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capability.notes
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Overview -------- As of Linux 2.2.0, the power of the superuser has been partitioned into a set of discrete capabilities (in other places, these capabilities are know as privileges). The contents of the libcap package are a library and a number of simple programs that are intended to show how an application/daemon can be protected (with wrappers) or rewritten to take advantage of this fine grained approach to constraining the danger to your system from programs running as 'root'. Notes on securing your system ----------------------------- Adopting a role approach to system security: changing all of the system binaries and directories to be owned by some user that cannot log on. You might like to create a user with the name 'system' who's account is locked with a '*' password. This user can be made the owner of all of the system directories on your system and critical system binaries too. Why is this a good idea? In a simple case, the CAP_FUSER capabilty is required for the superuser to delete files owned by a non-root user in a 'sticky-bit' protected non-root owned directory. Thus, the sticky bit can help you protect the /lib/ directory from an compromized daemon where the directory and the files it contains are owned by the system user. It can be protected by using a wrapper like execcap to ensure that the daemon is not running with the CAP_FUSER capability... Limiting the damage: If your daemon only needs to be setuid-root in order to bind to a low numbered port. You should restrict it to only having access to the CAP_NET_BIND_SERVICE capability. Coupled with not having any files on the system owned by root, it becomes significantly harder for such a daemon to damage your system. Note, you should think of this kind of trick as making things harder for a potential attacker to exploit a hole in a daemon of this type. Being able to bind to any privileged port is still a formidable privilege and can lead to difficult but 'interesting' man in the middle attacks -- hijack the telnet port for example and masquerade as the login program... Collecting passwords for another day. The /proc/ filesystem: This Linux-specific directory tree holds most of the state of the system in a form that can sometimes be manipulated by file read/writes. Take care to ensure that the filesystem is not mounted with uid=0, since root (with no capabilities) would still be able to read sensitive files in the /proc/ tree - kcore for example. [Patch is available for 2.2.1 - I just wrote it!]