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DFMI is an open-source security toolkit that its maintainers describe as a way to deliver payloads through Windows Installer packages. It is not a Windows feature. Its subject is the legitimate MSI CustomAction mechanism, which can run programs or scripts during installation; that flexibility means an MSI’s source, signature, and deployment context all matter.
What is DFMI?
The project expands DFMI as “Dont F(ool) My Installer” and describes it as a toolkit for fileless execution or covert payload delivery using MSI files. Its stated intended uses are authorized red-team engagements, penetration testing, and security research. Those are the maintainers’ capability and purpose claims, not independently verified test results. The DEF CON 34 program listing for an Aug. 7–8, 2026 session also summarizes the project’s three approaches: DEF CON DemoLabs: DFMI. The project documentation is available in the DFMI repository.
Why can an MSI run actions during setup?
Windows Installer supports Custom Actions for tasks that standard installation actions cannot handle. Microsoft documents that a custom action can launch an executable, call a DLL function, execute script text, or defer work during installation. Microsoft also notes that “Standard actions are sufficient to execute an installation in most cases,” so custom actions are an extension mechanism rather than a requirement for every installer. See Microsoft’s Custom Actions — Windows Installer documentation.
This is a legitimate mechanism used by software installers, not evidence by itself that a package is malicious. The security question is whether the package and its actions are expected, authorized, and obtained from a trusted source.
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How does DFMI describe its three approaches?
| Approach | Project-described behavior | Trust and signature consideration |
|---|---|---|
| Inject | Adds a CustomAction to a copy of an existing MSI. | The project says modifying a signed MSI invalidates its Authenticode signature; depending on policy, users may see an unknown-publisher warning. |
| Rogue MST | Provides a separate transform to be applied alongside an unchanged MSI; Windows Installer merges the transform at installation time. | The original MSI remains unchanged, but the project says the transform is unsigned and may be blocked by strict transform policy. A signature on the MSI does not validate a separate transform. |
| Stub | Creates a standalone MSI package from scratch. | The project says the stub is unsigned and may be flagged where MSI signing is enforced. |
These descriptions come from the project’s documentation and may change as it is updated. The repository says inject and stub generation can be performed on Windows or Linux, while Rogue MST generation requires Windows because it uses the Windows Installer COM API. This article does not provide steps for modifying or building such packages.
What does “fileless” mean here?
DFMI’s project description uses “fileless” for its delivery approach, but that label should not be read as proof that activity is invisible or leaves no artifacts. The repository documents limitations, including no direct AMSI or ETW bypass and no persistence mechanism. These are project-reported characteristics, not results from testing this article or a guarantee about how any particular security product will respond.
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How can you assess whether an MSI is trustworthy?
A signature is useful evidence about a specific artifact, but it is not a complete trust decision. Check the package’s origin and expected version, confirm that the signer is the publisher you expect, and consider whether any separately supplied transform or deployment instruction is also authorized.
- Verify provenance: Obtain installers through an approved publisher or software distribution channel. Compare the file with an expected release or internal record when available.
- Check signature status: Confirm that the MSI’s signature is valid and belongs to the expected publisher. An unsigned file or an invalidated signature needs investigation rather than an automatic assumption that it is safe or malicious.
- Protect internal distribution: Restrict who can write to software repositories, deployment shares, and other locations from which endpoints install software. Review access when staff or vendors change.
- Review deployment context: Confirm that the install was approved and that any transform is expected and sourced from a trusted channel. Do not treat a valid signature on the base MSI as proof about a separate transform.
- Monitor installer activity: Investigate unexpected MSI deployments and unusual CustomAction behavior, especially when they occur outside normal software-management workflows.
- Apply execution policy: Set and review organizational policy governing MSI installation and transform use so that authorized deployment paths are supported and unapproved ones are constrained.
What should defenders take away?
DFMI illustrates a broader installer-trust issue: a familiar package format can invoke extensible installation actions, and the artifact being trusted may not be the only input involved. Treat installer provenance, signature status, transform handling, repository permissions, and deployment controls as related checks. The existence of a CustomAction alone does not establish abuse; unexpected actions or an untrusted delivery path warrant investigation.
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