Openwall GNU/*/Linux - a small security-enhanced Linux distro for servers
[<prev] [next>] [day] [month] [year] [list]
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

Powered by blists - more mailing lists

Your e-mail address:

Please check out the Open Source Software Security Wiki, which is counterpart to this mailing list.

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.