BugTraq
GNU screen buffer overflow Nov 27 2003 01:29AM
Timo Sirainen (tss iki fi) (1 replies)
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 (at) uni-erlangen (dot) de [email concealed] (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");

[ reply ]
Re: GNU screen buffer overflow Dec 01 2003 11:41AM
Mariusz Woloszyn (emsi ipartners pl) (1 replies)
Re: GNU screen buffer overflow Dec 01 2003 09:48PM
Kyle Sallee (cromwell metalab unc edu) (1 replies)
Re: GNU screen buffer overflow Dec 03 2003 07:36PM
Pavel Kankovsky (peak argo troja mff cuni cz) (1 replies)
Re: GNU screen buffer overflow Dec 03 2003 09:17PM
Casper Dik (casper holland sun com)


 

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