*** UPDATE: Version 2.0 of EMET is now available.  Click here to read more about it. ***

 

Even as you read this, people around the world are hunting for vulnerabilities in software applications.  Odds are some of them will be successful.  Depending on their motives and what they find, your software and systems may be put at risk.  So how do you protect your software from unknown vulnerabilities that may or may not exist?  One option is to use security mitigations.

 

Microsoft offers a number of different mitigation technologies that are designed to make it more difficult for an attacker to exploit vulnerabilities in a given piece of software.  Take a look at Michael Howard’s article “Protecting Your Code with Visual C++ Defenses” (http://msdn.microsoft.com/en-us/magazine/cc337897.aspx) for a brief overview of some of these technologies.

 

To help on this front, we are announcing the initial release of a new utility called the Enhanced Mitigation Evaluation Toolkit (EMET).  Version 1.0.2 is now available, free of charge at the Microsoft Download Center (http://go.microsoft.com/fwlink/?LinkID=162309).  This utility builds on our current offerings in several key ways:

 

1.      Until now, many of the available mitigations have required for an application to be manually opted in and recompiled. EMET changes this by allowing a user to opt in applications via a simple command-line utility without recompilation. This is especially handy for deploying mitigations on software that was written before the mitigations were available and when source code is not available.

 

2.      EMET provides a higher degree of granularity by allowing mitigations to be applied on a per process basis. There is no need to enable an entire product or suite of applications. This is helpful in situations where a process is not compatible with a particular mitigation technology. When that happens, a user can simply turn EMET off for that process.

 

 

3.      Mitigations that have previously been limited to up-level versions of Microsoft Windows now ship with EMET and are available down-level. Users can benefit from these mitigations without the need to upgrade their systems.

 

4.      EMET is a living tool designed to be updated as new mitigation technologies become available. This provides a chance for users to try out and benefit from mitigations before they are included in the next versions of our products. It also gives users the opportunity to provide feedback and help guide the future of mitigation technologies in Microsoft products.

Supported Mitigations

This initial release of EMET is primarily focused on providing an extensible framework that will have future mitigations added to it.  A total of four mitigations are also being included with this release and are listed below.  We will provide announcements as future mitigations are added.  If you have ideas about mitigations you’d like to see (whether they already exist or not) feel free to contact us.

 

SEHOP

This mitigation performs Structured Exception Handling (SEH) chain validation and breaks SEH overwrite exploitation techniques.  Take a look at the following SRD blog post for more information: http://blogs.technet.com/srd/archive/2009/02/02/preventing-the-exploitation-of-seh-overwrites-with-sehop.aspx.  With this protection in place, the msvidctl exploit we already blogged about (http://blogs.technet.com/srd/archive/2009/07/28/msvidctl-ms09-032-and-the-atl-vulnerability.aspx) would have failed.

 

Dynamic DEP

Data Execution Prevention (DEP) is a memory protection mitigation that marks portions of a process’ memory non-executable.  This makes it more difficult to an attacker to exploit memory corruption vulnerabilities.  For more information on what DEP is and how it works, take a look at the two part SRD blog available at http://blogs.technet.com/srd/archive/2009/06/12/understanding-dep-as-a-mitigation-technology-part-1.aspx and http://blogs.technet.com/srd/archive/2009/06/12/understanding-dep-as-a-mitigation-technology-part-2.aspx.

 

 NULL page allocation

This blocks attackers from being able to take advantage of NULL dereferences in user mode.  It functions by allocating the first page of memory before the program starts.  Right now the exploitation techniques for these types of vulnerabilities are only theoretical.  However, this mitigation will protect you even if that changes.  Please note this protection does not impact kernel mode NULL dereferences as the current version of EMET only supports user mode mitigations.

 

 Heap spray allocation

Heap spraying is an attack technique that involves filling a process’ heap with specially crafted content (typically including shellcode) to aid in exploitation.  Right now, many attackers rely on their content being placed at a common set of memory addresses.

 

This mitigation is designed to pre-allocate those memory addresses and thus block these common attacks.  Please note that it only aims to break current exploit that take advantage of these common addresses.  It is not a general mitigation for the larger heap spraying attack.  That said, if attackers do change the addresses they use, EMET users can change the addresses

A Note about Application Compatibility

Security mitigations carry an application compatibility risk with them.  Some applications rely on precisely the behavior that the mitigations block.  For this reason mitigations are typically turned off by default and require opt-in from a developer before they are enabled.  While EMET allows users to override this, it is important to be aware of the risk.  EMET is intended for tech savvy users such as IT professionals and security researchers who can troubleshoot issues that these mitigations may introduce.  We also recommend testing your applications and use scenarios with these mitigations prior to deploying them on any production systems.

Feedback

We encourage you to download and try out the tool.  If you have any feedback on your experiences with the tool, you can reach us at switech@microsoft.com.

 

Special thanks to Matt Miller for his assistance with EMET.

-  Fermin J. Serna and Andrew Roths, MSRC Engineering