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Date: Tue, 23 Sep 2014 23:47:05 +0200
From: Nicolas RUFF <>
Subject: Multiple issues in libVNCserver

Hello list,

I am currently reviewing libVNCserver for potential security issues.

Project home is:

Here are my findings so far. Found bugs are either client- or server-side
(libVNCserver provides both). All bugs require authentication. However they
still pose a significant issue in the following scenarios:
* VNC server is running in password-less mode because authentication is
required afterwards (e.g. X11 login screen).
* Attacker is located in a man-in-the-middle position.
* Attacker has legitimate access to the VNC server but possibly not to the
host server (e.g. virtual machine, remote assistance to a customer).

Fixes are being committed - a few of them are still pending in the pull

CVE-2014-6051 Integer overflow in MallocFrameBuffer() on client side.

A malicious VNC server could advertise a very large screen size (by RFB
protocol, width and height are 16-bit integers), resulting in an integer
overflow during malloc() on client-side. Heap corruption, and possibly
remote code execution on client-side could ensue.

CVE-2014-6052 Lack of malloc() return value checking on client side.

malloc() return value was not checked on client-side during framebuffer
setup. A malicious VNC server that advertises a large enough screen size to
make malloc() fail could basically map the framebuffer at address 0, and
write anything-anywhere in client process memory using selective
FramebufferUpdate messages. This could certainly turn into remote code
execution on client-side.

CVE-2014-6053 Server crash on a very large ClientCutText message.

A malicious client could advertise a very large ClientCutText message size
(by RFB protocol, size is encoded on a 32-bit integer). malloc() is likely
to fail in that case; as malloc() return value is not checked, this will
most likely result in a server crash.

Note: this issue also affects RealVNC as per CVE-2010-5304.

CVE-2014-6054 Server crash when scaling factor is set to zero.

A malicious client could set the scaling factor to 0, which will result in
a server crash (division by zero).

CVE-2014-6055 Multiple stack overflows in File Transfer feature.

1/ The non-standard file transfer messages (UltraVNC feature) will blindly
strcpy() client-provided file and directory names into a stack-based buffer
of size MAX_PATH, resulting in multiple stack-based buffer overflows on

2/ Client-supplied FileTime attribute is copied into a stack-based
buffer of size 64 during rfbFileTransferOffer message parsing, resulting in
a stack-based buffer overflow on server-side.

- Nicolas RUFF

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