YARA-X 1.10.0: Using “yr fix warnings” to auto-correct brittle rules

YARA-X 1.10.0 adds a new subcommand that can automatically apply suggested fixes for certain compiler warnings. The command is invoked as yr fix warnings, and one common transformation replaces ambiguous 0 of (...) conditions with explicit none of (...). The tool edits your rule files in place, so use version control or work on copies first. (github.com)

Overview

YARA-X is a Rust rewrite of YARA with a modern CLI named yr. It targets high compatibility with existing rules while improving performance, safety, and developer ergonomics. (github.com)

Version 1.10.0 introduces the yr fix warnings command and new warning types (e.g., invariant_expr for impossible for-loop conditions). The release also includes WASM code-size optimizations and AST traversal improvements. (github.com)

The 0 of them ambiguity was resolved in classic YARA 4.3.0 to mean “exactly zero,” and both YARA and YARA-X documentation encourage using none instead for clarity. The new fixer leans into that guidance by rewriting 0 of (...) to none of (...) when safe. (yara.readthedocs.io)

Acquisition and Extraction (platform-specific)

• Linux/macOS/Windows: Download prebuilt YARA-X binaries from the official releases and unpack. The CLI binary is yr. (virustotal.github.io)
• macOS (Homebrew): brew install yara-x (current stable lists 1.10.0). (formulae.brew.sh)
• Python API (optional for automation): pip install yara-x (supports 3.9+ across major OSes). (virustotal.github.io)

Artifact Locations and Paths

• Rules typically live in .yar/.yara files. When you point yr scan or yr compile at a directory, YARA-X traverses it recursively and picks up those files. Mirror this layout for yr fix warnings runs. (virustotal.github.io)
• The per-user config file is ${HOME}/.yara-x.toml, which controls formatter, checks, and warnings. (virustotal.github.io)

Analysis and Correlation

Here’s a minimal, fixable case.

Before (ambiguous condition the compiler warns about):

rule FixableCountWarning {
  strings:
    $a1 = "malicious"
    $a2 = "badstuff"
  condition:
    0 of ($a*)
}

Run the fixer on a file or a rules folder (work on a branch or copy):

# Example: run in a repo checkout
yr fix warnings rules/family/

After (explicit intent, no ambiguity):

rule FixableCountWarning {
  strings:
    $a1 = "malicious"
    $a2 = "badstuff"
  condition:
    none of ($a*)
}

• The CLI name is yr (not yara). Confirm available commands with yr help. (virustotal.github.io)
• The fixer uses the same guidance documented for YARA/YARA-X rule conditions: prefer none of (...) to 0 of (...). (yara.readthedocs.io)
• In 1.10.0 you may also see new diagnostics like invariant_expr during compilation or checks; fix those manually if they don’t have an automatic edit. (github.com)

Suggested local workflow for rule hygiene:

• Format: yr fmt -c rules/ during commits to keep rules consistent. (virustotal.github.io)
• Fix: yr fix warnings ... to apply safe mechanical changes.
• Compile: yr compile rules/ to validate the full set. (virustotal.github.io)
• Scan a QA corpus with NDJSON output if you maintain tests. (CLI supports NDJSON in yr scan.) (virustotal.github.io)

Validation and Pitfalls

• In-place edits: yr fix warnings modifies the original files and does not create backups. Use a VCS branch or run on copies. (isc.sans.edu)
• Semantics: Replacing 0 of (...) with none of (...) aligns with YARA 4.3.0 semantics and removes ambiguity for maintainers and CI. Verify rule behavior against test fixtures. (yara.readthedocs.io)
• Config interactions: Your ${HOME}/.yara-x.toml can disable or escalate warnings; ensure your fixer run matches team policy. (virustotal.github.io)
• Not every warning is auto-fixable. For issues like unsatisfiable expressions or rule logic problems, you’ll still need a manual patch, optionally guided by YARA-X’s detailed diagnostics. (virustotal.github.io)

Reporting Notes (chain of custody, reproducibility)

• Record the exact tool version with yr --version and, for packaged installs, the package source (e.g., Homebrew shows stable 1.10.0). (formulae.brew.sh)
• Capture the commit hash before/after running the fixer and keep the diff in your change record.
• Store your config file (.yara-x.toml) in the repo to make warning and formatting policies reproducible across responders. (virustotal.github.io)

Tools

• YARA-X CLI (yr) and libraries. (virustotal.github.io)
• Git or another VCS to safely stage and review automated edits.
• Optional: YARA-X Python bindings for bulk validation and CI harnesses. (virustotal.github.io)

Takeaways

• Upgrade to YARA-X 1.10.0 and run yr fix warnings on a branch to normalize legacy patterns like 0 of (...) to none. (github.com)
• Add yr fmt -c and a compile step to your pre-commit or CI to keep rulebases clean and fast-failing. (virustotal.github.io)
• Keep ${HOME}/.yara-x.toml under version control and align warning policies before large-scale fixes. (virustotal.github.io)
• Treat auto-fixes as mechanical edits; still run test scans to confirm behavioral intent before merging. (virustotal.github.io)