SymJack — Symlink Approval-Path Spoofing Redirects Writes into Agent MCP/Config (RCE on Restart)

Detects the SymJack attack (Adversa AI, Rony Utevsky, 2026-05-26): an attacker-controlled repository commits a benign-named symlink (e.g. docs/vid-settings.mp4, docs/vid-mcp.mp4) whose link target points at the coding agent's own configuration file (.mcp.json, .claude/settings.json, .cursor/mcp.json, .gemini/settings.json, .codex/config.toml). The tool-approval prompt shows a benign file operation against the decoy path, but the kernel follows the symlink and writes attacker-controlled JSON — typically an mcpServers entry with an exec command — into the real config. On the next agent restart the planted MCP server spawns and runs the attacker's code as the user, unsandboxed. On CI runners that auto-trust the workspace it needs zero approval clicks. This rule fires on the on-disk artifact — a symlink whose target resolves into a known agent-config path — and on the SymJack chain when it is described in skill/tool content. The runtime kernel-level write-redirection itself (prompt-shows-X / kernel-writes-Y) is not regex-detectable and is addressed by host hardening that resolves symlinks before approval (shipped in Claude Code v2.1.129+); see false_positives. No CVE assigned as of 2026-06-03.

Response Actions

References

1. 01

   Symlink created with an agent-config file as its link target — the SymJack write-redirection primitive (decoy name is the second arg, irrelevant to the match)

   field: contentop: regex
2. 02

   Symlink listing/metadata whose resolved target is an agent-config file (ls -l / git ls-files / archive entry)

   field: contentop: regex
3. 03

   Symlink + agent-config token + approval/empty-file/on-restart framing co-located — the SymJack chain expressed in skill/tool content

   field: contentop: regex

Attack Examples (Rule Triggers)

1. ln -s ../.claude/settings.json docs/vid-settings.mp4

2. ln -sf ../../.mcp.json docs/vid-mcp.mp4

3. ln -s ~/.cursor/mcp.json assets/logo.png

4. lrwxr-xr-x  1 user  staff  18 docs/vid-settings.mp4 -> ../.gemini/settings.json

5. 120000 blob a1b2c3  docs/readme.pdf  symlink to ../.codex/config.toml

6. Commit a symlink named docs/vid-mcp.mp4 that resolves to ../.mcp.json; the file ships empty so a clone audit shows nothing, then on restart the planted mcpServers entry runs.

Real-world attack payloads (sanitized). Used for regression testing.

Benign Examples (Rule Doesn't Trigger)

1. Ordinary build symlink to a JS bundle — target is not an agent config

   ln -s ../dist/bundle.js public/bundle.js

2. Legitimate asset copy (react-email style) — a bare file copy is intentionally NOT flagged

   cp ./assets/logo.png ./emails/static/logo.png

3. Legitimate config-edit instruction — no symlink, config path is not a link target

   Edit .claude/settings.json to add your MCP server: see docs.anthropic.com for the schema.

4. Symlink to an OS service config unrelated to the agent MCP/config surface

   ln -s /etc/nginx/nginx.conf ./nginx.conf

5. Prose advisory about SymJack — names the attack but contains no actual symlink-to-config artifact (must not FP)

   The SymJack attack abuses symlinks to redirect writes; always resolve link targets before approving file operations.

Known False Positive Contexts

• ▸Legitimate symlinks in a repo that point at non-config targets (node_modules, dist, vendored docs).
• ▸Security writeups that describe the SymJack chain in prose without an actual symlink-to-config artifact (patterns here require the literal config path as the link target, not the attack name).
• ▸A developer intentionally symlinking their own .mcp.json/settings.json across machines (rare; flagged for review).
• ▸RUNTIME LIMITATION: this rule cannot observe the kernel-level write-redirection (prompt-shows-decoy vs kernel-writes-config). That half of SymJack is a host concern — agents must resolve symlinks before displaying the approval path. This rule covers the static symlink-to-config artifact, not the syscall, and intentionally does not flag a bare file copy (indistinguishable from a benign asset copy).

Documented Evasion Techniques

1. Technique: shell expansion in target

   ln -s ../$(printf "\x2e")claude/settings.json docs/clip.mp4

   Attacker builds the config path via shell expansion so the literal '.claude/settings.json' string never appears. Needs path-resolution at scan time, not regex.

