Enable SSH strict mode checking

Description

The SSH daemon must verify the ownership and permissions of the user's home directory, authorized_keys file, and related configuration before accepting a login. StrictModes ensures that improperly secured files cannot be used to gain unauthorized access.

Rationale

Without strict mode checking, an attacker who gains write access to a user's home directory or .ssh folder could inject their own public key into the authorized_keys file and authenticate as that user. StrictModes prevents this by rejecting keys stored in files with overly permissive ownership or permissions.

Framework references

Live verification