Open-source forensic tools under Rule 702

Does it matter whether a forensic tool is open source?

Not as a legal question. Rule 702 asks whether the principles and methods are reliable and whether the opinion reflects a reliable application of them to the facts of the case, and software licensing appears nowhere in either enquiry.

The question keeps getting asked because it is a proxy for a real one. What counsel actually want to know is whether relying on a free tool will look amateurish to a judge, and whether an opposing expert will make it look that way. Those are worth taking seriously as presentation problems. They are not reliability problems, and conflating them produces two bad outcomes: an examiner who reaches for a licence fee instead of a validation step, and a challenge aimed at a licence rather than at a finding.

The rulings bear this out from both directions, which is the useful part. A closed tool has been admitted where the examiner had never seen its source. An open tool has been admitted and named in the opinion as part of the reliability showing. The variable that moved in both was what the examiner could say about the output.

Why is “commercial” not a reliability finding?

Because a purchase order is not a test result. The elements courts have actually relied on when accepting a mainstream product are separable from the fact that it was sold, and reading them closely is instructive.

In Williford v. State the showing was that the product is generally accepted in the worldwide computer forensic community, that it is commercially available and therefore testable by anyone, that it has been tested, that it has been the subject of published comparative articles, and that it has a low potential rate of error. Note what commercial availability is doing in that list: it is the predicate for testability, not an independent virtue. Anyone can obtain the tool, so anyone can test it.

That reasoning applies with more force to an open-source project, not less. The tool can be obtained without a purchase, its behaviour can be tested, and its implementation can be read. Where a commercial product supports the inference “anyone could test what it does”, an open one supports “anyone can see how it does it”.

Commercial availability is the predicate for testability, not an independent virtue.

The claim that decays

The other common proffer is field standard status. In Sanders v. State the court accepted that a product was a field standard for forensic computer examination about which treatises had been published — a genuine reliability showing, and one with a shelf life. A tool that was the standard in 2006 may not be in the year of the report, so the argument has to be re-made for the version actually run rather than inherited from the tool’s reputation.

And the closed-source case that cuts the other way

In United States v. Chiaradio the district court acknowledged that the peer-to-peer tracing program had not been independently tested, and the source code was deliberately kept secret. The First Circuit affirmed admission anyway, relying on other indicia: the agent’s specialised experience with the program, and his ability to re-create its sessions by hand. Closed and untested, admitted on corroboration. That is the clearest available demonstration that the licence is not the variable.

Where is an open-source tool actually stronger?

In three specific places, each of which maps onto something a court has asked about.

• Testability in the literal sense. Daubert’s first factor asks whether the technique can be tested. For a closed parser, testing means comparing output against expectation and inferring the logic. For an open one, the logic is readable — how the write-ahead log is handled, which records are treated as recoverable, where an ambiguous structure is resolved one way rather than another. An examiner who has read that code can describe the behaviour rather than characterise it.
• The dual-tool comparison is free. Verification of a load-bearing finding by a second, independently developed parser is the answer to most tool objections, and it requires a second tool. Where the second tool costs nothing to obtain, there is no budget argument against doing it on every finding that matters. This is the most underrated practical consequence of the ecosystem.
• The opposing examiner can reproduce the work. Testability under Daubert means another competent examiner, given the same image and the same record, can repeat the steps. If those steps require a licence the other side does not hold, reproduction has a commercial obstacle in front of it. Where the tooling is free, that obstacle does not exist — and an examination another expert can actually repeat is a stronger examination.

The recognised open-source projects in host forensics are not obscure. The Sleuth Kit and Autopsy for filesystem analysis, Plaso and its log2timeline front end for timeline construction, RegRipper for registry parsing, Volatility for memory analysis, and ExifTool for file metadata are all long-established, widely used by working examiners, and documented in public. In In re Digital Music Antitrust Litigation the last of those was named in the opinion, and its acceptance in the field formed part of the reliability finding.

Where is it weaker, and what do you do about it?

The real problem is bespoke, not open

There is a category of tooling that genuinely struggles under Rule 702, and it is routinely confused with open source because both are non-commercial. A script written for this matter — to parse an unusual log format, to correlate two exports, to extract a field no product reads — has:

• no general acceptance, because nobody else has used it;
• no published comparison, because there is nothing to compare;
• no testing record, unless the examiner made one; and
• no behaviour anyone has measured, including on the edge cases in the data it was pointed at.

That is a factor-four and factor-three problem at once, and it is the situation the six named failure modes in the Daubert guide describe as a one-off script never run against known data.

The answer is not to stop writing them. Novel data formats appear faster than products support them, and refusing to look is worse than looking with something homemade. The answer is that the validation burden shifts entirely onto the examiner and has to be discharged deliberately.

Scale problems are handled the same way. In Arista Records v. Usenet.com the court accepted classification of infringing content by statistical sampling of files, weighing the approach against comparable sampling accepted in earlier litigation. Sampling is how an opinion reaches a corpus nobody can review file by file, and it is admissible when the sampling frame and the selection method are stated. An unexplained “representative selection” is the version that fails.

How do you build the reliability proffer either way?

The same five steps serve for an open tool, a commercial one, and a script, because none of them is about the licence.

1. 01Name the tool, the version, and the function it performed. Not a list of software in an appendix — which tool did which step of the work the opinion rests on.
2. 02Cite the testing record that exists, and say plainly where none does. A declaration that claims a testing record for a version that does not have one is worse than a declaration that says validation was performed in house and describes it.
3. 03Say who else uses it for this purpose. Acceptance in the field is a factual claim about practitioners, and it is answerable for any established project. It is not answerable for a script, which is why the script needs the other four steps to work harder.
4. 04Verify each load-bearing finding a second way. A second parser, a manual read, or a different artifact recording the same event. This is the step that answers the analytical-gap objection and the one the reported rulings reward most consistently.
5. 05State the tool’s limits before the other side does. Every parser has structures it handles badly. Naming them costs a sentence and removes the only surprise available on cross-examination.

One further point about the cross-examination that is coming. The standard gambit is “you do not know how this software works, do you?”, and courts have already answered it. The Vermont Supreme Court in State v. Pratt held that an investigator must have specialised knowledge in using the particular software but need not understand its underlying programming, and the D.C. Circuit in United States v. Morgan reasoned that requiring knowledge at the level of the software’s algorithms would make anyone using basic software an expert in its coding. An examiner working with open tooling is the one witness who can answer that question on its own terms, and should not be shy about it.

Frequently asked questions

Related reading

• NIST CFTT validation and admissibility

  What a published conformance test establishes about a named version, and how to validate in house when no report exists.

• What is a forensic tool's error rate?

  Parser disagreement is not tool defect, and the distinction decides which objection you are actually answering.

• The Daubert factors applied to forensic method

  Testability and general acceptance translated for a forensic examination, and where the bespoke-script problem sits.

• Source code review

  Where reading code is the examination rather than a means to it — provenance, copying, and the tiered access a protective order sets up.

• The Daubert Docket

  Filter by tool reliability to read the rulings where a court decided what a forensic tool's output could support.

Attorney advertising / expert services. General information about evidence law and forensic practice, not legal advice, and not a substitute for checking the rules and case law of your own forum.