Predictable Query IDs Pose Security Risks for DNS Servers

• Microsoft Windows NT Server versions 4.0

SYMPTOMS

An attacker can send a query to a DNS server asking for the IP address (for example, of www.microsoft.com.) The DNS server sends a recursive query on behalf of the client at which point the attacker floods the DNS server with responses indicating the IP address (for example, for www.microsoft.com it may return 127.0.0.1,) or any other incorrect response as desired by the attacker. The DNS server caches this result and subsequent queries for this site return the incorrect IP address. The cached entry can be set to live for an arbitrarily long time in the cache and is not purged until the DNS server is reset.

CAUSE

The Domain Name System (DNS) is used to resolve names with IP addresses and a DNS query can be sent to any DNS server. If the server does not have the authoritative answer to the query, it can ask other DNS servers in the DNS tree for the answer. This is called a recursive query. The result of a recursive query is cached by the originating DNS server to improve performance. A DNS server may have many outstanding recursive queries at any one moment in time and each outstanding query is identified by a query ID.

Microsoft Windows NT (and many other) DNS servers use a predictable sequence of query IDs when resolving recursive queries. If an attacker can determine the current sequence number, they can determine future sequence numbers. Knowledge of future sequence numbers helps an attacker flood a DNS server with spoofed responses to recursive queries. This makes a "cache pollution" attack easier to accomplish.

RESOLUTION

The Microsoft DNS Server has been modified to use random query IDs. Random query IDs reduce the effectiveness of this cache pollution attack. Obtain the following fix, or wait for the next Windows NT service pack.

This hotfix has been posted to the following Internet location:

   ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT40/
   hotfixes-postSP3/dns-fix

STATUS

Microsoft has confirmed this to be a problem in Windows NT version 4.0. A supported fix is now available, but has not been fully regression-tested and should be applied only to systems experiencing this specific problem. Unless you are severely impacted by this specific problem, Microsoft recommends that you wait for the next Service Pack that contains this fix. Contact Microsoft Technical Support for more information.