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Date: Mon, 10 Dec 2018 19:57:21 +0100
From: Solar Designer <solar@...nwall.com>
To: Pavel Cheremushkin <Pavel.Cheremushkin@...persky.com>
Cc: oss-security@...ts.openwall.com
Subject: Re: libvnc and tightvnc vulnerabilities

On Mon, Dec 10, 2018 at 04:08:00PM +0000, Pavel Cheremushkin wrote:
> These particular issues I was describing in my previous letter are located in source code of TightVNC vncviewer. Source code of TightVNC 1.3.10 vncviewer can be acquired though this link https://www.tightvnc.com/download/1.3.10/tightvnc-1.3.10_unixsrc.tar.gz and integer overflow that leads to a heap-buffer-overflow I was speaking about is located on the line 1220 inside file `vnc_unixsrc/vncviewer/rfbproto.c`. It is a fun fact that inside `libvncclient/rfbproto.c` the same code is located on line 2220, but all bugs connected with LibVNC I described in Github issues inside LibVNC repository.

Oh.  So you reported the instance of that one issue in LibVNC here:

https://github.com/LibVNC/libvncserver/issues/247

Upstream's fix appears to be to add casts to (uint64_t) before adding 1
in those many malloc() calls.  On platforms with larger than 32-bit
size_t, this should be sufficient against integer overflows since the
sizes are read from 32-bit protocol fields, but it isn't sufficient to
prevent maliciously large memory allocation on the client by a rogue
server.  On a platform with 32-bit size_t, this isn't even sufficient to
prevent the integer overflows.  If I haven't missed anything, it'd be
great if you open a new issue suggesting introduction of safety limits
prior to those malloc() lines.

The current code is:

  case rfbServerCutText:
  {
    char *buffer;

    if (!ReadFromRFBServer(client, ((char *)&msg) + 1,
			   sz_rfbServerCutTextMsg - 1))
      return FALSE;

    msg.sct.length = rfbClientSwap32IfLE(msg.sct.length);

    buffer = malloc((uint64_t)msg.sct.length+1);

    if (!ReadFromRFBServer(client, buffer, msg.sct.length)) {
      free(buffer);
      return FALSE;
    }

    buffer[msg.sct.length] = 0;

    if (client->GotXCutText)
      client->GotXCutText(client, buffer, msg.sct.length);

    free(buffer);

    break;
}

but per the commits referenced in issue #247 above, there are many more
instances of the "malloc(... + 1)" pattern, which were patched similarly
incompletely.

Thanks,

Alexander

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