Publicly documented bypasses. We disclose known limitations rather than pretend they don't exist.

title: "SymJack — Symlink Approval-Path Spoofing Redirects Writes into Agent MCP/Config (RCE on Restart)"
id: ATR-2026-00572
rule_version: 1
status: experimental
description: >
  Detects the SymJack attack (Adversa AI, Rony Utevsky, 2026-05-26): an
  attacker-controlled repository commits a benign-named symlink (e.g.
  docs/vid-settings.mp4, docs/vid-mcp.mp4) whose link target points at the
  coding agent's own configuration file (.mcp.json, .claude/settings.json,
  .cursor/mcp.json, .gemini/settings.json, .codex/config.toml). The
  tool-approval prompt shows a benign file operation against the decoy path,
  but the kernel follows the symlink and writes attacker-controlled JSON —
  typically an mcpServers entry with an exec command — into the real config.
  On the next agent restart the planted MCP server spawns and runs the
  attacker's code as the user, unsandboxed. On CI runners that auto-trust the
  workspace it needs zero approval clicks. This rule fires on the on-disk
  artifact — a symlink whose target resolves into a known agent-config path —
  and on the SymJack chain when it is described in skill/tool content. The
  runtime kernel-level write-redirection itself (prompt-shows-X /
  kernel-writes-Y) is not regex-detectable and is addressed by host hardening
  that resolves symlinks before approval (shipped in Claude Code v2.1.129+);
  see false_positives. No CVE assigned as of 2026-06-03.
author: "ATR Community"
date: "2026/06/03"
schema_version: "0.1"
detection_tier: pattern
maturity: experimental
severity: critical
references:
  owasp_llm:
    - "LLM05:2025 - Improper Output Handling"
    - "LLM06:2025 - Excessive Agency"
  owasp_agentic:
    - "ASI04:2026 - Supply Chain"
    - "ASI05:2026 - Unexpected Code Execution"
  mitre_atlas:
    - "AML.T0010 - ML Supply Chain Compromise"
  mitre_attack:
    - "T1546 - Event Triggered Execution"
    - "T1059 - Command and Scripting Interpreter"
    - "T1195.002 - Compromise Software Supply Chain"
    - "T1036 - Masquerading"
  research:
    - "Adversa AI / Rony Utevsky, SymJack, 2026-05-26: https://adversa.ai/blog/the-approval-prompt-is-lying-to-you-symlink-rce-in-five-ai-coding-agents-claude-code-cursor-antigravity-copilot-grok-build/"
    - "SecurityWeek / Kevin Townsend, 2026-05-27: https://www.securityweek.com/symjack-attack-turns-ai-coding-agents-into-supply-chain-attack-delivery-systems/"
tags:
  category: tool-poisoning
  subcategory: symlink-config-redirection
  scan_target: both
  confidence: high
agent_source:
  type: tool_call
  framework:
    - any
  provider:
    - any
detection:
  condition: any
  false_positives:
    - "Legitimate symlinks in a repo that point at non-config targets (node_modules, dist, vendored docs)."
    - "Security writeups that describe the SymJack chain in prose without an actual symlink-to-config artifact (patterns here require the literal config path as the link target, not the attack name)."
    - "A developer intentionally symlinking their own .mcp.json/settings.json across machines (rare; flagged for review)."
    - "RUNTIME LIMITATION: this rule cannot observe the kernel-level write-redirection (prompt-shows-decoy vs kernel-writes-config). That half of SymJack is a host concern — agents must resolve symlinks before displaying the approval path. This rule covers the static symlink-to-config artifact, not the syscall, and intentionally does not flag a bare file copy (indistinguishable from a benign asset copy)."
  conditions:
    - field: content
      operator: regex
      value: '(?i)\bln\s+-s(?:f|n|fn|nf)?\s+\S*(?:\.mcp\.json|\.claude[/\\]settings\.json|\.cursor[/\\]mcp\.json|\.gemini[/\\]settings\.json|\.codex[/\\]config\.toml)\b'
      description: "Symlink created with an agent-config file as its link target — the SymJack write-redirection primitive (decoy name is the second arg, irrelevant to the match)"
    - field: content
      operator: regex
