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rh6-pty-prob.txt
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rh6-pty-prob.txt
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[Mod: forwarded from BUGTRAQ -- alex]
Once again I've come up with another trivial Denial of Service flaw,
(wow,
I seem to be good at this Conseal Firewall, +++ath0, ppp byte-stuffing)
It's been a few months since my last DoS, so here you go:
Many of you RedHat 6.0 users who installed RedHat 6.0 rather than
upgrading may have noticed the new way RedHat displays remote TTY's.
Instead of the old fashioned /dev/ttyp<number>, it now uses
/dev/pts/<number>. There is a flaw in this new implementation that
local
users can exploit to cause minor disruption to anyone using X-windows on
the local machine.
This DoS is more of a nuisance than a "real problem" but it could
possibly
be used to cause some minor havok.
The way it works is simple. When whoever is using X opens up an "xterm"
(eterm, rxvt, nxterm...) a connection is made to the X server.
If you do a "who" you will see:
(RedHat 6.0, without upgrading from previous RedHat release)
wage pts/0 Jun 6 01:39 (:0.0)
Or on older versions:
wage ttyp0 Jun 6 01:39 (:0.0)
Now this is normal, but the problem lies within the permissions of that
device.
On older RedHat's if you did:
ls -l /dev/ttyp3 you would see:
crw------- 1 wage tty 3, 0 Jun 6 12:41 /dev/ttyp0
Which is normal and what it should look like.
For those of you who may be new to unix those letters at the beginning
of
the line indicate the permissions on the device.
For our output above, the line indicates it is a device (c), and that
the
OWNER has read and write permissions (rw)
Group has no permissions (---), and everyone has no permissions (---)
They basically go <type indicator><owner><group><everyone>
An example line of a device will ALL permissions set follows:
crwxrwxrwx
/ | \
Owner Group Everyone
This means that everyone has read/write/execute permissions to that
device.
So as you can see our ttyp0 can only be read or written to by it's owner
(and root).
In the case of RedHat 6.0 with regular remote connections (like telnet)
the standard permissions are as follows:
crw--w---- 1 ov3r tty 136, 0 Jun 6 12:32 /dev/pts/0
Here it's almost the same except that group "tty" also has write access.
The problem lies in the way that the permissions are set for local
connections with the X server using xterm.
if you do an ls -l /dev/pts/<the xterm's tty> (we will use pts/0)
You get:
crw--w--w- 1 ov3r ov3r 136, 0 Jun 6 12:32 /dev/pts/0
Notice how now "everyone" has write access to this terminal?
This leads to the hole that any local user can disrupt any xterminal
connected to the local machine. Simply typing "cat /dev/urandom >
/dev/pts/<number>" will flood the xterm with garbage data making it
impossible to use. Or we can also bring back the old "flash" attack and
flash the user's xterm by dumping ASCII escape characters to his
terminal.
This isn't a particularily "deadly" DoS attack, but can be used as a
nuisance OR perhaps even to trick the user into doing something he may
not want to do. (For example dumping "Login:" then "Password:" to the
terminal may trick the user into adding his login/password to a file or
to
his .bash_history).
--
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