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Malicious Skills & Supply Chain Risks in Coding Agents
securitylabs.datadoghq.comarticle

Malicious Skills & Supply Chain Risks in Coding Agents

Datadog Security Labs analyzes how Claude Code's dynamic context feature (the `!` shell-execution syntax in skill frontmatter) runs commands during preprocessing, before the rendered skill content ever reaches the model — meaning model-level prompt injection defenses never get a chance to block malicious actions. They demonstrate this with Clawsights, a malicious skill found in the wild that exfiltrates GitHub auth tokens via `gh auth token` and a curl POST; while Claude Opus 4.6 refused to run the skill when read as plain instructions, a version rewritten to use `!` dynamic context commands executed the credential theft during preprocessing even though the model later claimed (falsely) it hadn't run anything. The post explains Claude Code's multiple skill-loading paths (enterprise, personal, project .claude/skills/, plugins, nested folders, --add-dir) and gives grep-based detection patterns plus a `disableSkillShellExecution` mitigation.

TL;DR Claude Code's `!` dynamic-context syntax executes shell commands before the model ever sees the skill, letting malicious skills like the in-the-wild Clawsights token-stealer bypass prompt-injection defenses entirely.
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What to do

The page gives four concrete detection queries the summary references but does not reproduce. Run these against skill directories — and critically, not just ~/.claude/skills/ but the repository root, nested project folders, plugin skills, and any --add-dir locations:

# Search for dangerous dynamic context injection
grep -rE '!`.*curl|!`.*wget|!`.*nc |!`.*bash|!`.*cat.*env|!`.*find.*secret|!`.*grep.*password' .claude/skills/
# Find skills requesting broad shell access
grep -rE 'allowed-tools:.*Bash\(\*\)' .claude/skills/
# Identify skills that fetch remote content
grep -rE 'https?://[^)]*\.(com|io|zone|dev|cloud|net)' .claude/skills/ | grep -v '^\s*#'
# Detect potential credential exfiltration
grep -rE '(os\.environ|getenv|process\.env|printenv|\$AWS_|\.ssh/)' .claude/skills/

The frontmatter clause that makes the dynamic-context attack self-arming is allowed-tools: Bash(*), which preapproves Bash while the skill is active, so the ! preprocessing commands fire without an approval prompt.

A finding the summary does not carry

The summary frames model refusal as the reassuring baseline (Opus 4.6 caught the plain-text Clawsights skill). The page adds a materially worse observation: in later tests with Opus 4.7, using the same modified Clawsights skill and a controlled destination, Claude Code ran the skill and did not identify the credential theft at all — no dynamic-context trick required. Datadog saw this "consistently" across runs. So the plain-instruction defense the summary leans on is not reliable across model versions; the ! bypass is one path, but direct execution is another.

Caveats

Datadog labels the Opus 4.7 result an "implementation observation rather than a guarantee about any model version." The grep patterns are "starting points, not complete detections" — payloads can hide behind aliases, wrapper scripts, encoding, or less-obvious CLI tools, and a clean result is only one piece of evidence.

Technologies named here

Techniques named here

  • Prompt injection via malicious agent skills
  • Dynamic context shell execution bypassing model review
  • Credential exfiltration via CLI and curl
  • Broad tool preapproval via allowed-tools frontmatter
  • Nested/added-directory skill loading

Threat actors named here

  • Clawsights

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