yara-rule-authoring

by trailofbits

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SKILL.md About

---
name: yara-rule-authoring
description: >
  Guides authoring of high-quality YARA-X detection rules for malware identification.
  Use when writing, reviewing, or optimizing YARA rules. Covers naming conventions,
  string selection, performance optimization, migration from legacy YARA, and false
  positive reduction. Triggers on: YARA, YARA-X, malware detection, threat hunting,
  IOC, signature, crx module, dex module.
---

# YARA-X Rule Authoring

Write detection rules that catch malware without drowning in false positives.

> **This skill targets YARA-X**, the Rust-based successor to legacy YARA. YARA-X powers VirusTotal's production systems and is the recommended implementation. See [Migrating from Legacy YARA](#migrating-from-legacy-yara) if you have existing rules.

## Core Principles

1. **Strings must generate good atoms** — YARA extracts 4-byte subsequences for fast matching. Strings with repeated bytes, common sequences, or under 4 bytes force slow bytecode verification on too many files.

2. **Target specific families, not categories** — "Detects ransomware" catches everything and nothing. "Detects LockBit 3.0 configuration extraction routine" catches what you want.

3. **Test against goodware before deployment** — A rule that fires on Windows system files is useless. Validate against VirusTotal's goodware corpus or your own clean file set.

4. **Short-circuit with cheap checks first** — Put `filesize < 10MB and uint16(0) == 0x5A4D` before expensive string searches or module calls.

5. **Metadata is documentation** — Future you (and your team) need to know what this catches, why, and where the sample came from.

## When to Use

- Writing new YARA-X rules for malware detection
- Reviewing existing rules for quality or performance issues
- Optimizing slow-running rulesets
- Converting IOCs or threat intel into detection signatures
- Debugging false positive issues
- Preparing rules for production deployment
- Migrating legacy YARA rules to YARA-X
- Analyzing Chrome extensions (crx module)
- Analyzing Android apps (dex module)

## When NOT to Use

- Static analysis requiring disassembly → use Ghidra/IDA skills
- Dynamic malware analysis → use sandbox analysis skills
- Network-based detection → use Suricata/Snort skills
- Memory forensics with Volatility → use memory forensics skills
- Simple hash-based detection → just use hash lists

## YARA-X Overview

YARA-X is the Rust-based successor to legacy YARA: 5-10x faster regex, better errors, built-in formatter, stricter validation, new modules (crx, dex), 99% rule compatibility.

**Install:** `brew install yara-x` (macOS) or `cargo install yara-x`

**Essential commands:** `yr scan`, `yr check`, `yr fmt`, `yr dump`

## Platform Considerations

YARA works on any file type. Adapt patterns to your target:

| Platform | Magic Bytes | Bad Strings | Good Strings |
|----------|-------------|-------------|--------------|
| **Windows PE** | `uint16(0) == 0x5A4D` | API names, Windows paths | Mutex names, PDB paths |
| **macOS Mach-O** | `uint32(0) == 0xFEEDFACE` (32-bit), `0xFEEDFACF` (64-bit), `0xCAFEBABE` (universal) | Common Obj-C methods | Keylogger strings, persistence paths |
| **JavaScript/Node** | (none needed) | `require`, `fetch`, `axios` | Obfuscator signatures, eval+decode chains |
| **npm/pip packages** | (none needed) | `postinstall`, `dependencies` | Suspicious package names, exfil URLs |
| **Office docs** | `uint32(0) == 0x504B0304` | VBA keywords | Macro auto-exec, encoded payloads |
| **VS Code extensions** | (none needed) | `vscode.workspace` | Uncommon activationEvents, hidden file access |
| **Chrome extensions** | Use `crx` module | Common Chrome APIs | Permission abuse, manifest anomalies |
| **Android apps** | Use `dex` module | Standard DEX structure | Obfuscated classes, suspicious permissions |

### macOS Malware Detection

No dedicated Mach-O module exists yet. Use magic byte checks + string patterns:

