What does the skill need to write a good rule?

Distinctive characteristics of the sample: notable strings, byte sequences, imports, or behavioural artefacts from analysis. The more specific the input, the tighter the condition it can build. It can also start from a hash and the context around it.

How does it avoid false positives?

It favours conditions that combine several strings, applies file-size and file-type guards, and prefers content unique to the family over generic library strings. The draft notes where you should test against known-good files before deploying.

Is the YARA writing skill free to use?

Yes. It ships in the open-source, MIT-licensed CTI Skills pack, so you can run it, inspect how it builds rules, and modify it for your own malware-hunting workflow at no cost.

CTI Skill · Detection engineering

YARA Rule Writing.

/yara-writing

If you need to know how to write YARA rules that reliably catch a malware family without firing on everything else, this skill drafts them for you. Point it at sample characteristics or strings of interest and it produces a valid rule with meta, well-chosen string definitions, and a balanced condition that targets content rather than a single brittle hash.

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What it does.

The skill builds YARA rules that match files by their content: literal strings, hex byte sequences, regular expressions and the conditions that tie them together. It writes a meta block with author, description and references, defines named strings, and constructs a condition that balances detection against false positives, using file size guards, string counts and module checks where they help.

When to use it.

Use it after triaging a sample, when you want a durable detection that survives recompilation and minor variant changes better than a hash would. It suits analysts hunting a malware family across a corpus, detection engineers feeding rules into scanning pipelines, and responders who need a quick rule to sweep an estate during an incident.

What you get back.

A complete YARA rule in correct syntax: a descriptive rule name, a meta section documenting intent and sourcing, a strings section mixing text, hex and regex as appropriate, and a condition expression that combines them sensibly. The draft aims for precision, flagging the trade-offs between a tight rule and broad coverage so you can tune it for your environment.

How it fits your workflow.

It is one of the open-source CTI Skills that run inside an AI coding agent like Claude Code. You stay in the terminal, hand it the sample details, and get a reviewable rule you can test with the YARA CLI and commit to your detection repository. It works well right after a malware analysis or hash investigation step that has already surfaced the distinctive artefacts.

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Frequently Asked Questions.

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