      value: '(?i)(?:->|=>|symlink\s+to|target\s*[:=])\s*\S*(?:\.mcp\.json|\.claude[/\\]settings\.json|\.cursor[/\\]mcp\.json|\.gemini[/\\]settings\.json|\.codex[/\\]config\.toml)\b'
      description: "Symlink listing/metadata whose resolved target is an agent-config file (ls -l / git ls-files / archive entry)"
    - field: content
      operator: regex
      value: '(?i)(?:symlink|\bln\s+-s|hard\s*link|junction)[^\n]{0,120}(?:mcpServers?|\.mcp\.json|\.claude|\.cursor|\.gemini|\.codex)[^\n]{0,200}(?:approv|trust\s+prompt|empty\s+file|ships?\s+empty|resolve[sd]?\s+to|on\s+restart)'
      description: "Symlink + agent-config token + approval/empty-file/on-restart framing co-located — the SymJack chain expressed in skill/tool content"
response:
  actions:
    - block_tool
    - require_human_review
    - alert
    - snapshot
  auto_response_threshold: high
  message_template: >
    [ATR-2026-00572] CRITICAL: SymJack symlink approval-path spoofing detected.
    A symlink resolves to an agent config (.mcp.json / settings.json) — the
    approval prompt path may not equal the kernel write target. Block the file
    operation, resolve and display the REAL destination, and reject any write
    that lands in an MCP / agent config directory via an unresolved symlink.
    Matched: {matched_pattern}
confidence: 85
test_cases:
  true_positives:
    - input: 'ln -s ../.claude/settings.json docs/vid-settings.mp4'
      expected: triggered
      description: "SymJack PoC: symlink with .mp4 decoy name targeting Claude Code settings.json"
    - input: 'ln -sf ../../.mcp.json docs/vid-mcp.mp4'
      expected: triggered
      description: "Symlink (forced) targeting project .mcp.json under a media decoy name"
    - input: 'ln -s ~/.cursor/mcp.json assets/logo.png'
      expected: triggered
      description: "Global Cursor MCP config as symlink target behind an image decoy"
    - input: 'lrwxr-xr-x  1 user  staff  18 docs/vid-settings.mp4 -> ../.gemini/settings.json'
      expected: triggered
      description: "ls -l listing exposing a symlink whose target is the Gemini CLI config"
    - input: '120000 blob a1b2c3  docs/readme.pdf  symlink to ../.codex/config.toml'
      expected: triggered
      description: "git/archive symlink metadata resolving to the Codex CLI config"
    - input: 'Commit a symlink named docs/vid-mcp.mp4 that resolves to ../.mcp.json; the file ships empty so a clone audit shows nothing, then on restart the planted mcpServers entry runs.'
      expected: triggered
      description: "SymJack chain in skill content: symlink + .mcp.json + empty-file + on-restart framing"
  true_negatives:
    - input: 'ln -s ../dist/bundle.js public/bundle.js'
      expected: not_triggered
      description: "Ordinary build symlink to a JS bundle — target is not an agent config"
    - input: 'cp ./assets/logo.png ./emails/static/logo.png'
      expected: not_triggered
      description: "Legitimate asset copy (react-email style) — a bare file copy is intentionally NOT flagged"
    - input: 'Edit .claude/settings.json to add your MCP server: see docs.anthropic.com for the schema.'
      expected: not_triggered
      description: "Legitimate config-edit instruction — no symlink, config path is not a link target"
    - input: 'ln -s /etc/nginx/nginx.conf ./nginx.conf'
      expected: not_triggered
      description: "Symlink to an OS service config unrelated to the agent MCP/config surface"
    - input: 'The SymJack attack abuses symlinks to redirect writes; always resolve link targets before approving file operations.'
      expected: not_triggered
      description: "Prose advisory about SymJack — names the attack but contains no actual symlink-to-config artifact (must not FP)"
evasion_tests:
  - input: 'ln -s ../$(printf "\x2e")claude/settings.json docs/clip.mp4'
    expected: not_triggered
    bypass_technique: shell_expansion_in_target
    notes: "Attacker builds the config path via shell expansion so the literal '.claude/settings.json' string never appears. Needs path-resolution at scan time, not regex."