CERT/CC Vulnerability Note VU#837092

Overview

Acresso FLEXnet Connect executes scripts that are insecurely retrieved from a remote web server, which can allow a remote, unauthenticated attacker to execute arbitrary code on a vulnerable system.

Description

Acresso FLEXnet Connect is a software package that allows vendors to provide updates to applications. FLEXnet Connect-enabled software has the ability to

Check for updates from the software publisher
Receive update files and messages from the software publisher
Install software updates, including the ability to do so silently
Collect and transmit system information, such as machine name, operating system, IP address, or other hardware details, such as network or video card properties
Log and transmit each time an application is started, terminated, or when a specific feature within the application is used

Acresso FLEXnet Connect was formerly known as Macrovision FLEXnet Connect, and before that it was known as InstallShield Update Service.

The FLEXnet Connect client software communicates with centralized servers to check for updates and other product information on a periodic basis. Updates can also be triggered by using Internet Explorer to visit a web page that uses the FLEXnet Connect ActiveX control, which is provided by agent.exe. When connecting to the server, the client can receive special instructions (rules) to assist in evaluating whether an update is relevant. These instructions are provided by a GetRules.asp page on a web server. These rules are presented in a scripting language, such as VBScript.

FLEXnet Connect retrieves rules insecurely in that it uses unsigned and unencrypted communication using the HTTP protocol, which can allow an attacker to inject code that will be executed on the client system. This can happen in a number of ways, including

Compromising the FLEXnet Connect servers directly.
Filtering client system traffic through a malicious proxy.
Compromising DNS servers or otherwise modifying the host name lookup methodology of a client system.

Depending on how the vendor has configured the FLEXnet Connect components, the check for updates may occur on a periodic basis, every time an application is launched, when a user checks for updates manually, or if a web page that uses the FLEXnet ActiveX control is visited. Any software that has been packaged with the vulnerable InstallShield, Macrovision, or Acresso components may be vulnerable.

Impact

By modifying the rule script that is sent to a FLEXnet Connect client, a remote unauthenticated attacker may be able to execute arbitrary code on a vulnerable system.

Solution

Apply an update
This issue is addressed with the FLEXnet Connect 11.0.1 client, which comes with agent.exe version 11.1.100.17104. This version of FLEXnet Connect includes the ability to verify certificates that are provided by the FLEXnet Connect server. A FLEXnet Connect server that uses signed communication will add an X-FNC-Sig HTTP header to outgoing messages. This signature is designed to prevent the server response from being successfully modified by an attacker. The signature checking is also designed to ensure that the FLEXnet Connect client is connecting to an authentic FLEXnet Connect server, much in the same way that HTTPS helps to ensure the identity of a web site. Note that the originally-released version of the FLEXnet Connect 11.0.1 client, which came with agent.exe version 11.1.100.16604, did not completely address this vulnerability.

Note that the FLEXnet Connect 11.0.1 SDK does not enable secure communications by default, but the updates.installshield.com FLEXnet Connect server is currently distributing an update that enables this feature. This means that if a system's DNS has been hijacked or if the communications with the FLEXnet Connect update server are modified before this update can be retrieved, an attacker may be able to execute arbitrary code on the client system.

Because the fixed version of the FLEXnet Connect runtime is relatively new (it was digitally signed on September 26, 2008), it is likely to take some time before software that is packaged with FLEXnet Connect will receive the update and also configure FLEXnet Connect to verify signatures. For this reason, we recommend the following workarounds:

Block outbound requests that contain the string /GetRules.asp

It may be possible to prevent this vulnerability from being exploited by filtering outbound URLs that contain the string /GetRules.asp. Some filtering examples are below. These examples may not work in all cases, and may cause unintended side-effects.

iptables: iptables -A OUTPUT -m string --algo bm --string "/GetRules.asp" -j REJECT
Squid:
acl blockthisURL url_regex /GetRules.asp http_access deny blockthisURL
Snort: alert tcp any any <> any 80 (msg:"GetURL_rule"; sid:12346789; uricontent:"/GetRules.asp"; nocase;)

Disable the DWUpdateService ActiveX control in Internet Explorer

The vulnerable ActiveX control can be disabled in Internet Explorer by setting the kill bit for the following CLSIDs:

{551E5190-19C7-4626-9D54-FB20355E6467}

{5B7524C8-2446-40E9-9474-94A779DBA224}

{8D9BB053-FEE5-4411-B6F5-F1E37DDC3106}

{EE4E49B0-38EC-4C23-A7A6-2E190B5E3418}

{FFF2D28F-E4EE-44D9-8104-8E71556757F6}

More information about how to set the kill bit is available in Microsoft Support Document 240797. Alternatively, the following text can be saved as a .REG file and imported to set the kill bit for this control:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\ActiveX Compatibility\{551E5190-19C7-4626-9D54-FB20355E6467}]