CVE-2019-15693 (published 2019-12-26T15:15Z)

TigerVNC version prior to 1.10.1 is vulnerable to heap buffer overflow, which occurs in TightDecoder::FilterGradient. Exploitation of this vulnerability could potentially result into remote code execution. This attack appear to be exploitable via network connectivity.

Severity: MEDIUM
CVSS v2 base score: 6.5 (AV:N/AC:L/Au:S/C:P/I:P/A:P)
CVSS v3 base score: 7.2, HIGH (CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H)

Weakness information:
cwe_id: CWE-787
cwe_name: Out-of-bounds Write
description: The product writes data past the end, or before the beginning, of the intended buffer.
extended_description: 
            
            
             
			 
         
         

Fix information:
repo_url: https://github.com/CendioOssman/tigervnc
commit_details: repo_url: https://github.com/CendioOssman/tigervnc
author: Pierre Ossman
author_date: 2019-09-10 15:36:42+02:00
msg: Handle empty Tight gradient rects

We always assumed there would be one pixel per row so a rect with
a zero width would result in us writing to unknown memory.

This could theoretically be used by a malicious server to inject
code in to the viewer process.

Issue found by Pavel Cheremushkin from Kaspersky Lab.
repository: repo_url: https://github.com/CendioOssman/tigervnc
repo_name: CendioOssman/tigervnc
repo_language: C++
owner: CendioOssman
file_changes: filename: tightDecode.h
change_type: ModificationType.MODIFY
diff: @@ -56,15 +56,17 @@ TightDecoder::FilterGradient24(const rdr::U8 *inbuf,
   int rectWidth = r.width();
 
   for (y = 0; y < rectHeight; y++) {
-    /* First pixel in a row */
-    for (c = 0; c < 3; c++) {
-      pix[c] = inbuf[y*rectWidth*3+c] + prevRow[c];
-      thisRow[c] = pix[c];
-    }
-    pf.bufferFromRGB((rdr::U8*)&outbuf[y*stride], pix, 1);
+    for (x = 0; x < rectWidth; x++) {
+      /* First pixel in a row */
+      if (x == 0) {
+        for (c = 0; c < 3; c++) {
+          pix[c] = inbuf[y*rectWidth*3+c] + prevRow[c];
+          thisRow[c] = pix[c];
+        }
+        pf.bufferFromRGB((rdr::U8*)&outbuf[y*stride], pix, 1);
+        continue;
+      }
 
-    /* Remaining pixels of a row */
-    for (x = 1; x < rectWidth; x++) {
       for (c = 0; c < 3; c++) {
         est[c] = prevRow[x*3+c] + pix[c] - prevRow[(x-1)*3+c];
         if (est[c] > 0xff) {
@@ -103,17 +105,20 @@ void TightDecoder::FilterGradient(const rdr::U8* inbuf,
   int rectWidth = r.width();
 
   for (y = 0; y < rectHeight; y++) {
-    /* First pixel in a row */
-    pf.rgbFromBuffer(pix, &inbuf[y*rectWidth], 1);
-    for (c = 0; c < 3; c++)
-      pix[c] += prevRow[c];
+    for (x = 0; x < rectWidth; x++) {
+      /* First pixel in a row */
+      if (x == 0) {
+        pf.rgbFromBuffer(pix, &inbuf[y*rectWidth], 1);
+        for (c = 0; c < 3; c++)
+          pix[c] += prevRow[c];
 
-    memcpy(thisRow, pix, sizeof(pix));
+        memcpy(thisRow, pix, sizeof(pix));
 
-    pf.bufferFromRGB((rdr::U8*)&outbuf[y*stride], pix, 1);
+        pf.bufferFromRGB((rdr::U8*)&outbuf[y*stride], pix, 1);
+
+        continue;
+      }
 
-    /* Remaining pixels of a row */
-    for (x = 1; x < rectWidth; x++) {
       for (c = 0; c < 3; c++) {
         est[c] = prevRow[x*3+c] + pix[c] - prevRow[(x-1)*3+c];
         if (est[c] > 255) {

programming_language: C++