The currently spreading ransomware WannaCry exploits SMB vulnerabilities to infect new computer.
Is this exploitation possible over the internet?
How to check if my computer has SMB accessible over the internet or not?
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If your port 445/tcp is exposed to the outside then your SMB is accessible over internet. This usually requires port forwarding and your public/private IP must be reachable from the outside.
(Wannacry) as you might now is using "an exploit" and is not accessing SMB the regular way..
And for the sake of sharing, I guess you asked this question following what happened last Friday, Cisco released a snort rule for ETERNALBLUE (the SMB exploit) as SID 41978