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Re: screen 3.9.5 root vulnerability
From: Eugeny Kuzakov <coredumped () COREDUMPED NULL RU>
Date: Tue, 5 Sep 2000 09:16:36 +0700

On Tue, 5 Sep 2000, [latin1] Jouko PynnЖnen wrote:

FreeBSD port not affected for this problem after 1 sept 2000 because
it contains security patch for this problem.

$ cat /usr/ports/misc/screen/patches/patch-sec1
--- screen.c.orig       Fri Sep  1 17:58:35 2000
+++ screen.c    Fri Sep  1 17:57:35 2000
@@ -2311,7 +2311,7 @@
              else if (visual && !D_VB && (!D_status || !D_status_bell))
                  D_status_delayed = -1;
-                 Msg(0, VisualBellString);
+                 Msg(0, "%s", VisualBellString);
                  if (D_status)
                      D_status_bell = 1;

Date: Tue, 5 Sep 2000 01:28:01 +0300
From: "[latin1] Jouko PynnЖnen" <jouko () SOLUTIONS FI>
Subject: screen 3.9.5 root vulnerability


A vulnerability exists in the program "screen" version 3.9.5 and earlier.
If screen is installed setuid root, a local user may gain root privilege.
There are many systems where the program isn't setuid root by default, but
on many systems (afaik at least SuSE Linux, Red Hat 5.2 and earlier, *BSD ports
packages, Solaris, other commercial unices) it is, making them vulnerable.

To quickly check if your version is vulnerable, have these two lines in

vbell on
vbell_msg '%x'

Set TERM to vt100, start screen and press ctrl-G (you may need to issue the
command echo ^V^G to get a visual bell). If you see a hexadecimal number on
the last line, your version of screen is vulnerable. However it can't be
exploited unless the program is installed setuid root.


The bug is located in screen.c in function serv_select_fn():

              else if (visual && !D_VB && (!D_status || !D_status_bell))
                  D_status_delayed = -1;
                  Msg(0, VisualBellString);
                  if (D_status)

Msg() feeds the second argument to sprintf() and since VisualBellString is
user defineable, we have a classical format bug. From there, a malicious user
can either do the old trick and write over a return address in stack, or for
instance, write over the real_uid variable where screen saves the user id.
After zeroing this variable with the format string the user can just open
a new window with a root shell in it.

For this reason the bug is quite platform-independent; no shell code nor
executable stack is needed. The vulnerability has been tested on Linux, Intel
and ppc architectures.


NetBSD, FreeBSD, OpenBSD (screen is a part of the ports collection)
Red Hat Linux 5.2 and earlier, SuSE Linux, Solaris, many commercial unices


Red Hat Linux 6.0 and later, most other Linux distributions


Removing the setuid bit from the binary makes it impossible to be

chmod 111 /usr/local/bin/screen        # or /usr/bin/screen

BUT this may require some changes to the mode of screen's socket dir
(usually /tmp/screens). Consult screen documentation for more info.


Screen authors (and some OS vendors) have been informed and a new version
of screen can be retrieved from


and diffs relative to version 3.9.5:


Vendor patches for vulnerable systems have been released, or will be
released shortly.


Vulnerability discovered by: Jouko PynnЖnen

Jouko PynnЖnen           Online Solutions Ltd       Secure your Linux -
jouko () solutions fi                                  http://www.secmod.com

        Best wishes,
                    Eugeny Kuzakov, SA
                        ITBank, Omsk

All I want is a warm bed and a kind word and unlimited power
                -- Ashleigh Brilliant

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