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Re: Vulnerability in 4.4BSD Secure Levels Implementation
From: newsham () LAVA NET (Tim Newsham)
Date: Fri, 26 Jun 1998 08:41:01 -1000


Not propagating the immutable filesystem flag on an executable to its
address space, as you suggest is the correct and documented behaviour,
implies the following:

 - The syslogd daemon can be covertly compromised, so no useful
   information ever gets logged to the protected system logs.  But at
   least no-one can modify the useless information.

Be smart, niall, syslog can only be compromised after the system
has been compromised.

McKusick et al have this to say:

   Files marked immutable include those that are frequently the subject
   of attack by intruders (e.g., login and su).  The append-only flag
   is typically used for critical system logs.  If an intruder breaks
   in, he will be unable to cover his tracks.  Although simple in
   concept, these two features improve the security of a system
   dramatically.

Since the intruder cannot reverse time,  he cannot cover his tracks
in the system log.  Just as McKusick said!  (wow, he must have known
about the time thing too!)

Why do they advocate protecting login and su if such protection can
be trivially defeated using the same techniques we demonstrated in
the attack on inetd?  And why do they claim these features improve the
security of a system "dramatically" if they can be bypassed so easily?
Either they didn't read the chflags man page (hmm, I think they wrote it),
they advocate partial security (hmm, don't think so), or there is a bug.
I believe the latter is the case.

Because protecting login and su will protect the persistant system.
Yes, the running system may still be compromised.  Securelevels does
not address that issue.  (perhaps your stance could be summed up
as: "securelevels should protect the running system"?)

Propogation of the immutable flag is the logical and correct thing to do.
I agree that this behaviour is not explicitly documented, however it
is a reasonable expectation that people hold.  Secure levels become a
farce without it.

I can see why one might think this is desirable, but it's hardly the only
obvious alternative.  I wouldn't call securelevels minus this feature a
"farce" (that is, if securelevels plus this feature isn't considered
a farce as well :)

Niall

                                          Tim N.



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