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                             Luigi Auriemma

Application:  Feng
              http://live.polito.it/documentation/feng
Versions:     <= 0.1.15
Platforms:    *nix
Bugs:         A] first buffer-overflow in RTSP_valid_response_msg
              B] second buffer-overflow in RTSP_valid_response_msg
              C] crash in RTSP_remove_msg
              D] NULL pointer in parse_transport_header
              E] NULL pointer in parse_play_time_range
              F] NULL pointer in log_user_agent
              G] NULL pointer in Netembryo 0.0.4
Exploitation: remote
Date:         27 Dec 2007
Author:       Luigi Auriemma
              e-mail: aluigi@autistici.org
              web:    aluigi.org


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1) Introduction
2) Bugs
3) The Code
4) Fix


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===============
1) Introduction
===============


Feng is an open source RTSP/RTP streaming server written by the italian
team of the Politecnico di Torino University for the LScube project.


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=======
2) Bugs
=======

---------------------------------------------------
A] first buffer-overflow in RTSP_valid_response_msg
---------------------------------------------------

A buffer-overflow vulnerability is exploitable through a stack buffer
of 15 bytes called trash used as destination by sscanf without the
needed size limits.

>From rtsp/RTSP_state_machine.c:

int RTSP_valid_response_msg(unsigned short *status, char *msg, RTSP_buffer * rtsp)
// This routine is from BP.
{
    char ver[32], trash[15];
    unsigned int stat;
    unsigned int seq;
    int pcnt;        /* parameter count */

    *ver = *msg = '\0';
    /* assuming "stat" may not be zero (probably faulty) */
    stat = 0;

    pcnt =
        sscanf(rtsp->in_buffer, " %31s %u %s %s %u\n%255s ", ver, &stat,
           trash, trash, &seq, msg);
        ...


----------------------------------------------------
B] second buffer-overflow in RTSP_valid_response_msg
----------------------------------------------------

A buffer-overflow vulnerability is exploitable through a stack buffer
of 100 bytes called msg used as destination by sscanf limited to 255
bytes in the function RTSP_valid_response_msg.

>From rtsp/RTSP_state_machine.c:

int RTSP_handler(RTSP_buffer * rtsp)
{
    unsigned short status;
    char msg[100];
    ...
            op = RTSP_valid_response_msg(&status, msg, rtsp);
            ...

int RTSP_valid_response_msg(unsigned short *status, char *msg, RTSP_buffer * rtsp)
        ...
        sscanf(rtsp->in_buffer, " %31s %u %s %s %u\n%255s ", ver, &stat,
           trash, trash, &seq, msg);
        ...


---------------------------
C] crash in RTSP_remove_msg
---------------------------

An integer overflow is exploitable through the RTP packets.
Using a 16 bit size value of 0xffff is possible to force the server to
move 4294967293 bytes in the destination buffer:

RTSP_BUFFERSIZE = 65536
len             = 65539 (4 of hdrlen + 0xffff)
result          = -3

>From rtsp/RTSP_lowlevel.c:

void RTSP_remove_msg(int len, RTSP_buffer * rtsp)
{
    rtsp->in_size -= len;
    if (rtsp->in_size && len) {    /* discard the message from the in_buffer. */
        memmove(rtsp->in_buffer, &(rtsp->in_buffer[len]),
            RTSP_BUFFERSIZE - len);
        memset(&(rtsp->in_buffer[len]), 0, RTSP_BUFFERSIZE - len);
    }
}


-----------------------------------------
D] NULL pointer in parse_transport_header
-----------------------------------------

>From rtsp/RTSP_setup.c:

static RTSP_Error parse_transport_header(RTSP_buffer * rtsp,
                        ...
                        p = strstr(p, "=");
                        sscanf(p + 1, "%d", &(cli_ports.RTP));
                        p = strstr(p, "-");
                        sscanf(p + 1, "%d", &(cli_ports.RTCP));
                        ...
                    p = strstr(p, "=");
                    sscanf(p + 1, "%d", &rtp_ch);
                    ...
                    p = strstr(p, "=");
                    sscanf(p + 1, "%d", &rtp_ch);
                    ...


----------------------------------------
E] NULL pointer in parse_play_time_range
----------------------------------------

>From rtsp/RTSP_Play:

static RTSP_Error parse_play_time_range(RTSP_buffer * rtsp, play_args * args)
                ...
                q = strchr(q, '=');
                if (get_utc(&(args->playback_time), q + 1) != ERR_NOERROR) {
                ...


---------------------------------
F] NULL pointer in log_user_agent
---------------------------------

The log_user_agent function uses an unchecked strstr for finding the
end of the User-Agent value (a line field), but the server can handle
also carriage-return chars allowing an attacker to crash it using only
0x0d chars in his request.

>From rtsp/RTSP_utils.c:

void log_user_agent(RTSP_buffer * rtsp)
{
    char * p;

    if ((p = strstr(rtsp->in_buffer, HDR_USER_AGENT)) != NULL) {
        char cut[strlen(p)];
        strcpy(cut, p);
        p = strstr(cut, "\n");
        cut[strlen(cut) - strlen(p) - 1] = '\0';
        ...


----------------------------------
G] NULL pointer in Netembryo 0.0.4
----------------------------------

Although this vulnerability affects another software of the LScube
project I have preferred to include it here since only when used with
Feng this bug can be considered a security bug (Netembryo in fact is
used also in libnemesi which is a client-side library).

The usage of the ':' char after the backslash allows an attacker to
crash the server on which is used the Url_init() function of the
Netembryo library.
In short when an urlname like /: is used the port_begin and path_begin
pointers will point to the same location bypassing the check
"port_begin > path_begin", then the function will try to allocate
4294967294 bytes (-2 resulted by (size_t)(path_begin - port_begin - 1))
for url->port which will fail and so the resulted NULL pointer will be
used as destination in the subsequent strncpy.

>From utils/url.c:

int Url_init(Url * url, char * urlname)
{
    ...
    path_begin = strstr(hostname_begin, "/");
    if (path_begin == NULL) {
        path_len = 0;
    }
    else {
        ++path_begin;
    ...
    port_begin = strstr(hostname_begin, ":");
    if ((port_begin == NULL) || ((port_begin > path_begin) && (path_begin != NULL))) {
    ...
    if (port_len) {
        url->port = (char*)malloc(port_len+1);
        strncpy(url->port, port_begin, port_len);
        url->port[port_len] = '\0';
    }
    ...


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===========
3) The Code
===========


http://aluigi.org/poc/fengulo.zip

  nc SERVER 554 -v -v < bof1.txt
  ...
  nc SERVER 554 -v -v < netembry0.txt


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======
4) Fix
======


The bugs will be fixed in the next days.


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--- 
Luigi Auriemma
http://aluigi.org