Hmmm. I don't normally copy the whole contents of someone else's post (else it wouldn't be content curation), but this post from Stephen H. Blackwell left little choice:
Security update for FileMaker 13 versions.
Be sure to read the instructions first.
Important Security Update
Read the instructions first
That about covers it for the just released update. There is, however, an update coming out that also covers FM 12, but not for a couple of weeks:
This update is being released in two stages to minimize compatibility issues. FileMaker Server 13.0v9 and FileMaker Pro/Advanced 13.0v9 will be released first, with FileMaker Go 13.0.9, FileMaker Server 12 Security Update and FileMaker Pro/Advanced 12 Security Update following in approximately 2 weeks. FileMaker Go 12 will not be updated. FileMaker Go 13.0.9 will retain compatibility with the FileMaker Server 12 Security Update.It's an important update and...you know the rest.