文件上传漏洞靶场通关

#前言：记录一下学习。所用到的靶场在github上.

pass–01

​

通过观察发现这个源码是前端代码，而前端进行文件检测等于没有检测，我们进行抓包更改为php文件即可。

​

function checkFile() {
    var file = document.getElementsByName('upload_file')[0].value;
    if (file == null || file == "") {
        alert("请选择要上传的文件!");
        return false;
    }
    //定义允许上传的文件类型
    var allow_ext = ".jpg|.png|.gif";
    //提取上传文件的类型
    var ext_name = file.substring(file.lastIndexOf("."));
    //判断上传文件类型是否允许上传
    if (allow_ext.indexOf(ext_name + "|") == -1) {
        var errMsg = "该文件不允许上传，请上传" + allow_ext + "类型的文件,当前文件类型为：" + ext_name;
        alert(errMsg);
        return false;
    }
}

用蚁剑进行连接

pass–02

​

观察源码发现这个代码只对文件上传类型进行了限制，我们仍然可以抓包修改jpg 为php 文件

$is_upload = false;
$msg = null;
if (isset($_POST['submit'])) {
    if (file_exists(UPLOAD_PATH)) {
        if (($_FILES['upload_file']['type'] == 'image/jpeg') || ($_FILES['upload_file']['type'] == 'image/png') || ($_FILES['upload_file']['type'] == 'image/gif')) {
            $temp_file = $_FILES['upload_file']['tmp_name'];
            $img_path = UPLOAD_PATH . '/' . $_FILES['upload_file']['name']            
            if (move_uploaded_file($temp_file, $img_path)) {
                $is_upload = true;
            } else {
                $msg = '上传出错！';
            }
        } else {
            $msg = '文件类型不正确，请重新上传！';
        }
    } else {
        $msg = UPLOAD_PATH.'文件夹不存在,请手工创建！';
    }
}

进行连接

pass–03

这次对文件的上传进行了后缀的限制，可以看出这是个黑名单--不允许上传给出的文件后缀名. 将1.php名字改为1.php4(php1,php2,php3,php4,php5),phtml，

​

$is_upload = false;
$msg = null;
if (isset($_POST['submit'])) {
    if (file_exists(UPLOAD_PATH)) {
        $deny_ext = array('.asp','.aspx','.php','.jsp');
        $file_name = trim($_FILES['upload_file']['name']);
        $file_name = deldot($file_name);//删除文件名末尾的点
        $file_ext = strrchr($file_name, '.');
        $file_ext = strtolower($file_ext); //转换为小写
        $file_ext = str_ireplace('::$DATA', '', $file_ext);//去除字符串::$DATA
        $file_ext = trim($file_ext); //收尾去空

        if(!in_array($file_ext, $deny_ext)) {
            $temp_file = $_FILES['upload_file']['tmp_name'];
            $img_path = UPLOAD_PATH.'/'.date("YmdHis").rand(1000,9999).$file_ext;            
            if (move_uploaded_file($temp_file,$img_path)) {
                 $is_upload = true;
            } else {
                $msg = '上传出错！';
            }
        } else {
            $msg = '不允许上传.asp,.aspx,.php,.jsp后缀文件！';
        }
    } else {
        $msg = UPLOAD_PATH . '文件夹不存在,请手工创建！';
    }
}

