Mulitple Vendors DNS Spoofing Vulnerability

Today, Microsoft was just one vendor releasing a patch for its DNS server. The Internet Software Consortium ( published a very similar patch for its own DNS server, BIND.

Many other DNS servers are derived either form BIND or Microsoft's DNS server. Expect more similar announcements over the next couple days.

The Problem

The root cause is a fundamental, well known, weakness in the DNS protocol. DNS uses UDP, a stateless protocol. A DNS server will send a request in a single UDP packet, then wait for a response to come back. In order to match request and response, a number of parameters are checked:

  • who sent the response? Was it the DNS server we sent the request to?
  • for this particular response, do we have an outstanding request?
  • each request uses a unique and random query ID. The response has to use the same query ID.
  • The response has to be sent to the same port from which the request was sent.

Only if all this matches, the response is accepted. The first valid response wins. If an attacker is able to guess the query id and the source port, the attacker is able to send a fake response, which will be cached by the DNS server.

How likely is it to "guess" the query id and the source port? One would think, its not that easy. The query ID is 16 bits long, allowing for 65536 options. The source port could be anything about 1024 which again would allow for another 64512 options. It is easy to guess which DNS server is expected to reply, as it has to be a valid DNS server for the respective domain. A reasonable DNS server should respond in less then a second, allowing for about 1 second to send the spoofed response.

Ideally, one would think that it would take millions of packets per second to successfully spoof the response. However, the problem is in the details. A DNS server can not use any port to source the query. It may not use a port commonly used by outbound connections, or a port reserved by a server. This is an issue attacked by today's patches. As of today, DNS servers used a rather small set of ports to source requests. This is the actual new finding. The patch will increase the pool of source ports available to DNS queries. To make things worse: the real DNS server may be silenced using DDoS attacks.

Over the past few months, we had a couple patches (again both for Microsoft as well as for BIND) addressing the randomness of the query ID.

How bad is it?

If you run a caching DNS server, patch it soon. I wouldn't say "today, while ignoring sane patch management". But check with your vendor and follow their guidance. The world is not going to end today. It will in fact end in 2 1/2 years from today (different story ;-) ). But this is something you have to fix soon. Right now, the US-CERT advisory lists about 80 vulnerably products.

Eventually we all may have to break down and fix DNS. DNSSEC is an extension to DNS asking for cryptographic authentication. However, it requires a PKI infrastructure which at this point doesn't exist. There is not much to be gained from implementing DNSSEC on your own (but by all means: try it out and see how it works).

Where can I find out more?

Internet Software Consortium (BIND):
Dan Kaminski's Podcast:


Johannes B. Ullrich, Ph.D.
SANS Technology Institute -

I will be teaching next: Application Security: Securing Web Apps, APIs, and Microservices - SANS San Francisco Winter 2022


4601 Posts
ISC Handler
Jul 8th 2008

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