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25 November 2009, 12:52

Security update for BIND name server

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The Internet Systems Consortium (ISC), the company behind open source name server BIND, has released security updates to resolve a DNSSEC-related vulnerability. According to a report, under certain circumstances name servers which allow recursive queries extract information from the additional section of responses. This allows attackers to inject fake entries into the name server's cache, with the result that specific domains are resolved to incorrect IP addresses (e.g. phishing servers). Reportedly, all versions of BIND from 9.0.x to 9.6.x are affected.

The problem only occurs when a client sends recursive queries in which the DNSSec Ok (DO) and Checking Disabled (CD) flags are set in the header. According to ISC, this should never occur under normal circumstances and they are not aware of any client (stub) resolver which sends such queries. However, they are aware of at least one implementation of another DNS server, which has both of these flags set in its forwarded queries. DNSSEC-capable clients normally only set the "DNSSec Ok" flag, as is the case, for example, in Windows 7.

Although normal clients do not send queries of this type, it is certainly possible that a piece of malware might be able to manipulate a name server in this way using crafted queries within a corporate network. Users should therefore not hesitate to install updates 9.4.3-P4, 9.5.2-P1 or 9.6.1-P2. Administrators should also check whether servers permit recursive queries from external sources and if necessary block this option. The problem does not occur with authoritative-only name servers.

No updates are available for versions 9.0 to 9.3, as they are no longer supported. The beta version of BIND 9.7 is also vulnerable, but the problem is fixed in the forthcoming 9.7.0b3 release.

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