**Magic bytes:**
```yara
// Mach-O 32-bit
uint32(0) == 0xFEEDFACE
// Mach-O 64-bit
uint32(0) == 0xFEEDFACF
// Universal binary (fat binary)
uint32(0) == 0xCAFEBABE or uint32(0) == 0xBEBAFECA
```

**Good indicators for macOS malware:**
- Keylogger artifacts: `CGEventTapCreate`, `kCGEventKeyDown`
- SSH tunnel strings: `ssh -D`, `tunnel`, `socks`
- Persistence paths: `~/Library/LaunchAgents`, `/Library/LaunchDaemons`
- Credential theft: `security find-generic-password`, `keychain`

**Example pattern from Airbnb BinaryAlert:**
```yara
rule SUSP_Mac_ProtonRAT
{
    strings:
        // Library indicators
        $lib1 = "SRWebSocket" ascii
        $lib2 = "SocketRocket" ascii

// Behavioral indicators
        $behav1 = "SSH tunnel not launched" ascii
        $behav2 = "Keylogger" ascii

condition:
        (uint32(0) == 0xFEEDFACF or uint32(0) == 0xCAFEBABE) and
        any of ($lib*) and any of ($behav*)
}
```

### JavaScript Detection Decision Tree

```
Writing a JavaScript rule?
├─ npm package?
│  ├─ Check package.json patterns
│  ├─ Look for postinstall/preinstall hooks
│  └─ Target exfil patterns: fetch + env access + credential paths
├─ Browser extension?
│  ├─ Chrome: Use crx module
│  └─ Others: Target manifest patterns, background script behaviors
├─ Standalone JS file?
│  ├─ Look for obfuscation markers: eval+atob, fromCharCode chains
│  ├─ Target unique function/variable names (often survive minification)
│  └─ Check for packed/encoded payloads
└─ Minified/webpack bundle?
   ├─ Target unique strings that survive bundling (URLs, magic values)
   └─ Avoid function names (will be mangled)
```

**JavaScript-specific good strings:**
- Ethereum function selectors: `{ 70 a0 82 31 }` (transfer)
- Zero-width characters (steganography): `{ E2 80 8B E2 80 8C }`
- Obfuscator signatures: `_0x`, `var _0x`
- Specific C2 patterns: domain names, webhook URLs

**JavaScript-specific bad strings:**
- `require`, `fetch`, `axios` — too common
- `Buffer`, `crypto` — legitimate uses everywhere
- `process.env` alone — need specific env var names

## Essential Toolkit

| Tool | Purpose |
|------|---------|
| **yarGen** | Extract candidate strings: `yarGen.py -m samples/ --excludegood` → validate with `yr check` |
| **FLOSS** | Extract obfuscated/stack strings: `floss sample.exe` (when yarGen fails) |
| **yr CLI** | Validate: `yr check`, scan: `yr scan -s`, inspect: `yr dump -m pe` |
| **signature-base** | Study quality examples |
| **YARA-CI** | Goodware corpus testing before deployment |

Master these five. Don't get distracted by tool catalogs.

## Rationalizations to Reject

When you catch yourself thinking these, stop and reconsider.

| Rationalization | Expert Response |
|-----------------|-----------------|
| "This generic string is unique enough" | Test against goodware first. Your intuition is wrong. |
| "yarGen gave me these strings" | yarGen suggests, you validate. Check each one manually. |
| "It works on my 10 samples" | 10 samples ≠ production. Use VirusTotal goodware corpus. |
| "One rule to catch all variants" | Causes FP floods. Target specific families. |
| "I'll make it more specific if we get FPs" | Write tight rules upfront. FPs burn trust. |
| "This hex pattern is unique" | Unique in one sample ≠ unique across malware ecosystem. |
| "Performance doesn't matter" | One slow rule slows entire ruleset. Optimize atoms. |
| "PEiD rules still work" | Obsolete. 32-bit packers aren't relevant. |
| "I'll add more conditions later" | Weak rules deployed = damage done. |
| "This is just for hunting" | Hunting rules become detection rules. Same quality bar. |
| "The API name makes it malicious" | Legitimate software uses same APIs. Need behavioral context. |
| "any of them is fine for these common strings" | Common strings + any = FP flood. Use `any of` only for individually unique strings. |
| "This regex is specific enough" | `/fetch.*token/` matches all auth code. Add exfil destination requirement. |
| "The JavaScript looks clean" | Attackers poison legitimate code with injects. Check for eval+decode chains. |
| "I'll use .* for flexibility" | Unbounded regex = performance disaster + memory explosion. Use `.{0,30}`. |
| "I'll use --relaxed-re-syntax everywhere" | Masks real bugs. Fix the regex instead of hiding problems. |

