Vulnerability Note VU#707943

Microsoft Windows based applications may insecurely load dynamic libraries

If an attacker can cause an affected application to call LoadLibrary() while the application's current directory is set to one controlled by the attacker, that application may run the attacker's code from a specially named DLL also supplied in that directory. This can occur when the affected application opens a normal file typically associated with it from the attacker-controlled directory. The specific name of the DLL that an attacker would need to choose varies depending on the affected application.

II. Impact

In some cases of affected applications, an attacker who already has access to a local folder on the system could use this vulnerability in a local application running with elevated privileges to escalate their own privileges on the system.

III. Solution

The vulnerability described generically above can be manifest in a variety of software products. Please see the Vendor Information section of this document for information about specific applications that may be affected by this issue.

For Developers:

Ensure that applications do not load libraries from insecure locations

Developers of applications for the Windows platform should ensure that their applications call SetDllDirectory() with a blank path before calling LoadLibrary() to ensure that the DLL is not loaded from the current directory. More information about how to load libraries securely can be found in the following Microsoft articles: Dynamic-Link Library Security and Another technique for Fixing DLL Preloading attacks.

Disable loading of libraries from the current working directory

According to Microsoft Security Advisory 2269637:

Note This workaround requires installation of the tool described in Microsoft Knowledge Base Article 2264107.

Microsoft has released a tool which allows customers to disable the loading of libraries from remote network or WebDAV shares. This tool can be configured to disallow insecure loading on a per-application or a global system basis.

Customers who are informed by their vendor of an application being vulnerable can use this tool to help protect against attempts to exploit this issue.

After the update listed in KB article 2264107 has been installed, the following registry value can be used to remove the current working directory from the default DLL search order:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager]
"CWDIllegalInDllSearch"=dword:ffffffff

Note that making this change may cause some applications to not behave properly.

Disable the WebClient service

According to Microsoft Security Advisory 2269637:

Disabling the WebClient service helps protect affected systems from attempts to exploit this vulnerability by blocking the most likely remote attack vector through the Web Distributed Authoring and Versioning (WebDAV) client service. After applying this workaround, it will still be possible for remote attackers who successfully exploited this vulnerability to cause Microsoft Office Outlook to run programs located on the targeted user's computer or the Local Area Network (LAN), but users will be prompted for confirmation before opening arbitrary programs from the Internet.

To disable the WebClient Service, follow these steps:
1. Click Start, click Run, type Services.msc and then click OK.
2. Right-click WebClient service and select Properties.
3. Change the Startup type to Disabled. If the service is running, click Stop.
4. Click OK and exit the management application.

While this workaround does not remove the vulnerability, it does block an attack vector for this vulnerability.

Block outgoing SMB traffic

Block outgoing connections on ports 139/tcp, 139/udp, 445/tcp, and 445/udp at your network perimeter. Doing so will help prevent machines on the local network from connecting to SMB servers on the internet. While this does not remove the vulnerability, it does block an attack vector for this vulnerability.

Instances and variations of this vulnerability were independently discovered by a number of researchers, including Georgi Guninski; Simon Raner, Jure Skofic and Mitja Kolsek of ACROS Security; Taeho Kwon and Zhendong Su; H.D. Moore. Some vendor information comes from Secunia.

This document was written by Chad R Dougherty.

Other Information

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