ProFTPD mod_tls pre-authentication buffer overflow

[research@gleg.net]

Name:         ProFTPD mod_tls pre-authentication buffer overflow
Vendor:       http://www.proftpd.org
Release date: 28 Nov, 2006
Author:       Evgeny Legerov <research@gleg.net>

I. DESCRIPTION

A remote buffer overflow vulnerability has been found in mod_tls module of
ProFTPD server.
The vulnerability could allow a remote un-authenticated attacker to gain root
privileges.

II. DETAILS

Let's have a look at the code (ProFTPD version 1.3.0):

contrib/mod_tls.c:
"""
static char *tls_x509_name_oneline(X509_NAME *x509_name) {
  static char buf[256] = {'\0'};

  /* If we are using OpenSSL 0.9.6 or newer, we want to use
 * X509_NAME_print_ex()
   * instead of X509_NAME_oneline().
   */

#if OPENSSL_VERSION_NUMBER < 0x000906000L
  memset(&buf, '\0', sizeof(buf));
  return X509_NAME_oneline(x509_name, buf, sizeof(buf));
#else

  /* Sigh...do it the hard way. */
  BIO *mem = BIO_new(BIO_s_mem());
  char *data = NULL;
  long datalen = 0;
  int ok;

  if ((ok = X509_NAME_print_ex(mem, x509_name, 0, XN_FLAG_ONELINE)))
[1]  datalen = BIO_get_mem_data(mem, &data);

  if (data) {
    memset(&buf, '\0', sizeof(buf));
[2] memcpy(buf, data, datalen);
    buf[datalen] = '\0';
    buf[sizeof(buf)-1] = '\0';

    BIO_free(mem);
    return buf;
  }

  BIO_free(mem);
  return NULL;
#endif /* OPENSSL_VERSION_NUMBER >= 0x000906000 */
}
"""

The value of 'datalen' parameter is fully controlled by us (see [1]).
On line [2] we will be able to overflow the 'buf' buffer with our data.

III. VENDOR RESPONSE

Vendor has been notified on Nov 16, 2006 but ProFTPD 1.3.0a is still vulnerable.

IV. CREDIT

Discovered by Evgeny Legerov.

The vulnerability is a part of VulnDisco Pack Professional since Jan, 2006.