Zen MCP Server path-traversal blacklist bypass via non-canonical paths (CVE-2025-66689)

CVE-2025-66689 (CWE-22). The Zen MCP server's is_dangerous_path() blocks sensitive files with an exact-string blacklist but never canonicalizes the path before comparison, so a path that still resolves to a protected target slips through when it is wrapped in normalization tricks: a blacklisted prefix followed by ../ (/etc/shadow/../../../home/user/.ssh/id_rsa), a same-directory bounce (/home/user/.ssh/../.ssh/id_rsa), a /./ no-op (/etc/./shadow), or redundant slashes (/etc//shadow). The discriminator from the existing deep-../-chain rule (ATR-2026-00569) and the clean-credential-path rule (ATR-2026-00113) is that the SENSITIVE target token sits immediately adjacent to a normalization/traversal artefact — neither a clean credential path nor a bare ../ chain alone matches here.

Response Actions

References

1. 01

   Blacklisted sensitive prefix immediately followed by a ./ or ../ normalization hop that re-targets another sensitive file (Zen-MCP prefix-then-traversal bypass)

   field: contentop: regex
2. 02

   /etc/./shadow or /etc/../etc/shadow style same-dir or bounce normalization reaching an exact-blacklist target

   field: contentop: regex
3. 03

   .ssh/../.ssh/ same-directory bounce that re-resolves to an SSH private key while evading an exact ~/.ssh/id_rsa blacklist entry

   field: contentop: regex
4. 04

   Redundant FORWARD-slash variant (/etc//shadow , /.ssh//id_rsa) that defeats an exact-string blacklist while resolving to the protected file. Forward-slash only — backslash forms like \etc\\passwd are almost always doc/Windows-escaping in tutorials, not a unix redundant-slash bypass, and caused a wild FP.

   field: contentop: regex

Attack Examples (Rule Triggers)

1. {"path": "/etc/shadow/../../../home/user/.ssh/id_rsa"}

2. read_file("/home/user/.ssh/../.ssh/id_rsa")

3. requested file: /etc/./shadow

4. open("/etc//shadow")

5. {"file": "/root/.ssh/../.ssh/id_ed25519"}

6. path=/etc/../etc/passwd

7. load("/home/user/.aws/../.ssh/id_rsa")

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

Benign Examples (Rule Doesn't Trigger)

1. Legit absolute project source path, no normalization artefact or sensitive target

   path = "/home/user/project/main.py"

2. Normal relative import path, no sensitive credential/system target

   import "../../shared/utils"

3. Documentation sentence mentioning ../ with no sensitive target adjacent

   To go up a directory in the tutorial, use ../ before the folder name.

4. CVE patch-discussion prose, describes the fix without an inline non-canonical sensitive path

   CVE-2025-66689 is fixed by canonicalizing paths before the blacklist check in is_dangerous_path().

5. Research abstract describing the technique in prose, no literal payload

   We studied path-traversal bypasses where attackers reach SSH keys via redundant slashes and dot segments.

6. Clean ~/.ssh/id_rsa path with no traversal/normalization artefact (owned by ATR-2026-00113)

   ssh-keygen -lf ~/.ssh/id_rsa

7. Deep ../ chain to a build artefact, not a sensitive target (no overlap with ATR-2026-00569 either)

   relpath = "../../../build/output/lib.so"

8. Backup copy of passwd-like files via clean absolute paths, no normalization artefact

   cp /etc/passwd.bak /home/user/backup/passwd.copy

9. Security-education skill quoting a backslash-escaped \etc\passwd example (wild FP from skills-sh/sickn33 file-path-traversal.md) — backslash form is doc-escaping, not a unix redundant-slash bypass

   In this lab, requesting \etc\\passwd demonstrates the path-traversal example.