pass–04

通过源码可以看到对文件后缀进行了全面封杀。这时可以用到.htaccess 文件对上传的jpg 文件都解析为php文件

$is_upload = false;
$msg = null;
if (isset($_POST['submit'])) {
    if (file_exists(UPLOAD_PATH)) {
        $deny_ext = array(".php",".php5",".php4",".php3",".php2",".php1",".html",".htm",".phtml",".pht",".pHp",".pHp5",".pHp4",".pHp3",".pHp2",".pHp1",".Html",".Htm",".pHtml",".jsp",".jspa",".jspx",".jsw",".jsv",".jspf",".jtml",".jSp",".jSpx",".jSpa",".jSw",".jSv",".jSpf",".jHtml",".asp",".aspx",".asa",".asax",".ascx",".ashx",".asmx",".cer",".aSp",".aSpx",".aSa",".aSax",".aScx",".aShx",".aSmx",".cEr",".sWf",".swf",".ini");
        $file_name = trim($_FILES['upload_file']['name']);
        $file_name = deldot($file_name);//删除文件名末尾的点
        $file_ext = strrchr($file_name, '.');
        $file_ext = strtolower($file_ext); //转换为小写
        $file_ext = str_ireplace('::$DATA', '', $file_ext);//去除字符串::$DATA
        $file_ext = trim($file_ext); //收尾去空

        if (!in_array($file_ext, $deny_ext)) {
            $temp_file = $_FILES['upload_file']['tmp_name'];
            $img_path = UPLOAD_PATH.'/'.$file_name;
            if (move_uploaded_file($temp_file, $img_path)) {
                $is_upload = true;
            } else {
                $msg = '上传出错！';
            }
        } else {
            $msg = '此文件不允许上传!';
        }
    } else {
        $msg = UPLOAD_PATH . '文件夹不存在,请手工创建！';
    }
}

AddType 指令可以将给定的文件扩展名映射到指定的内容类型。 将jpg文件解析为php文件。 去网上查了一下 在进行上述实战时，不要使用新版本的phpstudy，新版本的phpstudy对.htaccess支持存在问题，推荐使用2018版本的phpstudy。
原创不易，我用的好像是新版所以上传之后解析不成php文件

pass–05

​

根据源码 可以看出只对文件后缀名进行了处理。去掉了文件名末尾的点，取了点后面的后缀名。并进行了小写转换。在和数组里面的后缀名匹配如果出现一样的不允许上传，根据这个我们可以使用1.php. . 先去掉末尾的点只剩下1.php. 他再取.后面的值只有空格。最后只剩下1.php. 便绕过了他的检测机制。

$is_upload = false;
$msg = null;
if (isset($_POST['submit'])) {
    if (file_exists(UPLOAD_PATH)) {
        $deny_ext = array(".php",".php5",".php4",".php3",".php2",".html",".htm",".phtml",".pht",".pHp",".pHp5",".pHp4",".pHp3",".pHp2",".Html",".Htm",".pHtml",".jsp",".jspa",".jspx",".jsw",".jsv",".jspf",".jtml",".jSp",".jSpx",".jSpa",".jSw",".jSv",".jSpf",".jHtml",".asp",".aspx",".asa",".asax",".ascx",".ashx",".asmx",".cer",".aSp",".aSpx",".aSa",".aSax",".aScx",".aShx",".aSmx",".cEr",".sWf",".swf",".htaccess");
        $file_name = trim($_FILES['upload_file']['name']);
        $file_name = deldot($file_name);//删除文件名末尾的点
        $file_ext = strrchr($file_name, '.');
        $file_ext = strtolower($file_ext); //转换为小写
        $file_ext = str_ireplace('::$DATA', '', $file_ext);//去除字符串::$DATA
        $file_ext = trim($file_ext); //首尾去空
        
        if (!in_array($file_ext, $deny_ext)) {
            $temp_file = $_FILES['upload_file']['tmp_name'];
            $img_path = UPLOAD_PATH.'/'.$file_name;
            if (move_uploaded_file($temp_file, $img_path)) {
                $is_upload = true;
            } else {
                $msg = '上传出错！';
            }
        } else {
            $msg = '此文件类型不允许上传！';
        }
    } else {
        $msg = UPLOAD_PATH . '文件夹不存在,请手工创建！';
    }
}

