http://www.kb.cert.org/vuls/id/800113

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Vulnerability Note VU#800113

I. Description
The Domain Name System (DNS) is responsible for translating host names to IP addresses (and vice versa) and is critical for the normal operation of internet-connected systems. DNS cache poisoning (sometimes referred to as cache pollution) is an attack technique that allows an attacker to introduce forged DNS information into the cache of a caching nameserver. DNS cache poisoning is not a new concept; in fact, there are published articles that describe a number of inherent deficiencies in the DNS protocol and defects in common DNS implementations that facilitate DNS cache poisoning. The following are examples of these deficiencies and defects:

* Insufficient transaction ID space
The DNS protocol specification includes a transaction ID field of 16 bits. If the specification is correctly implemented and the transaction ID is randomly selected with a strong random number generator, an attacker will require, on average, 32,768 attempts to successfully predict the ID. Some flawed implementations may use a smaller number of bits for this transaction ID, meaning that fewer attempts will be needed. Furthermore, there are known errors with the randomness of transaction IDs that are generated by a number of implementations. Amit Klein researched several affected implementations in 2007. These vulnerabilities are described in the following vulnerability notes:
o VU#484649 - Microsoft Windows DNS Server vulnerable to cache poisoning
o VU#252735 - ISC BIND generates cryptographically weak DNS query IDs
o VU#927905 - BIND version 8 generates cryptographically weak DNS query identifiers
* Multiple outstanding requests
Some implementations of DNS services contain a vulnerability in which multiple identical queries for the same resource record (RR) will generate multiple outstanding queries for that RR. This condition leads to the feasibility of a 'birthday attack,' which significantly raises an attacker's chance of success. This problem was previously described in VU#457875. A number of vendors and implementations have already added mitigations to address this issue.
* Fixed source port for generating queries
Some current implementations allocate an arbitrary port at startup (sometimes selected at random) and reuse this source port for all outgoing queries. In some implementations, the source port for outgoing queries is fixed at the traditional assigned DNS server port number, 53/udp.


Recent additional research into these issues and methods of combining them to conduct improved cache poisoning attacks have yielded extremely effective exploitation techniques. Caching DNS resolvers are primarily at risk--both those that are open (a DNS resolver is open if it provides recursive name resolution for clients outside of its administrative domain), and those that are not. These caching resolvers are the most common target for attackers; however, stub resolvers are also at risk.

Because attacks against these vulnerabilities all rely on an attacker's ability to predictably spoof traffic, the implementation of per-query source port randomization in the server presents a practical mitigation against these attacks within the boundaries of the current protocol specification. Randomized source ports can be used to gain approximately 16 additional bits of randomness in the data that an attacker must guess. In practice, implementers will be restricted to less than 65,535 in the actual number of source ports they can allocate (port numbers <1024 may be reserved, other ports may already be allocated, etc.); however, a significant amount of additional attack resiliency can be achieved. It is important to note that without changes to the DNS protocol, these mitigations cannot completely prevent cache poisoning. However, if properly implemented, they reduce the chances of success for an attacker by several orders of magnitude and make attacks impractical.