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Date: Thu, 27 Nov 2003 03:29:05 +0200
From: Timo Sirainen <tss@....fi>
To: bugtraq@...urityfocus.com
Subject: GNU screen buffer overflow

Summary
-------

Buffer overflow in GNU screen allows privilege escalation for local users.
Usually screen is installed either setgid-utmp or setuid-root.

It also has some potential for remote attacks or getting control of another
user's screen. The problem is that you have to transfer around 2-3 gigabytes
of data to user's screen to exploit this vulnerability.

4.0.1, 3.9.15 and older versions are vulnerable.

Details
-------

ansi.c:

	    case '0': case '1': case '2': case '3': case '4':
	    case '5': case '6': case '7': case '8': case '9':
	      if (curr->w_NumArgs < MAXARGS)
		{
		  if (curr->w_args[curr->w_NumArgs] < 100000000)
		    curr->w_args[curr->w_NumArgs] =
		      10 * curr->w_args[curr->w_NumArgs] + (c - '0');
		}
	      break;
	    case ';':
	    case ':':
	      curr->w_NumArgs++;
	      break;

w_NumArgs is signed integer, so after you've sent 2GB of ';' characters in
escape sequence it wraps to negative and the < MAXARGS protection fails.
Then it's only a matter of finding a position in memory where the next if
check passes and does something useful. I would guess there are multiple such
possibilities, but I didn't try to find any.

Window sizes
------------

I didn't really check this, but the code looked like there could be some
problems with large window sizes (eg. ESC[100000;100000t).

Vendor status
-------------

Sent a mail to screen@...-erlangen.de (16.10), no reply.
Sent a mail to screen mailing list (24.10), didn't help much.

Patch
-----

--- ansi.c.old  2003-11-15 18:04:12.000000000 +0200
+++ ansi.c      2003-11-15 18:04:51.000000000 +0200
@@ -559,7 +559,7 @@
            {
            case '0': case '1': case '2': case '3': case '4':
            case '5': case '6': case '7': case '8': case '9':
-             if (curr->w_NumArgs < MAXARGS)
+             if (curr->w_NumArgs >= 0 && curr->w_NumArgs < MAXARGS)
                {
                  if (curr->w_args[curr->w_NumArgs] < 100000000)
                    curr->w_args[curr->w_NumArgs] =
--- resize.c.old        2003-11-27 02:55:07.000000000 +0200
+++ resize.c    2003-11-27 02:58:33.000000000 +0200
@@ -682,6 +682,17 @@
   if (wi == 0)
     he = hi = 0;
 
+  if (wi > 1000)
+    {
+      Msg(0, "Window width too large, truncated");
+      wi = 1000;
+    }
+  if (he > 1000)
+    {
+      Msg(0, "Window height too large, truncated");
+      he = 1000;
+    }
+
   if (p->w_width == wi && p->w_height == he && p->w_histheight == hi)
     {
       debug("ChangeWindowSize: No change.\n");


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