshooting customer reports of problems.

Software Versions and Fixes

The following table summarizes the CSS software releases affected by the defect
described in this notice and provides scheduled dates on which the earliest
corresponding fixed releases will be available. Dates are tentative and subject
to change.

When selecting a release, keep in mind the following definitions.

* A maintenance release is the most heavily tested and highly recommended

* An interim release has much less testing than a maintenance release and
should be selected only if no other suitable release fixes the defect.

In all cases, customers should exercise caution to be certain the devices to be
upgraded contain sufficient memory and that current hardware and software
configurations will continue to be supported properly by the new release.

| Affected | | Estimated |
| Version | Fixed Version | Date |
| | | Available |
| | - | |
| | Interim Build | 2003-Apr-29 |
| | | |
| | - | 2003-May-30 |
| | Maintenance | |
| | Release | |
| | Upgrade to 6.10 - | |
| 5.03 | Maintenance | 2003-May-15 |
| | Release | |
| | - | |
| | Maintenance | Available |
| | Release | Now |
| | | |
| | - | Available |
| | Maintenance | Now |
| | Release | |

Obtaining Fixed Software

Cisco is offering free software upgrades to remedy this vulnerability for all
affected customers. Customers may only install and expect support for the
feature sets they have purchased.

Customers with contracts should obtain upgraded software through their regular
update channels. For most customers, this means that upgrades should be
obtained through the Software Center on Cisco's worldwide website at

Customers whose Cisco products are provided or maintained through prior or
existing agreement with third-party support organizations such as Cisco
Partners, authorized resellers, or service providers should contact that
support organization for assistance with the upgrade, which should be free of

Customers who purchase direct from Cisco but who do not hold a Cisco service
contract and customers who purchase through third-party vendors but are
unsuccessful at obtaining fixed software through their point of sale should get
their upgrades by contacting the Cisco Technical Assistance Center (TAC). In
those cases, customers may only upgrade to a later version of the same release
as indicated by the applicable row in the Software Versions and Fixes table.
TAC contacts are as follows:

* +1 800 553 2447 (toll free from within North America)

* +1 408 526 7209 (toll call from anywhere in the world)

* e-mail: [email protected]

See for additional
TAC contact information, including special localized telephone numbers and
instructions and e-mail addresses for use in various languages.

Please have your product serial number available and give the URL of this
notice as evidence of your entitlement to a free upgrade. Free upgrades for
non-contract customers must be requested through the TAC.

Please do not contact either "[email protected]" or "[email protected]"
for software upgrades.


The workaround for this issue is to disable Global Server Load Balancing and to
configure DNS records for the affected servers and domains on a separate
compliant DNS server until an upgrade to repaired versions can be installed.

Exploitation and Public Announcements

This vulnerability has been published by the CERT at CERT notes that this issue is not
new. The Cisco PSIRT is not aware of any malicious use of the vulnerabilities
described in this advisory, but because of the nature of this issue, it may
be unlikely that exploitation would be noticed or reported.

Status of This Notice: FINAL

This is a final notice. Although Cisco cannot guarantee the accuracy of all
statements in this notice, all the facts have been checked to the best of our
ability. Cisco does not anticipate issuing updated versions of this notice
unless there is some material change in the facts. Should there be a
significant change in the facts, Cisco may update this notice.


This notice will be posted on Cisco's worldwide website at

In addition to worldwide web posting, a text version of this notice is
clear-signed with the Cisco PSIRT PGP key and is posted to the following e-mail
and Usenet news recipients.

* [email protected]
* [email protected]
* [email protected]
* [email protected] (includes CERT/CC)
* [email protected]
* [email protected]
* Various internal Cisco mailing lists

Future updates of this advisory, if any, will be placed on Cisco's worldwide
website, but may or may not be actively announced on mailing lists or
newsgroups. Users concerned about this problem are encouraged to check the
above URL for any updates.

Revision History

| Revision | 2003-April-30 | Initial public |
| 1.0 | | release. |

Cisco Security Procedures

Complete information on reporting security vulnerabilities in Cisco products,
obtaining assistance with security incidents, and registering to receive
security information from Cisco, is available on Cisco's worldwide website at This includes
instructions for press inquiries regarding Cisco security notices. All Cisco
security advisories are available at

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This notice is Copyright 2003 by Cisco Systems, Inc. This notice may be
redistributed freely after the release date given at the top of the text,
provided that redistributed copies are complete and unmodified, and include all
date and version information.

Version: PGP 7.0


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