Application Verifier Dynamic Fault Injection

vfdynf.dll is an application verifier provider that implements unique-stack based systematic fault injection to simulate low resource scenarios.

The integration also works with the command line (the TEST type is DynFault):

appverif DynFault ... -for TARGET ... [-with [TEST.]PROPERTY=VALUE ...]

“Dynamic Fault Injection” (DynFault) is a replacement for “Low Resource Simulation” (LowRes) tests. LowRes is a probability-based (randomized) fault injection. In contrast, DynFault tracks stack hashes when determining where to inject faults. This provides better coverage when simulating low resource scenarios. DynFault injects failures for wait, heap, virtual memory, registry, file, event, section, and OLE string APIs. These are the same APIs as LowRes.

The ability to exclude modules in LowRes is limited. DynFault, in contrast, enables you to exclude stacks containing symbols matched by a set of regular expressions. Why is this helpful? I’ll provide an example, which was the impetus for me reversing the undocumented parts of verifier to implement this library. MSVC implemented debug iterators which are valuable to identify bugs but break noexcept contracts. For example, the default std::string constructor is marked noexcept but with debug iterators enabled an allocation could occur within it and throw an exception. The cpp exception handling then can’t locate a handler past noexcept. The contract is such that if an exception would cross that boundary the implementation should terminate the program. Hopefully you can see the problem with the limited functionality of LowRes (you can’t use it with debug iterators). To solve this DynFault has a property that allows you to define a list of regular expressions. When DynFault encounters a stack matching any expression in this list, that stack hash is excluded from fault injection. As an example, this regular expression tries to isolate stacks containing std::basic_string‘s default constructor:


The above regular expression will match on this stack:

testdynf.exe!operator new
testdynf.exe!std::_Container_base12::_Alloc_proxy<std::allocator<std::_Container_proxy> >
testdynf.exe!std::basic_string<char,std::char_traits<char>,std::allocator<char> >::basic_string<char,std::char_traits<char>,std::allocator<char> >

Enabling the best of both worlds – debug iterators and fault injection!

DynFault Properties (Options)

Name Type Description
GracePeriod DWORD Delays fault injection until after this period, in milliseconds.
SymbolSearchPath String Symbol search path used for dynamic fault injection and applying exclusions.
ExclusionsRegex MultiString Excludes stack from fault injection when one of these regular expression matches the stack.
DynamicFaultPeriod DWORD Clears dynamic stack fault injection tracking on this period, in milliseconds, zero does not clear tracking.
EnableFaultMask QWORD Mask of which fault types are enabled. Bit 1=Wait, 2=Heap, 3=VMem, 4=Reg, 5=File, 6=Event, 7=Section, 8=Ole.
FaultProbability DWORD Probability that a fault will be injected (0 – 1000000).
FaultSeed DWORD Seed used for fault randomization.


At this time there is no installer/script to automate installation. Here are the instructions to manually install the library:

  1. copy vfdynf.dll to C:\Windows\System32 (or SysWOW64 for x86 support on an x64 OS)
  2. add vfdynf.dll to the “Application Verifier Global Settings” “Verified Providers” list (again WOW6432Node when appropriate)
    • HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\{ApplicationVerifierGlobalSettings}
    • VerifierProviders

At this point vfdynf.dll is “registered” with application verifier and it should show up in the options via the command line or in the user interface.


The repo uses submodules, after cloning be sure to init and update the submodules. Project file are targeted to Visual Studio 2022.

git clone
cd .\vfdynf\
git submodule update --init --recursive
MSBuild .\vfdynf.sln


The following are used without modification. Credits to their authors.

  • Process Hacker Native API Headers Collection of Native API header files. Gathered from Microsoft header files and symbol files, as well as a lot of reverse engineering and guessing.

And, Grandfather Derpington 😉


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