## Decision Trees

### Is This String Good Enough?

```
Is this string good enough?
├─ Less than 4 bytes?
│  └─ NO — find longer string
├─ Contains repeated bytes (0000, 9090)?
│  └─ NO — add surrounding context
├─ Is an API name (VirtualAlloc, CreateRemoteThread)?
│  └─ NO — use hex pattern of call site instead
├─ Appears in Windows system files?
│  └─ NO — too generic, find something unique
├─ Is it a common path (C:\Windows\, cmd.exe)?
│  └─ NO — find malware-specific paths
├─ Unique to this malware family?
│  └─ YES — use it
└─ Appears in other malware too?
   └─ MAYBE — combine with family-specific marker
```

### When to Use "all of" vs "any of"

```
Should I require all strings or allow any?
├─ Strings are individually unique to malware?
│  └─ any of them (each alone is suspicious)
├─ Strings are common but combination is suspicious?
│  └─ all of them (require the full pattern)
├─ Strings have different confidence levels?
│  └─ Group: all of ($core_*) and any of ($variant_*)
└─ Seeing many false positives?
   └─ Tighten: switch any → all, add more required strings
```

**Lesson from production:** Rules using `any of ($network_*)` where strings included "fetch", "axios", "http" matched virtually all web applications. Switching to require credential path AND network call AND exfil destination eliminated FPs.

### When to Abandon a Rule Approach

Stop and pivot when:

- **yarGen returns only API names and paths** → See [When Strings Fail, Pivot to Structure](#when-strings-fail-pivot-to-structure)

- **Can't find 3 unique strings** → Probably packed. Target the unpacked version or detect the packer.

- **Rule matches goodware files** → Strings aren't unique enough. 1-2 matches = investigate and tighten; 3-5 matches = find different indicators; 6+ matches = start over.

- **Performance is terrible even after optimization** → Architecture problem. Split into multiple focused rules or add strict pre-filters.

- **Description is hard to write** → The rule is too vague. If you can't explain what it catches, it catches too much.

### Debugging False Positives

```
FP Investigation Flow:
│
├─ 1. Which string matched?
│     Run: yr scan -s rule.yar false_positive.exe
│
├─ 2. Is it in a legitimate library?
│     └─ Add: not $fp_vendor_string exclusion
│
├─ 3. Is it a common development pattern?
│     └─ Find more specific indicator, replace the string
│
├─ 4. Are multiple generic strings matching together?
│     └─ Tighten to require all + add unique marker
│
└─ 5. Is the malware using common techniques?
      └─ Target malware-specific implementation details, not the technique
```

### Hex vs Text vs Regex

```
What string type should I use?
│
├─ Exact ASCII/Unicode text?
│  └─ TEXT: $s = "MutexName" ascii wide
│
├─ Specific byte sequence?
│  └─ HEX: $h = { 4D 5A 90 00 }
│
├─ Byte sequence with variation?
│  └─ HEX with wildcards: { 4D 5A ?? ?? 50 45 }
│
├─ Pattern with structure (URLs, paths)?
│  └─ BOUNDED REGEX: /https:\/\/[a-z]{5,20}\.onion/
│
└─ Unknown encoding (XOR, base64)?
   └─ TEXT with modifier: $s = "config" xor(0x00-0xFF)
```