Known False Positive Contexts

• ▸Documentation or tutorials that mention ../ relative imports without a sensitive credential/system target adjacent to it
• ▸Legitimate absolute file paths to project source under /home/<user>/project that contain no /../ , /./ , or redundant-slash artefact
• ▸Security write-ups quoting the CVE that describe canonicalization in prose rather than presenting an inline non-canonical path to a sensitive file

title: Zen MCP Server path-traversal blacklist bypass via non-canonical paths (CVE-2025-66689)
id: ATR-2026-00578
rule_version: 1
status: experimental
description: >
  CVE-2025-66689 (CWE-22). The Zen MCP server's is_dangerous_path() blocks
  sensitive files with an exact-string blacklist but never canonicalizes the
  path before comparison, so a path that still resolves to a protected target
  slips through when it is wrapped in normalization tricks: a blacklisted
  prefix followed by ../ (/etc/shadow/../../../home/user/.ssh/id_rsa), a
  same-directory bounce (/home/user/.ssh/../.ssh/id_rsa), a /./ no-op
  (/etc/./shadow), or redundant slashes (/etc//shadow). The discriminator from
  the existing deep-../-chain rule (ATR-2026-00569) and the clean-credential-path
  rule (ATR-2026-00113) is that the SENSITIVE target token sits immediately
  adjacent to a normalization/traversal artefact — neither a clean credential
  path nor a bare ../ chain alone matches here.
author: "ATR Community (vulnerablemcp sync)"
date: 2026/06/12
schema_version: '0.1'
detection_tier: pattern
maturity: experimental
severity: high
references:
  owasp_llm:
  - "LLM07:2025 - System Prompt Leakage"
  - "LLM02:2025 - Sensitive Information Disclosure"
  owasp_agentic:
  - "ASI05:2026 - Unexpected Code Execution"
  - "ASI06:2026 - Memory and Context Poisoning"
  mitre_atlas:
  - "AML.T0057 - LLM Data Leakage"
  - "AML.T0056 - LLM Meta Prompt Extraction"
  cve:
  - CVE-2025-66689
  cwe:
  - CWE-22
  vulnerablemcp_id:
  - cve-2025-66689-zen-mcp-path-traversal
  external:
  - https://www.sentinelone.com/vulnerability-database/cve-2025-66689/
metadata_provenance:
  cve: vulnerablemcp-sync
  cwe: vulnerablemcp-sync
  vulnerablemcp: vulnerablemcp-sync
compliance:
  owasp_agentic:
    - id: ASI06:2026
      context: "OWASP Agentic ASI06:2026 is exercised by path-traversal blacklist bypass in Zen MCP Server that reads files outside the sandbox via non-canonical paths (CVE-2025-66689); this rule provides runtime detection of that technique."
      strength: primary
    - id: ASI05:2026
      context: "OWASP Agentic ASI05:2026 is exercised by path-traversal blacklist bypass in Zen MCP Server that reads files outside the sandbox via non-canonical paths (CVE-2025-66689); this rule provides runtime detection of that technique."
      strength: secondary
  owasp_llm:
    - id: LLM02:2025
      context: "OWASP LLM LLM02:2025 is exercised by path-traversal blacklist bypass in Zen MCP Server that reads files outside the sandbox via non-canonical paths (CVE-2025-66689); this rule is a detection implementation for that category."
      strength: primary
    - id: LLM07:2025
      context: "OWASP LLM LLM07:2025 is exercised by path-traversal blacklist bypass in Zen MCP Server that reads files outside the sandbox via non-canonical paths (CVE-2025-66689); this rule is a detection implementation for that category."
      strength: secondary
  eu_ai_act:
    - article: "15"
      context: "EU AI Act Article 15 (accuracy, robustness and cybersecurity) requires controls against path-traversal blacklist bypass in Zen MCP Server that reads files outside the sandbox via non-canonical paths (CVE-2025-66689); this rule provides runtime detection evidence for that obligation."
      strength: primary
    - article: "10"
      context: "EU AI Act Article 10 (data and data governance) requires controls against path-traversal blacklist bypass in Zen MCP Server that reads files outside the sandbox via non-canonical paths (CVE-2025-66689); this rule provides runtime detection evidence for that obligation."
      strength: secondary
  nist_ai_rmf:
    - function: Measure
      subcategory: MS.2.10
      context: "NIST AI RMF MS.2.10 (privacy risk examined and documented) is supported by this rule's detection of path-traversal blacklist bypass in Zen MCP Server that reads files outside the sandbox via non-canonical paths (CVE-2025-66689)."
      strength: primary
    - function: Measure
      subcategory: MS.2.7
      context: "NIST AI RMF MS.2.7 (security and resilience evaluated and documented) is supported by this rule's detection of path-traversal blacklist bypass in Zen MCP Server that reads files outside the sandbox via non-canonical paths (CVE-2025-66689)."
      strength: secondary
  iso_42001:
    - clause: "8.4"
      context: "ISO/IEC 42001 Clause 8.4 (AI system impact assessment) is operationalised by this rule's detection of path-traversal blacklist bypass in Zen MCP Server that reads files outside the sandbox via non-canonical paths (CVE-2025-66689)."
      strength: primary
    - clause: "9.1"
      context: "ISO/IEC 42001 Clause 9.1 (monitoring, measurement, analysis and evaluation) is operationalised by this rule's detection of path-traversal blacklist bypass in Zen MCP Server that reads files outside the sandbox via non-canonical paths (CVE-2025-66689)."
      strength: secondary

