Traefik v3.7.12 deprecates the previously introduced underscoreHeadersStrategy entrypoint option and introduces a new one called aliasHeadersStrategy that rejects even more potentially spoofed headers - not just with underscores.

Reminder to adjust your Traefik config.

Add an entry point option to handle request headers with aliasing names by rtribotte · Pull Request #13720 · traefik/traefik