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Microsoft on Monday issued a warning concerning a vulnerability in how Windows resolves hostnames, and is offering steps for systems administrators to work around the problem until a fix is issued.
The issue affects hostnames that do not include a fully qualified domain name (FQDN). A fully qualified domain name is a domain name that specifies the node's position in the DNS tree hierarchy and is often used for sub sections of domains or for functions. A fully qualified domain is distinguishable by a period after the top-level extension, like ".com."
This means third-level domains or deeper are vulnerable, such as mail.domainname.com or us.dbase.domainname.com. There are, however, mitigating factors, as detailed on the Microsoft Security Research Center alert page.