tags:
  category: context-exfiltration
  subcategory: path-normalization-bypass
  scan_target: runtime
  confidence: high
agent_source:
  type: mcp_exchange
  framework:
  - any
  provider:
  - any
detection:
  condition: any
  false_positives:
  - Documentation or tutorials that mention ../ relative imports without a sensitive credential/system target adjacent to it
  - Legitimate absolute file paths to project source under /home/<user>/project that contain no /../ , /./ , or redundant-slash artefact
  - Security write-ups quoting the CVE that describe canonicalization in prose rather than presenting an inline non-canonical path to a sensitive file
  conditions:
  - field: content
    operator: regex
    value: (?i)[\\/](?:etc[\\/](?:shadow|passwd|sudoers)|\.ssh|\.aws|\.config[\\/]gcloud|root)[\\/]\.{1,2}[\\/]{1,2}[^\s"'<>]{0,80}(?:id_(?:rsa|ed25519|ecdsa)|\.ssh|shadow|passwd|credentials|authorized_keys|\.pem|\.key)
    description: Blacklisted sensitive prefix immediately followed by a ./ or ../ normalization hop that re-targets another sensitive file (Zen-MCP prefix-then-traversal bypass)
  - field: content
    operator: regex
    value: (?i)[\\/]etc[\\/]\.{1,2}[\\/]{1,2}(?:etc[\\/]){0,1}(?:shadow|passwd|sudoers)\b
    description: /etc/./shadow or /etc/../etc/shadow style same-dir or bounce normalization reaching an exact-blacklist target
  - field: content
    operator: regex
    value: (?i)[\\/]\.ssh[\\/]\.\.[\\/]\.ssh[\\/](?:id_(?:rsa|ed25519|ecdsa)|authorized_keys|known_hosts)\b
    description: .ssh/../.ssh/ same-directory bounce that re-resolves to an SSH private key while evading an exact ~/.ssh/id_rsa blacklist entry
  - field: content
    operator: regex
    value: (?i)/etc/{2,}(?:shadow|passwd|sudoers)\b|/\.ssh/{2,}id_(?:rsa|ed25519|ecdsa)\b
    description: Redundant FORWARD-slash variant (/etc//shadow , /.ssh//id_rsa) that defeats an exact-string blacklist while resolving to the protected file. Forward-slash only — backslash forms like \etc\\passwd are almost always doc/Windows-escaping in tutorials, not a unix redundant-slash bypass, and caused a wild FP.
response:
  actions:
  - alert
  - block_tool
  message_template: >
    [ATR-2026-00578] Non-canonical path to a sensitive file detected
    (CVE-2025-66689, Zen MCP is_dangerous_path blacklist bypass). The path mixes
    a sensitive target (SSH key, /etc/shadow, credentials) with a normalization
    or traversal artefact (../ , /./ , redundant slash, blacklisted-prefix-then-..)
    that evades exact-string blacklists while still resolving to the protected
    file. Remediation: upgrade Zen MCP server to v9.8.2 or later; canonicalize
    paths (resolve symlinks, normalize . and ..) BEFORE any security check; prefer
    an allowlist of permitted directories over a sensitive-path blacklist.
  notify:
  - security_team
confidence: 85
wild_fp_rate: 0
test_cases:
  true_positives:
  - input: '{"path": "/etc/shadow/../../../home/user/.ssh/id_rsa"}'
    expected: triggered
    description: Blacklisted /etc/shadow prefix then ../ chain re-targeting an SSH private key (CVE-2025-66689 PoC)
  - input: read_file("/home/user/.ssh/../.ssh/id_rsa")
    expected: triggered
    description: .ssh/../.ssh/ same-directory bounce evading exact ~/.ssh/id_rsa blacklist
  - input: 'requested file: /etc/./shadow'
    expected: triggered
    description: /etc/./shadow no-op normalization reaching exact blacklist target
  - input: open("/etc//shadow")
    expected: triggered
