Security audit

S³ YARA Rule Authoring

Security checks for vulnerabilities and agentic risk

Overview

This skill is a benign guide for writing YARA-X detection rules; the malware terms are examples for signatures, not code it runs.

Use this skill when you want help authoring YARA-X rules. Before running package-manager install commands or scanning files, confirm the selected samples and directories are intentional.

A.I.G

Vulnerability Patterns

Skill Instruction HijackingAlters the agent's session goals or safety constraints when the skill loads
Agent Memory PoisoningWrites attacker-controlled rules into memory that affect later sessions
Remote Payload Retrieval and ExecutionFetches external code whose behavior can change after review
Embedded Malicious CodeShips malicious scripts inside the skill and executes them locally
Unauthorized Access and Privilege EscalationObtains permissions beyond the task's legitimate needs

SkillSpector

Vulnerability Patterns

YARA SignaturesMalware Match, Webshell Match, Cryptominer Match
Prompt InjectionInstruction Override, Hidden Instructions, Exfiltration Commands
Data ExfiltrationExternal Transmission, Env Variable Harvesting, File System Enumeration
Privilege EscalationExcessive Permissions, Sudo/Root Execution, Credential Access
Supply ChainUnpinned Dependencies, External Script Fetching, Obfuscated Code

Findings (1)

YARA rule 'c2_framework_indicators': Command-and-control framework indicators (Cobalt Strike, Metasploit, Sliver, etc.) [malware]

Critical

Category: YARA Match
Confidence: 85% confidence
Finding: YARA rule matched a known malware signature (reverse shell, backdoor, ransomware, C2 framework, or info stealer).
Content: Scanner excerpt · SKILL.md (reported line 59)May include surrounding context.
$api2 = "WriteProcessMemory" ascii $str1 = { 48 8B 05 ?? ?? ?? ?? 48 85 C0 } // hex with wildcards $pdb = /[A-Z]:\.\Release\..pdb/ nocase

text
condition: uint16(0) == 0x5A4D and filesize < 5MB and (2 of ($api*) and $str1) or $pdb

}

text
## Naming Convention `Family_Variant_Technique` — examples: - `Emotet_Loader_DocumentMacro` - `CobaltStrike_Beacon_x64` - `Generic_Cryptominer_XMRig` ## String Selection **Good strings (unique, specific):** - Mutex names, PDB paths, C2 URLs - Unique byte sequences from disassembly - Custom encryption constants - Uncommon API call sequences **Bad strings (too common, high FP):** - `http://`, `https://`, common API names alone - Single common words, short strings (<4 bytes) - Strings found in Windows system files ## Condition Patterns ```yara // Performance-ordered (cheap → expensive) condition: uint16(0) == 0x5A4D and // Magic bytes (instant) filesize < 10MB and

Static analysis

No suspicious patterns detected.