Random Japanese-language pages showing up in my site's Google index + favicon changed automatically

Over the last 2-3 days, I've noticed some strange issues when checking my site's index on Google Search Console:

• Several pages are getting indexed with Japanese-language titles and content that I never created — looks like e-commerce/product spam (random product listings, prices, star ratings, etc.).
• My favicon changed automatically without me touching any settings.
• These pages don't show up when I browse the site normally — only in the index/search results.

I haven't made any recent changes to my site, so I'm trying to figure out:

1. Has anyone else run into this exact issue?
2. Is this a known hack (I've seen it referred to as the "Japanese keyword hack")?
3. What's the best way to confirm it, clean it up, and stop it from recurring?

Any guidance on what to check first (plugins, file permissions, admin accounts, etc.) would be really appreciated. Happy to share more details if helpful.