pass–06

这个代码居然把转换小写功能去掉了，这样我们可以用大小写来绕过检测。

$is_upload = false;
$msg = null;
if (isset($_POST['submit'])) {
    if (file_exists(UPLOAD_PATH)) {
        $deny_ext = array(".php",".php5",".php4",".php3",".php2",".html",".htm",".phtml",".pht",".pHp",".pHp5",".pHp4",".pHp3",".pHp2",".Html",".Htm",".pHtml",".jsp",".jspa",".jspx",".jsw",".jsv",".jspf",".jtml",".jSp",".jSpx",".jSpa",".jSw",".jSv",".jSpf",".jHtml",".asp",".aspx",".asa",".asax",".ascx",".ashx",".asmx",".cer",".aSp",".aSpx",".aSa",".aSax",".aScx",".aShx",".aSmx",".cEr",".sWf",".swf",".htaccess",".ini");
        $file_name = trim($_FILES['upload_file']['name']);
        $file_name = deldot($file_name);//删除文件名末尾的点
        $file_ext = strrchr($file_name, '.');
        $file_ext = str_ireplace('::$DATA', '', $file_ext);//去除字符串::$DATA
        $file_ext = trim($file_ext); //首尾去空

        if (!in_array($file_ext, $deny_ext)) {
            $temp_file = $_FILES['upload_file']['tmp_name'];
            $img_path = UPLOAD_PATH.'/'.date("YmdHis").rand(1000,9999).$file_ext;
            if (move_uploaded_file($temp_file, $img_path)) {
                $is_upload = true;
            } else {
                $msg = '上传出错！';
            }
        } else {
            $msg = '此文件类型不允许上传！';
        }
    } else {
        $msg = UPLOAD_PATH . '文件夹不存在,请手工创建！';
    }
}

pass–07

可以拿下题代码跟上题代码进行比较看缺了什么 就对什么进行突破,很明显首尾去空被弄掉了，那就是这题的突破口

$is_upload = false;
$msg = null;
if (isset($_POST['submit'])) {
    if (file_exists(UPLOAD_PATH)) {
        $deny_ext = array(".php",".php5",".php4",".php3",".php2",".html",".htm",".phtml",".pht",".pHp",".pHp5",".pHp4",".pHp3",".pHp2",".Html",".Htm",".pHtml",".jsp",".jspa",".jspx",".jsw",".jsv",".jspf",".jtml",".jSp",".jSpx",".jSpa",".jSw",".jSv",".jSpf",".jHtml",".asp",".aspx",".asa",".asax",".ascx",".ashx",".asmx",".cer",".aSp",".aSpx",".aSa",".aSax",".aScx",".aShx",".aSmx",".cEr",".sWf",".swf",".htaccess",".ini");
        $file_name = $_FILES['upload_file']['name'];
        $file_name = deldot($file_name);//删除文件名末尾的点
        $file_ext = strrchr($file_name, '.');
        $file_ext = strtolower($file_ext); //转换为小写
        $file_ext = str_ireplace('::$DATA', '', $file_ext);//去除字符串::$DATA
        
        if (!in_array($file_ext, $deny_ext)) {
            $temp_file = $_FILES['upload_file']['tmp_name'];
            $img_path = UPLOAD_PATH.'/'.date("YmdHis").rand(1000,9999).$file_ext;
            if (move_uploaded_file($temp_file,$img_path)) {
                $is_upload = true;
            } else {
                $msg = '上传出错！';
            }
        } else {
            $msg = '此文件不允许上传';
        }
    } else {
        $msg = UPLOAD_PATH . '文件夹不存在,请手工创建！';
    }
}

