oCERT-2014-007 libvncserver multiple issues


Virtual Network Computing (VNC) is a graphical sharing system based on the Remote Frame Buffer (RFB) protocol.

The LibVNCServer project, an open source library for implementing VNC compliant communication, suffers from a number of bugs that can be potentially exploited with security impact.

Various implementation issues resulting in remote code execution and/or DoS conditions on both the VNC server and client side have been discovered.

  1. A malicious VNC server can trigger incorrect memory management handling by advertising a large screen size parameter to the VNC client. This would result in multiple memory corruptions and could allow remote code execution on the VNC client.
  2. A malicious VNC client can trigger multiple DoS conditions on the VNC server by advertising a large screen size, ClientCutText message length and/or a zero scaling factor parameter.
  3. A malicious VNC client can trigger multiple stack-based buffer overflows by passing a long file and directory names and/or attributes (FileTime) when using the file transfer message feature.

It should be noted that every described issue represents a post-authentication bug, therefore the server side conditions can be anonymously leveraged only if the VNC server is configured to allow unauthenticated sessions.

Affected version:

LibVNCServer <= 0.9.9

Fixed version:

LibVNCServer, N/A

Credit: vulnerability report received from Nicolas Ruff of Google Security Team <nruff AT google.com>.

CVE: CVE-2014-6051 (1), CVE-2014-6052 (1), CVE-2014-6053 (2), CVE-2014-6054 (2), CVE-2014-6055 (3)


2014-09-05: vulnerability report received
2014-09-16: contacted affected vendors
2014-09-22: contacted additional affected vendors
2014-09-25: advisory release

(1) https://github.com/newsoft/libvncserver/commit/045a044e8ae79db9244593fbce154cdf6e843273
(2) https://github.com/newsoft/libvncserver/commit/6037a9074d52b1963c97cb28ea1096c7c14cbf28
(2) https://github.com/newsoft/libvncserver/commit/05a9bd41a8ec0a9d580a8f420f41718bdd235446
(3) https://github.com/newsoft/libvncserver/commit/06ccdf016154fde8eccb5355613ba04c59127b2e
(3) https://github.com/newsoft/libvncserver/commit/f528072216dec01cee7ca35d94e171a3b909e677