D3FEND Top Ten – #2

We’re nearly finished with our D3FEND Top Ten, so we’ve turned to a particularly intricate example from the framework: D3-EHPV, Exception Handler Pointer Validation.

What is Exception Handler Pointer Validation?

Exception Pointer Handler Validation, also known as Exception Handler Validation, is a security technique that verifies the validity of a referenced exception handler pointer. When a process encounters an exception, it calls an exception handler to handle the exception. The exception handler’s location can be determined differently based on the operating system. In Windows, the exception registration record, containing a pointer to the next exception registration record and a pointer to the handler, is stored at the start of the Thread Information Block.

The validation process checks if the exception handler is present in a list of valid exception handlers before it is called. If the handler is not found in the list, other defensive techniques like Process Termination or Executable Blacklisting may be invoked. However, this validation does not ensure the integrity or security of the exception handler’s code, but only confirms that it could be a valid exception handler called by the program.

When would I use Exception Handler Point Validation?

The source of valid exception handlers can be generated at runtime, during compilation, or as a binary patch. If the program file can be altered by an attacker, they could bypass the security measures by replacing it with their desired program. Additionally, if an attacker has already overwritten the code of a valid exception handler through other program functionality, this validation would not prevent arbitrary code execution.

SafeSEH, a security feature, can be applied to executable files or modules to protect against certain types of exception handler exploits. However, if SafeSEH is not applied to all modules, attackers may still call unprotected modules’ code as exception handlers, leading to potential vulnerabilities.

How does Exception Handler Point Validation link to the ATT&CK framework?

The “Exception Pointer Handler Validation” technique can be linked to the MITRE ATT&CK framework in the following ways:

  1. T1187 – “Forced Authentication”
    In the ATT&CK framework, T1187 refers to the technique of forcing the target system to authenticate to a service provider with credentials obtained through alternative means. While the direct connection between “Exception Pointer Handler Validation” and “Forced Authentication” may not be immediately apparent, the key here lies in how attackers can exploit weaknesses in exception handling mechanisms to gain unauthorized access and perform actions that would otherwise require proper authentication.

2. T1507 – “Dynamic-link Library Injection”
T1507 in the ATT&CK framework involves the injection of malicious code into a running process by abusing dynamic-link library (DLL) loading mechanisms. In some cases, attackers can manipulate exception handling to facilitate the injection of malicious DLLs. By exploiting vulnerabilities in the exception handler chain, attackers may gain the ability to execute their malicious code within the context of a legitimate process, leading to potential escalation of privileges or code execution.

3. T1055 – “Process Injection”
T1055 encompasses a variety of techniques where adversaries inject code into a running process, often to evade defenses or gain execution in a different context. While not directly related, there may be cases where attackers abuse exception handling mechanisms to facilitate process injection. This could be achieved by corrupting exception handler pointers or leveraging buffer overflows to overwrite return addresses and redirect control flow to the injected code.

What tools can I use for Exception Handler Pointer Validation?

Although exception handler pointer validation is better understood as a framework than as a set of tools. But that doesn’t mean that we can’t use some helpful tools to make the process easier. Here are some of our favorites:

  1. SafeSEH: SafeSEH is a security feature available in Microsoft’s Visual C++ compiler. It aims to protect against certain types of structured exception handler (SEH) overwrite exploits. SafeSEH validates exception handler pointers at runtime, ensuring that they point to valid exception handlers registered within the application’s SEH chain. This protection can help prevent malicious code execution through SEH overwrite attacks.
  2. Stack Canaries: Stack canaries are random values placed on the stack between local variables and the saved return address. They act as a guard against buffer overflow attacks that could overwrite the return address and affect the exception handler chain. When an exception occurs, the stack canary is checked for integrity before unwinding the stack and calling the exception handler.
  3. Static Code Analysis: Static code analysis tools can help identify potential vulnerabilities in the codebase, including issues related to exception handling. By analyzing the code without executing it, these tools can flag potential problems such as buffer overflows, uninitialized exception handlers, or invalid pointers.
  4. Code Review and Security Best Practices: Manual code review and adherence to secure coding practices are crucial in identifying and mitigating potential issues related to exception handler validation. Developers should be aware of the risks associated with exception handling and ensure that proper checks are in place to validate handler pointers.
  5. Runtime Protection Tools: Some runtime protection tools and application security solutions may include features to monitor and validate exception handler pointers during program execution. These tools can help detect and prevent exploits that target exception handling mechanisms.

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