The default action is deny. If an SELinux policy rule does not exist to allow access, such as for a process opening a file, access is denied.

SELinux can confine Linux users. A number of confined SELinux users exist in SELinux policy. Linux users can be mapped to confined SELinux users to take advantage of the security rules and mechanisms applied to them. For example, mapping a Linux user to the SELinux user_u user, results in a Linux user that is not able to run (unless configured otherwise) set user ID (setuid) applications, such as sudo and su, as well as preventing them from executing files and applications in their home directory - if configured, this prevents users from executing malicious files from their home directories.

Process separation is used. Processes run in their own domains, preventing processes from accessing files used by other processes, as well as preventing processes from accessing other processes. For example, when running SELinux, unless otherwise configured, an attacker can not compromise a Samba server, and then use that Samba server as an attack vector to read and write to files used by other processes, such as databases used by MySQL®.

SELinux helps limit the damage made by configuration mistakes. Domain Name System (DNS) servers often replicate information between each other in what is known as a zone transfer. Attackers can use zone transfers to update DNS servers with false information. When running the Berkeley Internet Name Domain (BIND) as a DNS server in Red Hat Enterprise Linux, even if an administrator forgets to limit which servers can perform a zone transfer, the default SELinux policy prevents zone files ^[3] from being updated via zone transfers, by the BIND named daemon itself, and by other processes.

Refer to the Red Hat® Magazine article, Risk report: Three years of Red Hat Enterprise Linux 4^[4], for exploits that were restricted due to the default SELinux targeted policy in Red Hat® Enterprise Linux® 4.

Refer to the LinuxWorld.com article, A seatbelt for server software: SELinux blocks real-world exploits^[5], for background information about SELinux, and information about various exploits that SELinux has prevented.

Refer to James Morris's SELinux mitigates remote root vulnerability in OpenPegasus blog post for information about an exploit in OpenPegasus that was mitigated by SELinux as shipped with Red Hat Enterprise Linux 4 and 5.