All versions of l2tpd contain a bss-based buffer overflow. After
circumventing some minor obstacles (i.e., faking a L2TP tunnel
establishment) the overflow can be triggered by sending a specially
crafted packet.

The crucial code can be found in write_packet() in control.c:

    static unsigned char wbuf[MAX_RECV_SIZE];
    int pos = 0;
    [..]
    e = PPP_FLAG;
    wbuf[pos++] = e;
    for (x = 0; x < buf->len; x++)
    {
        e = *((char *) buf->start + x);
        if ((e < 0x20) || (e == PPP_ESCAPE) || (e == PPP_FLAG))
        {
            /* Escape this */
            e = e ^ 0x20;
            wbuf[pos++] = PPP_ESCAPE;
        }
        wbuf[pos++] = e;

    }
    wbuf[pos++] = PPP_FLAG;

Nota bene: buf->len can be upto 4080 = 4096 (=: MAX_RECV_SIZE) - 16.

It might be hard or even impossible to exploit this buffer overflow.

Thomas Walpuski