    description: Redundant-slash variant defeating exact-string match on /etc/shadow
  - input: '{"file": "/root/.ssh/../.ssh/id_ed25519"}'
    expected: triggered
    description: root .ssh bounce to ed25519 private key
  - input: path=/etc/../etc/passwd
    expected: triggered
    description: /etc/../etc/passwd bounce normalization to /etc/passwd
  - input: load("/home/user/.aws/../.ssh/id_rsa")
    expected: triggered
    description: Blacklisted .aws prefix then ../ hop re-targeting an SSH key
  true_negatives:
  - input: path = "/home/user/project/main.py"
    expected: not_triggered
    description: Legit absolute project source path, no normalization artefact or sensitive target
  - input: import "../../shared/utils"
    expected: not_triggered
    description: Normal relative import path, no sensitive credential/system target
  - input: To go up a directory in the tutorial, use ../ before the folder name.
    expected: not_triggered
    description: Documentation sentence mentioning ../ with no sensitive target adjacent
  - input: CVE-2025-66689 is fixed by canonicalizing paths before the blacklist check in is_dangerous_path().
    expected: not_triggered
    description: CVE patch-discussion prose, describes the fix without an inline non-canonical sensitive path
  - input: We studied path-traversal bypasses where attackers reach SSH keys via redundant slashes and dot segments.
    expected: not_triggered
    description: Research abstract describing the technique in prose, no literal payload
  - input: ssh-keygen -lf ~/.ssh/id_rsa
    expected: not_triggered
    description: Clean ~/.ssh/id_rsa path with no traversal/normalization artefact (owned by ATR-2026-00113)
  - input: relpath = "../../../build/output/lib.so"
    expected: not_triggered
    description: Deep ../ chain to a build artefact, not a sensitive target (no overlap with ATR-2026-00569 either)
  - input: cp /etc/passwd.bak /home/user/backup/passwd.copy
    expected: not_triggered
    description: Backup copy of passwd-like files via clean absolute paths, no normalization artefact
  - input: 'In this lab, requesting \etc\\passwd demonstrates the path-traversal example.'
    expected: not_triggered
    description: Security-education skill quoting a backslash-escaped \etc\passwd example (wild FP from skills-sh/sickn33 file-path-traversal.md) — backslash form is doc-escaping, not a unix redundant-slash bypass
_llm_authored:
  model: claude (gstack subagent)
  generalization_note: >
    Generalizes the Zen-MCP is_dangerous_path() exact-blacklist bypass beyond the
    two literal PoCs (/home/user/.ssh/../.ssh/id_rsa and /etc/./shadow) by keying
    on the structural invariant of the bug class: a SENSITIVE target token
    (id_rsa/ed25519/ecdsa, /etc/shadow|passwd|sudoers, credentials, authorized_keys,
    .pem/.key) appearing ADJACENT to a path-normalization artefact (blacklisted
    prefix then ../ , same-dir .ssh/../.ssh/ bounce, /./ no-op, /etc/../etc bounce,
    or redundant slashes). This is deliberately disjoint from ATR-2026-00569
    (requires 2+ consecutive ../ hops to a system target — a clean deep chain, no
    prefix/bounce/normalization adjacency) and from ATR-2026-00113 (clean
    credential paths with no traversal), so a bare ../ chain or a clean
    ~/.ssh/id_rsa does not trigger this rule. Bounded {0,80}/{1,2} spans keep the
    regex linear and avoid catastrophic backtracking.
  note: Generation-time authoring; verified by deterministic gate. Runtime detection is pure regex. Human review required before merge.