pass–08

代码没有首先去掉末尾的点，而先取的最后一个点后面的内容，如我们上传一个1.php. 他就会取点后面的内容在进行小写转换，绕过了黑名单检测

$is_upload = false;
$msg = null;
if (isset($_POST['submit'])) {
    if (file_exists(UPLOAD_PATH)) {
        $deny_ext = array(".php",".php5",".php4",".php3",".php2",".html",".htm",".phtml",".pht",".pHp",".pHp5",".pHp4",".pHp3",".pHp2",".Html",".Htm",".pHtml",".jsp",".jspa",".jspx",".jsw",".jsv",".jspf",".jtml",".jSp",".jSpx",".jSpa",".jSw",".jSv",".jSpf",".jHtml",".asp",".aspx",".asa",".asax",".ascx",".ashx",".asmx",".cer",".aSp",".aSpx",".aSa",".aSax",".aScx",".aShx",".aSmx",".cEr",".sWf",".swf",".htaccess",".ini");
        $file_name = trim($_FILES['upload_file']['name']);
        $file_ext = strrchr($file_name, '.');
        $file_ext = strtolower($file_ext); //转换为小写
        $file_ext = str_ireplace('::$DATA', '', $file_ext);//去除字符串::$DATA
        $file_ext = trim($file_ext); //首尾去空
        
        if (!in_array($file_ext, $deny_ext)) {
            $temp_file = $_FILES['upload_file']['tmp_name'];
            $img_path = UPLOAD_PATH.'/'.$file_name;
            if (move_uploaded_file($temp_file, $img_path)) {
                $is_upload = true;
            } else {
                $msg = '上传出错！';
            }
        } else {
            $msg = '此文件类型不允许上传！';
        }
    } else {
        $msg = UPLOAD_PATH . '文件夹不存在,请手工创建！';
    }
}

pass–09

观察源代码发现, 删除了 $data ，根据这个我们可以利用。
问了下chatgpt，这个是根据windowsNTFS文件流的特点进行绕过黑名单检测。这个文件流有默认数据和备用数据.
使用::$data  第一个冒号表示使用默认数据流，第二个冒号就是使用数据流的名称。默认情况下，文件中的数据存储在默认数据流中，而不使用冒号指定的具体数据流名称。因此，如果你只使用一个冒号，文件上传将使用默认数据流。
说简单点就是你上传了一个1.php::$data,winsows还是把他看成了1.php

$is_upload = false;
$msg = null;
if (isset($_POST['submit'])) {
    if (file_exists(UPLOAD_PATH)) {
        $deny_ext = array(".php",".php5",".php4",".php3",".php2",".html",".htm",".phtml",".pht",".pHp",".pHp5",".pHp4",".pHp3",".pHp2",".Html",".Htm",".pHtml",".jsp",".jspa",".jspx",".jsw",".jsv",".jspf",".jtml",".jSp",".jSpx",".jSpa",".jSw",".jSv",".jSpf",".jHtml",".asp",".aspx",".asa",".asax",".ascx",".ashx",".asmx",".cer",".aSp",".aSpx",".aSa",".aSax",".aScx",".aShx",".aSmx",".cEr",".sWf",".swf",".htaccess",".ini");
        $file_name = trim($_FILES['upload_file']['name']);
        $file_name = deldot($file_name);//删除文件名末尾的点
        $file_ext = strrchr($file_name, '.');
        $file_ext = strtolower($file_ext); //转换为小写
        $file_ext = trim($file_ext); //首尾去空
        
        if (!in_array($file_ext, $deny_ext)) {
            $temp_file = $_FILES['upload_file']['tmp_name'];
            $img_path = UPLOAD_PATH.'/'.date("YmdHis").rand(1000,9999).$file_ext;
            if (move_uploaded_file($temp_file, $img_path)) {
                $is_upload = true;
            } else {
                $msg = '上传出错！';
            }
        } else {
            $msg = '此文件类型不允许上传！';
        }
    } else {
        $msg = UPLOAD_PATH . '文件夹不存在,请手工创建！';
    }
}

