CVE-2020-26117 (Other, published 2020-09-27T04:15Z)
Description: value: In rfb/CSecurityTLS.cxx and rfb/CSecurityTLS.java in TigerVNC before 1.11.0, viewers mishandle TLS certificate exceptions. They store the certificates as authorities, meaning that the owner of a certificate could impersonate any server after a client had added an exception.
Severity: MEDIUM (CVSS v3 base score: 8.1)
Weakness: CWE-295 \u2014 Improper Certificate Validation
The product does not validate, or incorrectly validates, a certificate.
Repository: https://github.com/TigerVNC/tigervnc
Files changed:
common/rfb/CSecurityTLS.cxx
Commit message: Properly store certificate exceptions

The previous method stored the certificates as authorities, meaning that
the owner of that certificate could impersonate any server it wanted
after a client had added an exception.

Handle this more properly by only storing exceptions for specific
hostname/certificate combinations, the same way browsers or SSH does
things.


Vulnerable code:
    /* Load previously saved certs */
    char *homeDir = NULL;
    int err;
    if (getvnchomedir(&homeDir) == -1)
      vlog.error("Could not obtain VNC home directory path");
    else {
      CharArray caSave(strlen(homeDir) + 19 + 1);
      sprintf(caSave.buf, "%sx509_savedcerts.pem", homeDir);
      delete [] homeDir;

      err = gnutls_certificate_set_x509_trust_file(cert_cred, caSave.buf,
                                                   GNUTLS_X509_FMT_PEM);
      if (err < 0)
        vlog.debug("Failed to load saved server certificates from %s", caSave.buf);
    }

    char buf[255];
    snprintf(buf, sizeof(buf), "Hostname (%s) does not match any certificate, "
			       "do you want to continue?", client->getServerName());
    buf[sizeof(buf) - 1] = '\0';
    if (!msg->showMsgBox(UserMsgBox::M_YESNO, "hostname mismatch", buf))
      throw AuthFailureException("hostname mismatch");
  vlog.debug("Saved server certificates don't match");
  if (gnutls_x509_crt_print(crt, GNUTLS_CRT_PRINT_ONELINE, &info)) {
    /*
     * GNUTLS doesn't correctly export gnutls_free symbol which is
     * a function pointer. Linking with Visual Studio 2008 Express will
     * fail when you call gnutls_free().
     */
#if WIN32
    free(info.data);
#else
    gnutls_free(info.data);
#endif
    throw AuthFailureException("Could not find certificate to display");
  size_t out_size = 0;
  char *out_buf = NULL;
  char *certinfo = NULL;
  int len = 0;

  vlog.debug("certificate issuer unknown");

  len = snprintf(NULL, 0, "This certificate has been signed by an unknown "
                          "authority:\n\n%s\n\nDo you want to save it and "
                          "continue?\n ", info.data);
  if (len < 0)
    throw AuthFailureException("certificate decoding error");

  vlog.debug("%s", info.data);

  certinfo = new char[len];

  snprintf(certinfo, len, "This certificate has been signed by an unknown "
                          "authority:\n\n%s\n\nDo you want to save it and "
                          "continue? ", info.data);
  for (int i = 0; i < len - 1; i++)
    if (certinfo[i] == ',' && certinfo[i + 1] == ' ')
      certinfo[i] = '\n';
  if (!msg->showMsgBox(UserMsgBox::M_YESNO, "certificate issuer unknown",
		       certinfo)) {
    delete [] certinfo;
    throw AuthFailureException("certificate issuer unknown");
  delete [] certinfo;