### Is the Sample Packed? (Check First)

Before writing any string-based rule:

```
Is the sample packed?
├─ Entropy > 7.0?
│  └─ Likely packed — find unpacked layer first
├─ Few/no readable strings?
│  └─ Likely packed — use entropy, PE structure, or packer signatures
├─ UPX/MPRESS/custom packer detected?
│  └─ Target the unpacked payload OR detect the packer itself
└─ Readable strings available?
   └─ Proceed with string-based detection
```

**Expert guidance:** Don't write rules against packed layers. The packing changes; the payload doesn't.

### When Strings Fail, Pivot to Structure

If yarGen returns only API names and generic paths:

```
String extraction failed — what now?
├─ High entropy sections?
│  └─ Use math.entropy() on specific sections
├─ Unusual imports pattern?
│  └─ Use pe.imphash() for import hash clustering
├─ Consistent PE structure anomalies?
│  └─ Target section names, sizes, characteristics
├─ Metadata present?
│  └─ Target version info, timestamps, resources
└─ Nothing unique?
   └─ This sample may not be detectable with YARA alone
```

**Expert guidance:** "One can try to use other file properties, such as metadata, entropy, import hashes or other data which stays constant." — Kaspersky Applied YARA Training

## Expert Heuristics

**String selection:** Mutex names are gold; C2 paths silver; error messages bronze. Stack strings are almost always unique. If you need >6 strings, you're over-fitting.

**Condition design:** Start with `filesize <`, then magic bytes, then strings, then modules. If >5 lines, split into multiple rules.

**Quality signals:** yarGen output needs 80% filtering. Rules matching <50% of variants are too narrow; matching goodware are too broad.

**Modifier discipline:**
- **Never use `nocase` or `wide` speculatively** — only when you have confirmed evidence the case/encoding varies in samples
- `nocase` doubles atom generation; `wide` doubles string matching — both have real costs
- "If you don't have a clear reason for using those modifiers, don't do it" — Kaspersky Applied YARA

**Regex anchoring:**
- Regex without a 4+ byte literal substring **evaluates at every file offset** — catastrophic performance
- Always anchor regex to a distinctive literal: `/mshta\.exe http:\/\/.../` not `/http:\/\/.../`
- If you can't anchor, consider hex pattern with wildcards instead

**Loop discipline:**
- Always bound loops with filesize: `filesize < 100KB and for all i in (1..#a) : ...`
- Unbounded `#a` can be thousands in large files — exponential slowdown

**YARA-X tips:** `$_unused` to suppress warnings; `private $s` to hide from output; `yr check` + `yr fmt` before every commit.

### When to Use Modules vs. Byte Checks

```
Should I use a module or raw bytes?
├─ Need imphash/rich header/authenticode?
│  └─ Use PE module — too complex to replicate
├─ Just checking magic bytes or simple offsets?
│  └─ Use uint16/uint32 — faster, no module overhead
├─ Checking section names/sizes?
│  └─ PE module is cleaner, but add magic bytes filter FIRST
├─ Checking Chrome extension permissions?
│  └─ Use crx module — string parsing is fragile
└─ Checking LNK target paths?
   └─ Use lnk module — LNK format is complex
```

**Expert guidance:** "Avoid the magic module — use explicit hex checks instead" — Neo23x0. Apply this principle: if you can do it with uint32(), don't load a module.

## YARA-X New Features

Key additions from recent releases:

- **Private patterns** (v1.3.0+): `private $helper = "pattern"` — matches but hidden from output
- **Warning suppression** (v1.4.0+): `// suppress: slow_pattern` inline comments
- **Numeric underscores** (v1.5.0+): `filesize < 10_000_000` for readability
- **Built-in formatter**: `yr fmt rules/` to standardize formatting
- **NDJSON output**: `yr scan --output-format ndjson` for tooling

## YARA-X Tooling Workflow

YARA-X provides diagnostic tools legacy YARA lacks:

**Rule development cycle:**
```bash
# 1. Write initial rule
# 2. Check syntax with detailed errors
yr check rule.yar

# 3. Format consistently
yr fmt -w rule.yar

# 4. Dump module output to inspect file structure (no dummy rule needed)
yr dump -m pe sample.exe --output-format yaml

# 5. Scan with timing info
time yr scan -s rule.yar corpus/
```

**When to use `yr dump`:**
- Investigating what PE/ELF/Mach-O fields are available
- Debugging why module conditions aren't matching
- Exploring new modules (crx, lnk, dotnet) before writing rules

**YARA-X diagnostic advantage:** Error messages include precise source locations. If `yr check` points to line 15, the issue is actually on line 15 (unlike legacy YARA).

## Chrome Extension Analysis (crx module)

The `crx` module enables detection of malicious Chrome extensions. Requires YARA-X v1.5.0+ (basic), v1.11.0+ for `permhash()`.

**Key APIs:** `crx.is_crx`, `crx.permissions`, `crx.permhash()`

**Red flags:** `nativeMessaging` + `downloads`, `debugger` permission, content scripts on `<all_urls>`

```yara
import "crx"

rule SUSP_CRX_HighRiskPerms {
    condition:
        crx.is_crx and
        for any perm in crx.permissions : (perm == "debugger")
}
```

See [crx-module.md](references/crx-module.md) for complete API reference, permission risk assessment, and example rules.

## Android DEX Analysis (dex module)

The `dex` module enables detection of Android malware. Requires YARA-X v1.11.0+. **Not compatible with legacy YARA's dex module** — API is completely different.

**Key APIs:** `dex.is_dex`, `dex.contains_class()`, `dex.contains_method()`, `dex.contains_string()`

**Red flags:** Single-letter class names (obfuscation), `DexClassLoader` reflection, encrypted assets

```yara
import "dex"

rule SUSP_DEX_DynamicLoading {
    condition:
        dex.is_dex and
        dex.contains_class("Ldalvik/system/DexClassLoader;")
}
```

See [dex-module.md](references/dex-module.md) for complete API reference, obfuscation detection, and example rules.

## Migrating from Legacy YARA

YARA-X has 99% rule compatibility, but enforces stricter validation.

**Quick migration:**
```bash
yr check --relaxed-re-syntax rules/  # Identify issues
# Fix each issue, then:
yr check rules/  # Verify without relaxed mode
```

**Common fixes:**
| Issue | Legacy | YARA-X Fix |
|-------|--------|------------|
| Literal `{` in regex | `/{/` | `/\{/` |
| Invalid escapes | `\R` silently literal | `\\R` or `R` |
| Base64 strings | Any length | 3+ chars required |
| Negative indexing | `@a[-1]` | `@a[#a - 1]` |
| Duplicate modifiers | Allowed | Remove duplicates |

> **Note:** Use `--relaxed-re-syntax` only as a diagnostic tool. Fix issues rather than relying on relaxed mode.

## Quick Reference

### Naming Convention

```
{CATEGORY}_{PLATFORM}_{FAMILY}_{VARIANT}_{DATE}
```

**Common prefixes:** `MAL_` (malware), `HKTL_` (hacking tool), `WEBSHELL_`, `EXPL_`, `SUSP_` (suspicious), `GEN_` (generic)

**Platforms:** `Win_`, `Lnx_`, `Mac_`, `Android_`, `CRX_`

**Example:** `MAL_Win_Emotet_Loader_Jan25`

See [style-guide.md](references/style-guide.md) for full conventions, metadata requirements, and naming examples.

### Required Metadata

Every rule needs: `description` (starts with "Detects"), `author`, `reference`, `date`.

```yara
meta:
    description = "Detects Example malware via unique mutex and C2 path"
    author = "Your Name <email@example.com>"
    reference = "https://example.com/analysis"
    date = "2025-01-29"
```

### String Selection

**Good:** Mutex names, PDB paths, C2 paths, stack strings, configuration markers
**Bad:** API names, common executables, format specifiers, generic paths

See [strings.md](references/strings.md) for the full decision tree and examples.

