possible security bug if uid of nobody is 65535 or -1

[iang () cs berkeley edu (Ian Goldberg)]

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I've seen the user "nobody" on some systems have a uid of -1 or 65535.
(Slackware Linux is such an example.) On most such systems, this will
have interesting interactions with syscalls like setreuid() and chown(),
for which an argument of -1 means "no change".

A program that is setuid root, but uses setreuid() to swap its real and
effective uids will thus remain root if run by the "nobody" user.
Also note that it is easy to run programs as nobody on systems on which
CGI scripts are available (the default is to run them as nobody).

   - Ian

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