777 grants full write access to everyone. Any compromised process or local user can overwrite executable code, inject malicious shell commands, or delete mission-critical assets.

644 (rw-r--r--) is for standard files (HTML, CSS, images) where execution is not needed. 755 (rwxr-xr-x) is for directories and executable scripts where execute (+x) allows entering directories and executing scripts.

On Linux, the execute bit on directories controls whether users can traverse/enter (cd into) the directory and access files inside. Without execute, users cannot access any files even if they have read rights.

The umask (user file-creation mask) determines default permissions for new files and directories. A standard umask of 022 subtracts group and other write rights, creating files with 644 and folders with 755.