Microsoft Security Advisory (2269637) Insecure Library Loading Could Allow Remote Code Execution

Microsoft Security Advisory (2269637)
Insecure Library Loading Could Allow Remote Code Execution
Published: August 23, 2010

Version: 1.0
General Information
Executive Summary

Microsoft is aware that research has been published detailing a remote attack vector for a class of vulnerabilities that affects how applications load external libraries.

This issue is caused by specific insecure programming practices that allow so-called "binary planting" or "DLL preloading attacks". These practices could allow an attacker to remotely execute arbitrary code in the context of the user running the vulnerable application when the user opens a file from an untrusted location.

This issue is caused by applications passing an insufficiently qualified path when loading an external library. Microsoft has issued guidance to developers in the MSDN article, Dynamic-Link Library Security, on how to correctly use the available application programming interfaces to prevent this class of vulnerability. Microsoft is also actively reaching out to third-party vendors through the Microsoft Vulnerability Research Program to inform them of the mitigations available in the operating system. Microsoft is also actively investigating which of its own applications may be affected.

In addition to this guidance, Microsoft is releasing a tool that allows system administrators to mitigate the risk of this new attack vector by altering the library loading behavior system-wide or for specific applications. This advisory describes the functionality of this tool and other actions that customers can take to help protect their systems.

Mitigating Factors:
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This issue only affects applications that do not load external libraries securely. Microsoft has previously published guidelines for developers in the MSDN article, Dynamic-Link Library Security, that recommend alternate methods to load libraries that are safe against these attacks.
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For an attack to be successful, a user must visit an untrusted remote file system location or WebDAV share and open a document from this location that is then loaded by a vulnerable application.
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The file sharing protocol SMB is often disabled on the perimeter firewall. This limits the possible attack vectors for this vulnerability.
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Advisory Details
Affected and Non-Affected Software

Microsoft is investigating whether any of its own applications are affected by insecure library loading vulnerabilities and will take appropriate action to protect its customers.
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Frequently Asked Questions

What is the scope of the advisory?
Microsoft is aware of research published by a number of security researchers that describes a new remote attack vector for this known class of vulnerabilities. Applications are affected when they insufficiently qualify the path of an external library.

What causes this threat?
This exploit may occur when applications do not directly specify the fully qualified path to a library it intends to load. Depending on how the application is developed, Windows, instructed by the application, will search specific locations in the file system for the necessary library, and will load the file if found.

Some Application Programming Interfaces (API), such as SearchPath, use a search order that is intended for documents and not application libraries. Applications that use this API may try to load the library from the Current Working Directory (CWD), which may be controlled by an attacker. Other APIs may also lead to similar behavior, when used in specific ways described in the MSDN article, Dynamic-Link Library Security.

In the case of network shares, such as WebDAV or SMB, an attacker who can write to this location could upload a specially crafted library. In this scenario, the application attempts to load the specially crafted library, which can then execute arbitrary code on the client system in the security context of the logged-on user.

What might an attacker use this vulnerability to do?
An attacker who successfully exploited this vulnerability could gain the same user rights as a logged-on user. If the user is logged on with administrative user rights, an attacker who successfully exploited this vulnerability could take complete control of an affected system. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights.

In some cases, an attacker who already has access to a local folder on the system could use a DLL preloading vulnerability in a local application running with elevated privileges to elevate his access to the system.

How could an attacker exploit this vulnerability?
This vulnerability requires that the attacker convince the user to open a file using a vulnerable program, from a remote network location. When the application loads one of its required or optional libraries, the vulnerable application may attempt to load the library from the remote network location. If the attacker provides a specially crafted library at this location, the attacker may succeed at executing arbitrary code on the user's machine.

What are the remote attack vectors for this vulnerability?
This vulnerability can be exploited over network file systems such as (but not limited to) WebDAV and SMB. An attacker can offer a file for download over any such protocol. If the application used to open this file does not load external libraries securely, the user opening that file could be exposed to this vulnerability.

Is this a security vulnerability that requires Microsoft to issue a security update?
This vulnerability may require third-party vendors to issue a security update for their respective affected applications. As part of this security advisory, Microsoft is releasing an optional mitigation tool that helps customers address the risk of the remote attack vector through a per-application and global configuration setting.

Microsoft is also investigating whether any of its own applications are affected by DLL preloading vulnerabilities and will take appropriate action to protect its customers.

What is a Dynamic Link Library (DLL)?
A DLL is a library that contains code and data that can be used by more than one program at the same time. For example, in Windows operating systems, the Comdlg32 DLL performs common dialog box related functions. Therefore, each program can use the functionality that is contained in this DLL to implement an Open dialog box. This helps promote code reuse and efficient memory usage.

By using a DLL, a program can be modularized into separate components. For example, an accounting program may be sold by module. Each module can be loaded into the main program at run time if that module is installed. Because the modules are separate, the load time of the program is faster, and a module is only loaded when that functionality is requested.

What is Web-based Distributed Authoring and Versioning (WebDAV)?
Web-based Distributed Authoring and Versioning (WebDAV) extends the HTTP/1.1 protocol to allow clients to publish, lock, and manage resources on the Web. Integrated into IIS, WebDAV allows clients to do the following:
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Manipulate resources in a WebDAV publishing directory on your server. For example, users who have been assigned the correct rights can copy and move files around in a WebDAV directory.
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Modify properties associated with certain resources. For example, a user can write to and retrieve a file's property information.
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Lock and unlock resources so that multiple users can read a file concurrently.
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Search the content and properties of files in a WebDAV directory.

