Vulnerability: LibVNC through 0.9.12 contains a heap out-of-bounds write vulnerability in libvncserver/rfbserver.c. The fix for CVE-2018-15127 was incomplete.
Vulnerability ID: CVE-2018-20750
CWE: CWE-787
Severity score: 6.4
Project: https://github.com/LibVNC/libvncserver
Published: 2019-01-30
File: libvncserver/rfbserver.c
Commit message: Limit lenght to INT_MAX bytes in rfbProcessFileTransferReadBuffer()

This ammends 15bb719c03cc70f14c36a843dcb16ed69b405707 fix for a heap
out-of-bound write access in rfbProcessFileTransferReadBuffer() when
reading a transfered file content in a server. The former fix did not
work on platforms with a 32-bit int type (expected by rfbReadExact()).

CVE-2018-15127
<https://github.com/LibVNC/libvncserver/issues/243>
<https://github.com/LibVNC/libvncserver/issues/273>
Commit author: Petr Písař

Vulnerable version (before fix):
 #include <errno.h> /* strftime() */ #include <time.h>  #ifdef LIBVNCSERVER_WITH_WEBSOCKETS #include "rfbssl.h" char *rfbProcessFileTransferReadBuffer(rfbClientPtr cl, uint32_t length)        0XFFFFFFFF, i.e. SIZE_MAX for 32-bit systems. On 64-bit systems, a length of 0XFFFFFFFF        will safely be allocated since this check will never trigger and malloc() can digest length+1        without problems as length is a uint32_t.     */     if(length == SIZE_MAX) { 	rfbErr("rfbProcessFileTransferReadBuffer: too big file transfer length requested: %u", (unsigned int)length); 	rfbCloseClient(cl); 	return NULL;

Fixed version (after fix):
 #include <errno.h> /* strftime() */ #include <time.h> /* INT_MAX */ #include <limits.h>  #ifdef LIBVNCSERVER_WITH_WEBSOCKETS #include "rfbssl.h" char *rfbProcessFileTransferReadBuffer(rfbClientPtr cl, uint32_t length)        0XFFFFFFFF, i.e. SIZE_MAX for 32-bit systems. On 64-bit systems, a length of 0XFFFFFFFF        will safely be allocated since this check will never trigger and malloc() can digest length+1        without problems as length is a uint32_t.        We also later pass length to rfbReadExact() that expects a signed int type and        that might wrap on platforms with a 32-bit int type if length is bigger        than 0X7FFFFFFF.     */     if(length == SIZE_MAX || length > INT_MAX) { 	rfbErr("rfbProcessFileTransferReadBuffer: too big file transfer length requested: %u", (unsigned int)length); 	rfbCloseClient(cl); 	return NULL;

Patch:
patch: @@ -88,6 +88,8 @@
 #include <errno.h>
 /* strftime() */
 #include <time.h>
+/* INT_MAX */
+#include <limits.h>
 
 #ifdef LIBVNCSERVER_WITH_WEBSOCKETS
 #include "rfbssl.h"
@@ -1472,8 +1474,11 @@ char *rfbProcessFileTransferReadBuffer(rfbClientPtr cl, uint32_t length)
        0XFFFFFFFF, i.e. SIZE_MAX for 32-bit systems. On 64-bit systems, a length of 0XFFFFFFFF
        will safely be allocated since this check will never trigger and malloc() can digest length+1
        without problems as length is a uint32_t.
+       We also later pass length to rfbReadExact() that expects a signed int type and
+       that might wrap on platforms with a 32-bit int type if length is bigger
+       than 0X7FFFFFFF.
     */
-    if(length == SIZE_MAX) {
+    if(length == SIZE_MAX || length > INT_MAX) {
 	rfbErr("rfbProcessFileTransferReadBuffer: too big file transfer length requested: %u", (unsigned int)length);
 	rfbCloseClient(cl);
 	return NULL;
path: libvncserver/rfbserver.c