Microsoft Security Bulletin MS10-097 - Important

Mitigation refers to a setting, common configuration, or general best-practice, existing in a default state, that could reduce the severity of exploitation of a vulnerability. The following mitigating factors may be helpful in your situation:

• 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.
• The file sharing protocol, Server Message Block (SMB), is often disabled on the perimeter firewall. This limits the possible attack vectors for this vulnerability.

Workaround refers to a setting or configuration change that does not correct the underlying vulnerability but would help block known attack vectors before you apply the update. Microsoft has tested the following workarounds and states in the discussion whether a workaround reduces functionality:

• Disable loading of libraries from WebDAV and remote network shares

  Note See Microsoft Knowledge Base Article 2264107 to deploy a workaround tool that 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.

  Note See Microsoft Knowledge Base Article 2264107 to use the automated Microsoft Fix it solution to deploy the registry key to block loading of libraries for SMB and WebDAV shares. Note that this Fix it solution does require you to install the workaround tool also described in Microsoft Knowledge Base Article 2264107 first. This Fix it solution only deploys the registry key and requires the workaround tool in order to be effective. We recommend that administrators review the KB article closely prior to deploying this Fix it solution.

• 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 the system 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, perform the following 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.

  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, perform the following 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 Automatic. If the service is not running, click Start.
  4. Click OK and exit the management application.
• 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:

  • Applications that use SMB (CIFS)
  • Applications that use mailslots or named pipes (RPC over SMB)
  • Server (File and Print Sharing)
  • Group Policy
  • Net Logon
  • Distributed File System (DFS)
  • Terminal Server Licensing
  • Print Spooler
  • Computer Browser
  • Remote Procedure Call Locator
  • Fax Service
  • Indexing Service
  • Performance Logs and Alerts
  • Systems Management Server
  • 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.

What is the scope of the vulnerability? 
This is a remote code execution vulnerability. 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.

What causes the vulnerability? 
This vulnerability is caused when the Internet Connection Signup Wizard improperly restricts the path used for loading external libraries.

What is the Internet Connection Signup Wizard?
The Internet Connection Signup Wizard enables users to configure a connection on a new computer, or to use an existing Internet service account that they had on a previous computer. The wizard also helps users whose Internet settings have been corrupted. The automatic configuration feature restores settings — the user only needs to supply the user ID, password, and the name of the point of presence (POP) server.

What might an attacker use the vulnerability to do? 
An attacker who successfully exploited this vulnerability could run arbitrary code as the logged-on user. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. If a user is logged-on with administrative user rights, an attacker could take complete control of the affected system. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.

How could an attacker exploit the vulnerability? 
An attacker could convince a user to open a legitimate .ins or .isp file that is located in the same network directory as a specially crafted dynamic link library (DLL) file. Then, while opening the file, the Internet Connection Signup Wizard could attempt to load the DLL file and execute any code it contained.

In an e-mail attack scenario, an attacker could exploit the vulnerability by sending a legitimate .ins or .isp attachment to a user, and convincing the user to place the attachment into a directory containing a specially crafted DLL file and to open the .ins or .isp file. Then, while opening the .ins or .isp file, the Internet Connection Signup Wizard could attempt to load the DLL file and execute any code it contained.

In a network scenario, an attacker could place a legitimate .ins or .isp file and a specially crafted DLL in a UNC or WebDAV location and then convince the user to open the legitimate file. An attacker could also host a Web site, and refer to the legitimate file from this Web site, causing the vulnerability to be triggered through a Web browser.

What systems are primarily at risk from the vulnerability? 
This vulnerability mainly affects workstations where users may be expected to open attachments from untrusted network locations. Servers could also be at risk if administrators allow users to log on to servers and to run programs. However, best practices strongly discourage allowing this.

What does the update do? 
The update addresses this vulnerability by correcting the manner in which the Internet Connection Signup Wizard loads external libraries.

When this security bulletin was issued, had this vulnerability been publicly disclosed? 
Yes. This vulnerability has been publicly disclosed. It has been assigned Common Vulnerability and Exposure number CVE-2010-3144.

When this security bulletin was issued, had Microsoft received any reports that this vulnerability was being exploited? 
No. Microsoft had seen examples of proof of concept code published publicly but had not received any information to indicate that this vulnerability had been publicly used to attack customers when this security bulletin was originally issued.

Reference Table

The following table contains the security update information for this software. You can find additional information in the subsection, Deployment Information, in this section.

Note For supported versions of Windows XP Professional x64 Edition, this security update is the same as supported versions of the Windows Server 2003 x64 Edition security update.

Reference Table

The following table contains the security update information for this software. You can find additional information in the subsection, Deployment Information, in this section.