### Condition Patterns

**Order conditions for short-circuit:**
1. `filesize < 10MB` (instant)
2. `uint16(0) == 0x5A4D` (nearly instant)
3. String matches (cheap)
4. Module checks (expensive)

See [performance.md](references/performance.md) for detailed optimization patterns.

## Workflow

1. **Gather samples** — Multiple samples; single-sample rules are brittle
2. **Extract candidates** — `yarGen -m samples/ --excludegood`
3. **Validate quality** — Use decision tree; yarGen needs 80% filtering
4. **Write initial rule** — Follow template with proper metadata
5. **Lint and test** — `yr check`, `yr fmt`, linter script
6. **Goodware validation** — VirusTotal corpus or local clean files
7. **Deploy** — Add to repo with full metadata, monitor for FPs

See [testing.md](references/testing.md) for detailed validation workflow and FP investigation.

For a comprehensive step-by-step guide covering all phases from sample collection to deployment, see [rule-development.md](workflows/rule-development.md).

## Common Mistakes

| Mistake | Bad | Good |
|---------|-----|------|
| API names as indicators | `"VirtualAlloc"` | Hex pattern of call site + unique mutex |
| Unbounded regex | `/https?:\/\/.*/` | `/https?:\/\/[a-z0-9]{8,12}\.onion/` |
| Missing file type filter | `pe.imports(...)` first | `uint16(0) == 0x5A4D and filesize < 10MB` first |
| Short strings | `"abc"` (3 bytes) | `"abcdef"` (4+ bytes) |
| Unescaped braces (YARA-X) | `/config{key}/` | `/config\{key\}/` |

## Performance Optimization

**Quick wins:** Put `filesize` first, avoid `nocase`, bounded regex `{1,100}`, prefer hex over regex.

**Red flags:** Strings <4 bytes, unbounded regex (`.*`), modules without file-type filter.

See [performance.md](references/performance.md) for atom theory and optimization details.

## Reference Documents

| Topic | Document |
|-------|----------|
| Naming and metadata conventions | [style-guide.md](references/style-guide.md) |
| Performance and atom optimization | [performance.md](references/performance.md) |
| String types and judgment | [strings.md](references/strings.md) |
| Testing and validation | [testing.md](references/testing.md) |
| Chrome extension module (crx) | [crx-module.md](references/crx-module.md) |
| Android DEX module (dex) | [dex-module.md](references/dex-module.md) |

## Workflows

| Topic | Document |
|-------|----------|
| Complete rule development process | [rule-development.md](workflows/rule-development.md) |

## Example Rules

The `examples/` directory contains real, attributed rules demonstrating best practices:

| Example | Demonstrates | Source |
|---------|--------------|--------|
| [MAL_Win_Remcos_Jan25.yar](examples/MAL_Win_Remcos_Jan25.yar) | PE malware: graduated string counts, multiple rules per family | Elastic Security |
| [MAL_Mac_ProtonRAT_Jan25.yar](examples/MAL_Mac_ProtonRAT_Jan25.yar) | macOS: Mach-O magic bytes, multi-category grouping | Airbnb BinaryAlert |
| [MAL_NPM_SupplyChain_Jan25.yar](examples/MAL_NPM_SupplyChain_Jan25.yar) | npm supply chain: real attack patterns, ERC-20 selectors | Stairwell Research |
| [SUSP_JS_Obfuscation_Jan25.yar](examples/SUSP_JS_Obfuscation_Jan25.yar) | JavaScript: obfuscator detection, density-based matching | imp0rtp3, Nils Kuhnert |
| [SUSP_CRX_SuspiciousPermissions.yar](examples/SUSP_CRX_SuspiciousPermissions.yar) | Chrome extensions: crx module, permissions | Educational |

## Scripts

```bash
uv run {baseDir}/scripts/yara_lint.py rule.yar      # Validate style/metadata
uv run {baseDir}/scripts/atom_analyzer.py rule.yar  # Check string quality
```

See [README.md](../../README.md#scripts) for detailed script documentation.