再去访问php不要::$data

我们可以去根目录查看这个文件发现他默认就是1.php

pass–10

还是使用. .进行绕过

$is_upload = false;
$msg = null;
if (isset($_POST['submit'])) {
    if (file_exists(UPLOAD_PATH)) {
        $deny_ext = array(".php",".php5",".php4",".php3",".php2",".html",".htm",".phtml",".pht",".pHp",".pHp5",".pHp4",".pHp3",".pHp2",".Html",".Htm",".pHtml",".jsp",".jspa",".jspx",".jsw",".jsv",".jspf",".jtml",".jSp",".jSpx",".jSpa",".jSw",".jSv",".jSpf",".jHtml",".asp",".aspx",".asa",".asax",".ascx",".ashx",".asmx",".cer",".aSp",".aSpx",".aSa",".aSax",".aScx",".aShx",".aSmx",".cEr",".sWf",".swf",".htaccess",".ini");
        $file_name = trim($_FILES['upload_file']['name']);
        $file_name = deldot($file_name);//删除文件名末尾的点
        $file_ext = strrchr($file_name, '.');
        $file_ext = strtolower($file_ext); //转换为小写
        $file_ext = str_ireplace('::$DATA', '', $file_ext);//去除字符串::$DATA
        $file_ext = trim($file_ext); //首尾去空
        
        if (!in_array($file_ext, $deny_ext)) {
            $temp_file = $_FILES['upload_file']['tmp_name'];
            $img_path = UPLOAD_PATH.'/'.$file_name;
            if (move_uploaded_file($temp_file, $img_path)) {
                $is_upload = true;
            } else {
                $msg = '上传出错！';
            }
        } else {
            $msg = '此文件类型不允许上传！';
        }
    } else {
        $msg = UPLOAD_PATH . '文件夹不存在,请手工创建！';
    }
}

pass–11

这个代码str_ireplace函数我们可以利用双写去绕过

$is_upload = false;
$msg = null;
if (isset($_POST['submit'])) {
    if (file_exists(UPLOAD_PATH)) {
        $deny_ext = array("php","php5","php4","php3","php2","html","htm","phtml","pht","jsp","jspa","jspx","jsw","jsv","jspf","jtml","asp","aspx","asa","asax","ascx","ashx","asmx","cer","swf","htaccess","ini");

        $file_name = trim($_FILES['upload_file']['name']);
        $file_name = str_ireplace($deny_ext,"", $file_name);
        $temp_file = $_FILES['upload_file']['tmp_name'];
        $img_path = UPLOAD_PATH.'/'.$file_name;        
        if (move_uploaded_file($temp_file, $img_path)) {
            $is_upload = true;
        } else {
            $msg = '上传出错！';
        }
    } else {
        $msg = UPLOAD_PATH . '文件夹不存在,请手工创建！';
    }
}

pass–12

第12关和之前的关不同是白名单的上传机制，只允许上传的他指定的文件。但12用到的技巧就是00截断。这个只适用于PHP5.3.4之前的版本.且
要关闭php  magic_quotes_gpc（魔术引号开关）函数 他会对用户传递的 get post cookie 进行转义，所以做12关时将他关闭。
在一些编程语言和操作系统中，空字节被用作字符串的终止符。这意味着在空字节之后的所有内容都会被忽略，导致截断字符串的效果。
查看源码可以看到，他用了一个get参数传参这个是可以控制的，我们假设上传一个1.php%00那么他后面的数据就会被忽略掉。
为什么是%00因为get请求用的是url编码，直接00无法解析的。

$is_upload = false;
$msg = null;
if(isset($_POST['submit'])){
    $ext_arr = array('jpg','png','gif');
    $file_ext = substr($_FILES['upload_file']['name'],strrpos($_FILES['upload_file']['name'],".")+1);
    if(in_array($file_ext,$ext_arr)){
        $temp_file = $_FILES['upload_file']['tmp_name'];
        $img_path = $_GET['save_path']."/".rand(10, 99).date("YmdHis").".".$file_ext;

        if(move_uploaded_file($temp_file,$img_path)){
            $is_upload = true;
        } else {
            $msg = '上传出错！';
        }
    } else{
        $msg = "只允许上传.jpg|.png|.gif类型文件！";
    }
}

