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7.4 Trust

There are many implicit trust relationships in computer systems. It is crucial to understand them. If you do not understand where you are placing your trust, your trust can be exploited by attackers who have thought more carefully than you have.

For example, any NFS server of users' home-directories trusts the root user on the hosts which mount those directories. Some bad accidents are prevented by mapping root to the user nobody on remote systems, but this is not security, only convenience. The root user can always use `su' to becomes any user in its password file and access/change any data within those filesystems. The .rlogin and hosts.equiv files on Unix machines grant root (or other user) privileges to other hosts without the need for authentication.

If you are collecting software from remote servers, you should make sure that they come from a machine that you trust, particularly if they are files which could lead to privileged access to your system. Even checksums are no good unless they also are trustworthy. For example, it would be an extremely foolish idea to copy a binary program such as /bin/ps from a host you know nothing about. This program runs with root privileges. If someone were to replace that version of ps with a Trojan horse command, you would have effectively opened your system to attack. Most users trust anonymous FTP servers where they collect free software. In any remote copy you are setting up an implicit trust relationship. First of all you trust integrity of the host you are collecting files from. Secondly you trust that they have the same username database with regard to access control. The root user on the collecting host has the same rights to read files as the root user on the server. The same applies to any matched user name.

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