YARA Rule Writing for DFIR

What SEC409 teaches: writing YARA rules that still fire next month, on files you have not seen, without burying the estate in false positives. Built on YARA-X, with the 4.x deployment reality the rules actually land in. Start here.

The YARA language: text strings and their modifiers, hex patterns with wildcards and jumps, regular expressions and their cost, the condition logic that turns candidate patterns into an assertion, and metadata that survives being read by somebody else.

Operating yr scan properly: what a target is and why directory scanning is not recursive by default, reading output at scale, threads and timeouts and size bounds, the controls that prove a negative, rule sets and namespaces, and the hunt loop.

Choosing patterns by what it would cost the author to change them: the durability question, both measurement columns, extraction as a workflow, the false friends your eye picks first, and the build-environment trap.

Reading a file with yr dump before writing any condition: pe fields and the bitmask trap, imports and imphash, entropy baselines, signatures and the Rich header, and what structural rules actually cost.

Formats where the binary model does not apply: why a correct rule returns nothing on a document or an encoded script, what each container keeps readable, webshell precision, ELF differences, and the three questions that transfer.

What happens after a rule is written: finding which rule in a set dominates a scan, the tuning loop, baselines and regression, diagnosing a rule that has gone quiet, reviewing rules you did not write, and the maintenance cadence.

What changes when a rule leaves your machine: the two engines and where they disagree, surfaces and thresholds, deploying a set, running a hunt, sharing rules honestly, and where the course leaves you.

The lookup layer: five sheets covering patterns and modifiers, scanning and output, choosing patterns, structure and modules, and non-binary formats, each entry stating what to read and what it does not prove.

Three runbooks: writing a rule from a sample end to end, running a hunt whose negative result means something, and reviewing a rule set you did not write.

Building the environment every measurement in this course depends on: the yr binary, two specimen families, a clean corpus, a PE corpus, the non-binary specimens, and a controls directory, with a verification script that fails loudly.

Two cases worked end to end including the wrong turns: a packed PE where the obvious signal points the wrong way, and a document-and-script chain where a correct rule matches nothing.

Three procedures for the moments something fires: a rule hit and you have to decide what it establishes, a rule has gone quiet and you cannot tell why, and a set stopped producing hits after an update.

The whole course condensed to one operating page: the command for each stage, the figure it produced, and the one judgment that stage turns on.

The tools, the documentation, the frameworks, and what to check first when something in the course behaves differently from the page.

YARA Rule Writing end-of-course exam: a hunt that returns zero hits including on the host holding the sample, testing whether you can separate did not match from was never evaluated and diagnose three independent failures under incident pressure.