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.

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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.

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The Microsoft DNS Server has been modified to use random query IDs. Random query IDs reduce the effectiveness of this cache pollution attack.

To resolve this problem, obtain the latest service pack for Windows NT 4.0 or Windows NT Server 4.0, Terminal Server Edition. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

For your convenience, the English version of this post-SP3 hotfix has been posted to the following Internet location. However, Microsoft recommends that you install Windows NT 4.0 Service Pack 4 to correct this problem.

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Microsoft has confirmed this problem could result in some degree of security vulnerability in Windows NT version 4.0. This problem was first corrected in Windows NT 4.0 Service Pack 4.0 and Windows NT Server 4.0, Terminal Server Edition Service Pack 4.

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