  if (gnutls_x509_crt_export(crt, GNUTLS_X509_FMT_PEM, NULL, &out_size)
      != GNUTLS_E_SHORT_MEMORY_BUFFER)
    throw AuthFailureException("certificate issuer unknown, and certificate "
                               "export failed");
  // Save cert
  out_buf =  new char[out_size];
  if (gnutls_x509_crt_export(crt, GNUTLS_X509_FMT_PEM, out_buf, &out_size) < 0)
    throw AuthFailureException("certificate issuer unknown, and certificate "
                               "export failed");
  char *homeDir = NULL;
  if (getvnchomedir(&homeDir) == -1)
    vlog.error("Could not obtain VNC home directory path");
  else {
    FILE *f;
    CharArray caSave(strlen(homeDir) + 1 + 19);
    sprintf(caSave.buf, "%sx509_savedcerts.pem", homeDir);
    delete [] homeDir;
    f = fopen(caSave.buf, "a+");
    if (!f)
      msg->showMsgBox(UserMsgBox::M_OK, "certificate save failed",
                      "Could not save the certificate");
    else {
      fprintf(f, "%s\n", out_buf);
      fclose(f);
    }
  delete [] out_buf;

Fixed code:

  char *homeDir;
  size_t len;
    CharArray text;
    text.format("Hostname (%s) does not match the server certificate, "
                "do you want to continue?", client->getServerName());
    if (!msg->showMsgBox(UserMsgBox::M_YESNO,
                         "Certificate hostname mismatch", text.buf))
      throw AuthFailureException("Certificate hostname mismatch");
  /* Certificate is fine, except we don't know the issuer, so TOFU time */
  homeDir = NULL;
  if (getvnchomedir(&homeDir) == -1) {
    throw AuthFailureException("Could not obtain VNC home directory "
                               "path for known hosts storage");
  CharArray dbPath(strlen(homeDir) + 16 + 1);
  sprintf(dbPath.buf, "%sx509_known_hosts", homeDir);
  delete [] homeDir;
  err = gnutls_verify_stored_pubkey(dbPath.buf, NULL,
                                    client->getServerName(), NULL,
                                    GNUTLS_CRT_X509, &cert_list[0], 0);
  /* Previously known? */
  if (err == GNUTLS_E_SUCCESS) {
    vlog.debug("Server certificate found in known hosts file");
    gnutls_x509_crt_deinit(crt);
    return;
  if ((err != GNUTLS_E_NO_CERTIFICATE_FOUND) &&
      (err != GNUTLS_E_CERTIFICATE_KEY_MISMATCH)) {
    throw AuthFailureException("Could not load known hosts database");
  }
  if (gnutls_x509_crt_print(crt, GNUTLS_CRT_PRINT_ONELINE, &info))
    throw AuthFailureException("Could not find certificate to display");
  len = strlen((char*)info.data);
  for (size_t i = 0; i < len - 1; i++) {
    if (info.data[i] == ',' && info.data[i + 1] == ' ')
      info.data[i] = '\n';
  }
  /* New host */
  if (err == GNUTLS_E_NO_CERTIFICATE_FOUND) {
    CharArray text;

    vlog.debug("Server host not previously known");
    vlog.debug("%s", info.data);

    text.format("This certificate has been signed by an unknown "
                "authority:\n\n%s\n\nSomeone could be trying to "
                "impersonate the site and you should not "
                "continue.\n\nDo you want to make an exception "
                "for this server?", info.data);

    if (!msg->showMsgBox(UserMsgBox::M_YESNO,
                         "Unknown certificate issuer",
                         text.buf))
      throw AuthFailureException("Unknown certificate issuer");
  } else if (err == GNUTLS_E_CERTIFICATE_KEY_MISMATCH) {
    CharArray text;

    vlog.debug("Server host key mismatch");
    vlog.debug("%s", info.data);

    text.format("This host is previously known with a different "
                "certificate, and the new certificate has been "
                "signed by an unknown authority:\n\n%s\n\nSomeone "
                "could be trying to impersonate the site and you "
                "should not continue.\n\nDo you want to make an "
                "exception for this server?", info.data);

    if (!msg->showMsgBox(UserMsgBox::M_YESNO,
                         "Unexpected server certificate",
                         text.buf))
      throw AuthFailureException("Unexpected server certificate");
  if (gnutls_store_pubkey(dbPath.buf, NULL, client->getServerName(),
                          NULL, GNUTLS_CRT_X509, &cert_list[0], 0, 0))
    vlog.error("Failed to store server certificate to known hosts database");