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[linux-alert] rlogin authentication bug



I'm sorry for forwarding message from other mailing list,
but I consider the message as urgent.

People throw stones at us, and they are right.
Linux-PAM-0.58preG (and probably Linux-PAM-0.59preA) are still vulnerable.

A fix:
--- Linux-PAM-0.58.orig/modules/pam_rhosts/pam_rhosts_auth.c	Mon Aug  4 04:14:14 1997
+++ Linux-PAM-0.58/modules/pam_rhosts/pam_rhosts_auth.c	Wed Oct  1 11:43:14 1997
@@ -524,25 +524,32 @@
 {
     struct hostent *hp;
     int answer = 1;                             /* default to failure */
-    u_long addr;
-    char **ap;
+    u_long *addrs;
+    int n, i;
 
     opts->last_error = (char *) 0;
     hp               = gethostbyname(rhost);         /* identify host */
 
     if (hp != NULL) {
-	ap = hp->h_addr_list;
-	while (*ap) {                     /* loop though address list */
-	    memcpy (&addr, *ap, sizeof(addr));
-
-	    /* check user on remote host */
-	    if (pam_iruserok(pamh, opts, addr, superuser, ruser, luser, rhost)
-		== 0) {
-		answer = 0;                                /* success */
-		break;
-	    }
-	    ++ap;
-	}
+
+        /* loop though address list */
+        for (n = 0; hp->h_addr_list[n]; n++);
+        D(("rhosts: %d addresses", n))
+
+        if (n) {
+            addrs = malloc (n * sizeof(*addrs));
+            for (i = 0; i < n; i++)
+                memcpy (addrs+i, hp->h_addr_list[i], sizeof(*addrs));
+
+            for (i = 0; i < n && answer; i++) {
+                D(("rhosts: address %d is %04x", i, addrs[i]))
+                answer = pam_iruserok(pamh, opts, addrs[i], superuser,
+                                      ruser, luser, rhost);
+                         /* answer == 0 means success */
+            }
+
+            free (addrs);
+        }
     }
 
     return answer;


Forwarded message:
> 
> Hi,
> 
> I have discovered a bug in the rhost module of Linux-PAM-0.57.  This
> bug leads to a vulnerability in the remote login authentication, with
> the effect that ordinary user accounts may not be password protected.  
> 
> There is only one case when the bug causes a vulnerability:
> 
> The problem case is when the user's .rhost file has the name of a machine
> with *more than one IP address* as the *final* entry.
> 
> To be explicit, here are two example .rhost files:  One of them
> leaves an account wide open, the other does not.
> 
> CAUSES PROBLEMS                  DOES NOT CAUSE PROBLEMS  
> ----------------                 -----------------------
> 
> karman.tam.uiuc.edu              vn.nas.nasa.gov
> vn.nas.nasa.gov                  karman.tam.uiuc.edu
> 
> Note that vn.nas.nasa.gov has two IP addresses.  Thus, as the final
> entry of a .rhost file, it leaves the user's account wide open, i.e.
> any other user of the same name can log in to that account from any
> machine without a password.
> 
> The problem is in the rhost module of PAM.  The module calls
> gethostbyname() in two locations, once for the connecting machine,
> then once for each named entry in the .rhosts file.  The call returns
> a structure with a pointer to a list of IP addresses stored as unsigned
> longs.  Since there may be multiple IP addresses, the PAM code uses a
> while loop over the addresses of the connecting machine, then another 
> while loop over the addresses of the machines named in .rhosts.  
> These while loops are terminated by a null pointer in the address list.
> Apparently gethostbyname does not allocate new memory for the unsigned
> longs each time it is called.  Thus the calls to gethostbyname() for
> the .rhosts entries clobber the subsequent IP addresses of the connecting
> machine.  If the final .rhost entry is a name with multiple IP addresses,
> the the null terminator gets replaced with the second IP address of the 
> final entry in the .rhost file.  The second iteration of the while loop
> on the connecting machine now sets the IP number of the connecting machine
> to the IP number of the final entry in the .rhosts file.  A match follows
> and the account is open. 
> 
> 
> WHO IS AFFECTED?
> 
> Unfortunately, the effects of this vulnerability reach beyond the Linux
> community.  For example, users named langford have been able to walk 
> into my Linux account for months.  From that Linux account, user langford
> could freely rlogin to other departmental machines and NASA government 
> machines.  In particular, administrators of sites with multiple IP 
> addresses might consider disabling rlogin access to their machines, with
> the understanding that not all Linux users will fix the rlogin problem
> immediately.
> 
> HOW TO FIX IT?
> 
> I removed the loop that goes over IP numbers of the connecting machine.
> IMO this is not particularly important.  The only case is if I have .rhost
> entry by IP number, not name, of a machine I connect *from* with multiple
> IP addresses.  If I refer to the connecting machine by *name* in the .rhost
> file, the remaining while loop takes care of everything for me.  One of the
> IP addresses will still match.  For me this fix is suitable, although I 
> hope someone can fix the PAM module in a better way.
> 
> Until the PAM module is updated, users should check that the last entry
> of their .rhost file only has one IP address.  They can use the command
> nslookup, for example.
> 
> 
> 
> Jacob Langford
> langford@uiuc.edu
> 



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