我们直接删除多余的，访问1.php

pass–13

这个和上一个比较，请求方式发生了变化成了post请求。但原理还是一样的

$is_upload = false;
$msg = null;
if(isset($_POST['submit'])){
    $ext_arr = array('jpg','png','gif');
    $file_ext = substr($_FILES['upload_file']['name'],strrpos($_FILES['upload_file']['name'],".")+1);
    if(in_array($file_ext,$ext_arr)){
        $temp_file = $_FILES['upload_file']['tmp_name'];
        $img_path = $_POST['save_path']."/".rand(10, 99).date("YmdHis").".".$file_ext;

        if(move_uploaded_file($temp_file,$img_path)){
            $is_upload = true;
        } else {
            $msg = "上传失败";
        }
    } else {
        $msg = "只允许上传.jpg|.png|.gif类型文件！";
    }
}

可以在php后面写写一个a 转成16进制就是61 在改成00就可以了。00是显示不出来的。

pass–14

第14关用到的是图片马，制作一个图片马先
copy 123.jpg/b + 1.php/a  555.jpg   /b 表示二进制  /a 表示二进制马 然后重命名为555.jpg

可以看到这两个图片并没有差别

用16进制查看可以看到我们写的php 一句话木马 

这个上传会出现问题，应该是文件太大了。建议使用大小较小的图片

进行上传，配合文件包含漏洞查看图片木马是否成功，至于文件包含漏洞就是一个文件里面包含了多个文件。可以查看这个文件里面的文件
可以看到file变量是可控的，我们利用这个变量传入我们图片地址的参数进行查看是否成功

图片地址

进行传参查看图片

pass–15

和14关一样使用图片马即可。 getimagesize函数 会对目标文件的16进制去进行一个读取，去读取头几个字符串是不是符合图片的要求的

function isImage($filename){
    $types = '.jpeg|.png|.gif';
    if(file_exists($filename)){
        $info = getimagesize($filename);
        $ext = image_type_to_extension($info[2]);
        if(stripos($types,$ext)>=0){
            return $ext;
        }else{
            return false;
        }
    }else{
        return false;
    }
}

$is_upload = false;
$msg = null;
if(isset($_POST['submit'])){
    $temp_file = $_FILES['upload_file']['tmp_name'];
    $res = isImage($temp_file);
    if(!$res){
        $msg = "文件未知，上传失败！";
    }else{
        $img_path = UPLOAD_PATH."/".rand(10, 99).date("YmdHis").$res;
        if(move_uploaded_file($temp_file,$img_path)){
            $is_upload = true;
        } else {
            $msg = "上传出错！";
        }
    }
}

pass–16

exif_imagetype() 读取一个图像的第一个字节并检查其签名。 也只是检查第一个字节。 做本关之前需要开启php exif 模块. 因为这个函数是这个模块内置的。

function isImage($filename){
    //需要开启php_exif模块
    $image_type = exif_imagetype($filename);
    switch ($image_type) {
        case IMAGETYPE_GIF:
            return "gif";
            break;
        case IMAGETYPE_JPEG:
            return "jpg";
            break;
        case IMAGETYPE_PNG:
            return "png";
            break;    
        default:
            return false;
            break;
    }
}

$is_upload = false;
$msg = null;
if(isset($_POST['submit'])){
    $temp_file = $_FILES['upload_file']['tmp_name'];
    $res = isImage($temp_file);
    if(!$res){
        $msg = "文件未知，上传失败！";
    }else{
        $img_path = UPLOAD_PATH."/".rand(10, 99).date("YmdHis").".".$res;
        if(move_uploaded_file($temp_file,$img_path)){
            $is_upload = true;
        } else {
            $msg = "上传出错！";
        }
    }
}

图片马上传，使用文件包含漏洞地址进行文件读取。

pass–17

图片二次渲染，简单说就是上传一张图片后他在渲染一次，相当于照着图片重新画了一次。对于二次渲染绕过使用gif格式比较简单，找第二次渲染没有改变的位置，建议第四行插入php木马。二次渲染函数：imagecreatefromjpeg