## Quality Checklist

Before deploying any rule:

- [ ] Name follows `{CATEGORY}_{PLATFORM}_{FAMILY}_{VARIANT}_{DATE}` format
- [ ] Description starts with "Detects" and explains what/how
- [ ] All required metadata present (author, reference, date)
- [ ] Strings are unique (not API names, common paths, or format strings)
- [ ] All strings have 4+ bytes with good atom potential
- [ ] Base64 modifier only on strings with 3+ characters
- [ ] Regex patterns have escaped `{` and valid escape sequences
- [ ] Condition starts with cheap checks (filesize, magic bytes)
- [ ] Rule matches all target samples
- [ ] Rule produces zero matches on goodware corpus
- [ ] `yr check` passes with no errors
- [ ] `yr fmt --check` passes (consistent formatting)
- [ ] Linter passes with no errors
- [ ] Peer review completed

## Resources

### Quality YARA Rule Repositories

Learn from production rules. These repositories contain well-tested, properly attributed rules:

| Repository | Focus | Maintainer |
|------------|-------|------------|
| [Neo23x0/signature-base](https://github.com/Neo23x0/signature-base) | 17,000+ production rules, multi-platform | Florian Roth |
| [Elastic/protections-artifacts](https://github.com/elastic/protections-artifacts) | 1,000+ endpoint-tested rules | Elastic Security |
| [reversinglabs/reversinglabs-yara-rules](https://github.com/reversinglabs/reversinglabs-yara-rules) | Threat research rules | ReversingLabs |
| [imp0rtp3/js-yara-rules](https://github.com/imp0rtp3/js-yara-rules) | JavaScript/browser malware | imp0rtp3 |
| [InQuest/awesome-yara](https://github.com/InQuest/awesome-yara) | Curated index of resources | InQuest |

### Style & Performance Guides

| Guide | Purpose |
|-------|---------|
| [YARA Style Guide](https://github.com/Neo23x0/YARA-Style-Guide) | Naming conventions, metadata, string prefixes |
| [YARA Performance Guidelines](https://github.com/Neo23x0/YARA-Performance-Guidelines) | Atom optimization, regex bounds |
| [Kaspersky Applied YARA Training](https://yara.readthedocs.io/) | Expert techniques from production use |

### Tools

| Tool | Purpose |
|------|---------|
| [yarGen](https://github.com/Neo23x0/yarGen) | Extract candidate strings from samples |
| [FLOSS](https://github.com/mandiant/flare-floss) | Extract obfuscated and stack strings |
| [YARA-CI](https://yara-ci.cloud.virustotal.com/) | Automated goodware testing |
| [YaraDbg](https://yaradbg.dev) | Web-based rule debugger |

### macOS-Specific Resources

| Resource | Purpose |
|----------|---------|
| Apple XProtect | Production macOS rules at `/System/Library/CoreServices/XProtect.bundle/` |
| [objective-see](https://objective-see.org/) | macOS malware research and samples |
| [macOS Security Tools](https://github.com/0xmachos/macos-security-tools) | Reference list |

### Multi-Indicator Clustering Pattern

Production rules often group indicators by type:

```yara
strings:
    // Category A: Library indicators
    $a1 = "SRWebSocket" ascii
    $a2 = "SocketRocket" ascii

// Category B: Behavioral indicators
    $b1 = "SSH tunnel" ascii
    $b2 = "keylogger" ascii nocase

// Category C: C2 patterns
    $c1 = /https:\/\/[a-z0-9]{8,16}\.onion/

condition:
    filesize < 10MB and
    any of ($a*) and any of ($b*)  // Require evidence from BOTH categories
```

**Why this works:** Different indicator types have different confidence levels. A single C2 domain might be definitive, while you need multiple library imports to be confident. Grouping by `$a*`, `$b*`, `$c*` lets you express graduated requirements.