Understanding Structured Exception Handling Overwrite Protection (SEHOP)

What does SEHOP do

Structured Exception Handling Overwrite Protection (SEHOP) is a security safeguard setting within Windows designed to prevent malicious actors from exploiting the Structured Exception Handler (SEH) overwrite. By preventing this exploit, SEHOP helps to ensure programs run smoothly and securely. 

What is a SEH overwrite attack

Structured Exception Handler (SEH) is a mechanism within software that’s responsible to keep the program running smoothly in the event of an error. One of the ways the SEH works is by having alternative paths or instructions (exceptions) to be used when their primary instructions cannot be actioned. 

One of the ways an overwrite attack works is by taking advantage of the software's SEH. The attacker manipulates the software into thinking there is an error and tricks the software into executing malicious code in an attempt to stop the error. This is also known as a buffer overflow attack.

How does SEHOP work

SEHOP works by checking for vulnerabilities and verifying the integrity of the SEH chain before executing any exception handling code. These checks are done at run-time and are designed to protect applications regardless of whether or not they were compiled with safeguards such as /SAGESEH.

These additional layers of security ensure the exception handling process hasn’t been tampered with. It is important to note that once enabled, certain versions of Cygwin, Skype, and Armadillo-protected applications may not work correctly.

How to enable SEHOP

To enable structured exception handling overwrite protection via GP, set the following UI path to

Enabled:

Computer Configuration\Policies\Administrative Templates\MS Security

Guide\Enable Structured Exception Handling Overwrite Protection (SEHOP)

Note: This Group Policy path does not exist by default. An additional Group Policy

template (SecGuide.admx/adml) is required – it is available from Microsoft at this link.

More information is available at MSKB 956607: How to enable Structured Exception

Handling Overwrite Protection (SEHOP) in Windows operating systems

Default value 

Disabled for 32-bit processes.

Recommended setting 

The recommended state for this setting is: Enabled.

SEHOP Best Practices 

SEHOP enhances security by blocking exploits that could allow malicious code execution. Server hardening complements SEHOP by applying improved security measures like disabling unnecessary services and enforcing strict access controls, reducing the attack surface and mitigating threats. Together, they provide a robust defense strategy, protecting critical applications and data from various vulnerabilities and sophisticated attacks.