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Critical Unauthenticated RCE Flaws in CUPS Printing Systems
(blog.qualys.com)
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Based on this...
...it seems the exploit can be triggered either remotely through your CUPS instance listening on port 631, or locally by interacting with a malicious/compromised print server.
So if I understand correctly, shutting down that port wouldn't be enough by itself. You would also have to keep your system from initiating contact with such a server, such as by using a public printer, or conceivably even just browsing printers at a cafe/business/school. I haven't read the exploit details, so I don't know which interactions are safe, if any.