What is Microsoft Server Message Block (SMB) protocol?
Microsoft Server Message Block (SMB) Protocol is a Microsoft network file sharing protocol used in Microsoft Windows. For more information on SMB, see MSDN article, Microsoft SMB Protocol and CIFS Protocol Overview.

Where can developers find guidance on how to avoid this issue?
Microsoft has published the MSDN article, Dynamic-Link Library Security, which describes the various Application Programming Interfaces (APIs) available on Windows that allow developers to correctly and securely load external libraries.

Microsoft is working with developers through the Microsoft Vulnerability Research Program to share information with them on how to prevent this vulnerability in their products. Software vendors and ISVs that have questions on the mitigations available in Windows for this issue are invited to contact [email protected] for additional mitigation information.
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Mitigating Factors and Suggested Actions

Mitigating Factors

Mitigation refers to a setting, common configuration, or general best-practice, existing in a default state, that could reduce the severity of exploitation of this issue. The following mitigating factors may be helpful in your situation:
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This issue only affects applications that do not load external libraries securely. Microsoft has previously published guidelines for developers that recommend alternate methods to load libraries that are safe against these attacks.
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For an attack to be successful, a user must visit an untrusted remote file system location or WebDAV share and open a document from this location that is then loaded by a vulnerable application.
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The file sharing protocol SMB is often disabled on the perimeter firewall. This limits the possible attack vectors for this vulnerability.
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Workarounds

Workaround refers to a setting or configuration change that does not correct the underlying issue but would help block known attack vectors before a security update is available. Microsoft has tested the following workarounds and states in the discussion whether a workaround reduces functionality:
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Disable loading of libraries from WebDAV and remote network shares

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.
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Disable the WebClient service

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 is still possible for remote attackers who successfully exploit 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:

Click Start, click Run, type Services.msc and then click OK.

Right-click WebClient service and select Properties.

Change the Startup type to Disabled. If the service is running, click Stop.

Click OK and exit the management application.

Impact of workaround. When the WebClient service is disabled, Web Distributed Authoring and Versioning (WebDAV) requests are not transmitted. In addition, any services that explicitly depend on the Web Client service will not start, and an error message will be logged in the System log. For example, WebDAV shares will be inaccessible from the client computer.

How to undo the workaround.

To re-enable the WebClient Service, follow these steps:

Click Start, click Run, type Services.msc and then click OK.

Right-click WebClient service and select Properties.

Change the Startup type to Automatic. If the service is not running, click Start.

Click OK and exit the management application.
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Block TCP ports 139 and 445 at the firewall

These ports are used to initiate a connection with the affected component. Blocking TCP ports 139 and 445 at the firewall will help protect systems that are behind that firewall from attempts to exploit this vulnerability. Microsoft recommends that you block all unsolicited inbound communication from the Internet to help prevent attacks that may use other ports. For more information about ports, see the TechNet article, TCP and UDP Port Assignments.

Impact of workaround. Several Windows services use the affected ports. Blocking connectivity to the ports may cause various applications or services to not function. Some of the applications or services that could be impacted are listed below:
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Applications that use SMB (CIFS)
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Applications that use mailslots or named pipes (RPC over SMB)
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Server (File and Print Sharing)
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Group Policy
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Net Logon
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Distributed File System (DFS)
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Terminal Server Licensing
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Print Spooler
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Computer Browser
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Remote Procedure Call Locator
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Fax Service
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Indexing Service
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Performance Logs and Alerts
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Systems Management Server
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License Logging Service

How to undo the workaround. Unblock TCP ports 139 and 445 at the firewall. For more information about ports, see TCP and UDP Port Assignments.

Additional Suggested Actions
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Install updates from third-party vendors that address insecure library loading

Third-party vendors may release updates that address insecure library loading in their products. Microsoft recommends that customers contact their vendor if they have any questions whether or not a specific application is affected by this issue, and monitor for security updates released by these vendors.
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Protect Your Computer

We continue to encourage customers to follow our Protect Your Computer guidance of enabling a firewall, getting software updates and installing antivirus software. Customers can learn more about these steps by visiting Protect Your Computer.

For more information about staying safe on the Internet, visit Microsoft Security Central.
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Keep Windows updated

All Windows users should apply the latest Microsoft security updates to help make sure that their computers are as protected as possible. If you are not sure whether your software is up to date, visit Windows Update, scan your computer for available updates, and install any high-priority updates that are offered to you. If you have Automatic Updates enabled, the updates are delivered to you when they are released, but you have to make sure you install them.

Other Information
Microsoft Active Protections Program (MAPP)

To improve security protections for customers, Microsoft provides vulnerability information to major security software providers in advance of each monthly security update release. Security software providers can then use this vulnerability information to provide updated protections to customers via their security software or devices, such as antivirus, network-based intrusion detection systems, or host-based intrusion prevention systems. To determine whether active protections are available from security software providers, please visit the active protections Web sites provided by program partners, listed in Microsoft Active Protections Program (MAPP) Partners.

Feedback
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You can provide feedback by completing the Microsoft Help and Support form, Customer Service Contact Us.

Support
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Customers in the United States and Canada can receive technical support from Security Support. For more information about available support options, see Microsoft Help and Support.
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International customers can receive support from their local Microsoft subsidiaries. For more information about how to contact Microsoft for international support issues, visit International Support.
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Microsoft TechNet Security provides additional information about security in Microsoft products.

Disclaimer

The information provided in this advisory is provided "as is" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.
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Revisions
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V1.0 (August 23, 2010) Advisory published.