Filesystem:/private/etc/fstab

fstab controls the read/write access of the root and media partitions. A barebones jailbreak can be put in place simply by modifying this file to grant full read/write access to the user.

File Contents
Note the empty line. This is required for the file to be parsed correctly.

Before jailbreak
/dev/disk0s1s1 / hfs ro 0 1 /dev/disk0s1s2 /private/var hfs rw 0 2

After jailbreak
/dev/disk0s1s1 / hfs rw 0 1 /dev/disk0s1s2 /private/var hfs rw 0 2