Hackers have already extracted passwords from macOS High Sierra

Apple just launched macOS High Sierra, a Mac update that brings over several security improvements, including a new ad tracker blocker in Safari. But it also carried over a significant safety issue that would let any rogue app steal all the passwords you’ve saved in Keychain without your knowledge or any user interaction.

If you think that sticking with Sierra, for the time being, will fix it, you should know that older macOS versions are also susceptible to the same attack. You’re much better off upgrading to the latest macOS version.