Is it dangerous to use Linked Server? (SQL Server 2008)
You are creating a path to another location, be it SQL Server, Oracle, DB2 whatever.  So the security risks are there but can be whatever you make of them.  For example if you set up a linked server to a SQL Server and tell it to impersonate SA for every connection then the security risks are massive.  If however you tell it to only use the users current security context the risks are less so.  With any additional "option" that you turn on or use you increase your security risks by a certain amount.  You just need to make sure that you do your due diligence about how you set it up.

Personally I prefer to use the "Be made using the login's current security context" option.  No SQL Server IDs will be able to use it and only AD logins that are on both machines will be able to do so.  If you want to use the option "Be made using this security context" option then make sure that the user you are giving out has minimal permissions and that those are permissions you are willing to give out to EVERYONE.