Unix file permissions and the umask

What it is

Permissions are three triplets (owner, group, others) of read (4), write (2) and execute (1), written in octal (0644) or symbolically (u=rw,go=r), as the chmod manual describes. Execute on a directory means "may traverse"; read on a directory means "may list". The umask subtracts bits from the default mode of newly created files (typically 0022, giving 0644 files and 0755 directories). setgid on a directory makes new files inherit its group; the sticky bit on a shared directory lets only owners delete their files.

Why it matters

World-readable configuration files leak credentials to every process on the host; a world-writable script run by root is a privilege escalation; a service running as root turns any bug into full compromise.

How to apply

• Store secrets and private keys with mode 0600 (or 0640 with a dedicated group) owned by the service user; SSH and many tools refuse looser modes.
• Run services as a dedicated unprivileged user; give them write access only to their data directories.
• Set a restrictive umask (0077 or 0027) in service units and deployment scripts that create sensitive files.
• Audit with find / -perm -o+w -type f for world-writable files and stat for specific paths; fix with chmod and chown, never with blanket 777.

Pitfalls

Copying files as root then forgetting to change ownership. Container volumes mounting host directories with mismatched UIDs. Access control lists (getfacl) that extend the classic bits invisibly.