SUMMARY: Bug

From: Piotr Bienias <bienias_at_ae.katowice.pl>
Date: Mon, 19 Jul 1999 13:16:06 +0200

Hi!

My original post was (see archive of this list):

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http://www.xray.mpe.mpg.de/mailing-lists/tru64-unix-managers/1999-07/msg00255.html
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I only intended to show that o+w permissions on /dev/ttys could cause
problems.
I.e.: victim's terminal /dev/ttyp1 has 622 perms. (u+rw,g+w,o+w)
someone executes the following command:
echo `perl -e 'print "A"x1000'` > /dev/ttyp1 ( or endless loop ;-) )
or
tput reset > /dev/ttyp1
If perms are 620 (o-w) it won't be possible to mess up victim's
terminal.
IMHO. "mesg y" should only change group permissions (crw--w----) and
nothing
more. "write", "wall" and "talk" don't need o+w perms.
I received the following answers:
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From:  Pirie Hart <pirie_at_u.washington.edu> (the first mail)
An explicit "mesg y" command grants ALL users the ability to send a
message with the write and talk commands (hence permission of
crw--w--w-).
"mesg n" turns off access so only the user him/herself can send
write/talk
(displayed as crw-------).  The initial setting of mesg grants access to
the user and group, but not to all other users (crw--w----).
The only "bug" is that there is no option to restore the default setting
once it is changed with "mesg y" or "mesg n".
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From:  Pirie Hart <pirie_at_u.washington.edu> (the second one)
As documented in the manual, mesg yes "permits other users to send
messages to your terminal."  This means ALL other users, not just user
and
group.
The "who -T" command displays a "+" if the terminal is "writable by
anyone" or "-" if "writable only by the superuser or the terminal's
owner".
I understand (and concur with, by the way) your concern about granting
access of other users to your terminal.  However, the situation is not a
"bug".  It works exactly as documented in the manual.
If you are so inclined, you could write a script to remove others' write
permission:
     #
     mesg y ; chmod o-w /dev/`who -m | awk '{print $2}'`
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From: John P Speno <speno_at_isc.upenn.edu>
Well, it isn't fixed in patch kit #3, but it is fixed in 4.0f and 5.0.
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Thanks and best regards,
--
PB

Received on Mon Jul 19 1999 - 11:21:12 NZST