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This question already has an answer here:

New to network security here.

Is exploiting known/zero day vulnerabilities on "listening" services after scanning for open ports the only way or at least the most common way to compromise a system?

Of course we're not talking about completely different things like DDOS, malware and whatnot.

Just wondering if opening up ports and running server services has any other security risks.

marked as duplicate by SilverlightFox, StackzOfZtuff, Question Overflow, RoraΖ, WhiteWinterWolf Aug 10 '15 at 13:15

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Opening ports, especially to untrusted networks can have a number of risks (one of which, as you've correctly identified is 0-day vulnerabilities)

Depending on the service that's listening the configuration could be a larger source of concern. Things like default or easily guessable credentials are the source of many security issues and these are generally quite easily exploited as the information about what the default usernames/passwords for various services are is easily available.

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