Mastering Linux Security and Hardening - Second Edition

The security implications of the SUID and SGID permissions

As useful as it may be to have SUID or SGID permissions on your executable files, we should consider it as just a necessary evil. While having SUID or SGID set on certain operating system files is essential for the operation of your Linux system, it becomes a security risk when users set SUID or SGID on other files. The problem is that, if intruders find an executable file that belongs to the root user and has the SUID bit set, they can use that to exploit the system. Before they leave, they might leave behind their own root-owned file with an SUID set, which will allow them to easily gain entry to the system the next time they encounter it. If the intruder's SUID file isn't found, ...