$is_upload = false;
$msg = null;
if (isset($_POST['submit'])){
    // 获得上传文件的基本信息，文件名，类型，大小，临时文件路径
    $filename = $_FILES['upload_file']['name'];
    $filetype = $_FILES['upload_file']['type'];
    $tmpname = $_FILES['upload_file']['tmp_name'];

    $target_path=UPLOAD_PATH.'/'.basename($filename);

    // 获得上传文件的扩展名
    $fileext= substr(strrchr($filename,"."),1);

    //判断文件后缀与类型，合法才进行上传操作
    if(($fileext == "jpg") && ($filetype=="image/jpeg")){
        if(move_uploaded_file($tmpname,$target_path)){
            //使用上传的图片生成新的图片
            $im = imagecreatefromjpeg($target_path);

            if($im == false){
                $msg = "该文件不是jpg格式的图片！";
                @unlink($target_path);
            }else{
                //给新图片指定文件名
                srand(time());
                $newfilename = strval(rand()).".jpg";
                //显示二次渲染后的图片（使用用户上传图片生成的新图片）
                $img_path = UPLOAD_PATH.'/'.$newfilename;
                imagejpeg($im,$img_path);
                @unlink($target_path);
                $is_upload = true;
            }
        } else {
            $msg = "上传出错！";
        }

    }else if(($fileext == "png") && ($filetype=="image/png")){
        if(move_uploaded_file($tmpname,$target_path)){
            //使用上传的图片生成新的图片
            $im = imagecreatefrompng($target_path);

            if($im == false){
                $msg = "该文件不是png格式的图片！";
                @unlink($target_path);
            }else{
                 //给新图片指定文件名
                srand(time());
                $newfilename = strval(rand()).".png";
                //显示二次渲染后的图片（使用用户上传图片生成的新图片）
                $img_path = UPLOAD_PATH.'/'.$newfilename;
                imagepng($im,$img_path);

                @unlink($target_path);
                $is_upload = true;               
            }
        } else {
            $msg = "上传出错！";
        }

    }else if(($fileext == "gif") && ($filetype=="image/gif")){
        if(move_uploaded_file($tmpname,$target_path)){
            //使用上传的图片生成新的图片
            $im = imagecreatefromgif($target_path);
            if($im == false){
                $msg = "该文件不是gif格式的图片！";
                @unlink($target_path);
            }else{
                //给新图片指定文件名
                srand(time());
                $newfilename = strval(rand()).".gif";
                //显示二次渲染后的图片（使用用户上传图片生成的新图片）
                $img_path = UPLOAD_PATH.'/'.$newfilename;
                imagegif($im,$img_path);

                @unlink($target_path);
                $is_upload = true;
            }
        } else {
            $msg = "上传出错！";
        }
    }else{
        $msg = "只允许上传后缀为.jpg|.png|.gif的图片文件！";
    }
}

我们可以用之前的方法把一句话木马加入到图片里面，再看渲染后的样子.

末尾的一句话木马已经没有了

蓝色部分就是二次渲染之后没有改变的值

在第5行中插入木马

进行上传，使用文件包含漏洞访问图片，查看是否成功

pass–18

第18关用到的是条件竞争，根据源码是先上传后检测文件，再重命名。条件竞争原理就是我们只要一直上传超过他删除文件的速度，就可以绕过了。

$is_upload = false;
$msg = null;

if(isset($_POST['submit'])){
    $ext_arr = array('jpg','png','gif');
    $file_name = $_FILES['upload_file']['name'];
    $temp_file = $_FILES['upload_file']['tmp_name'];
    $file_ext = substr($file_name,strrpos($file_name,".")+1);
    $upload_file = UPLOAD_PATH . '/' . $file_name;

    if(move_uploaded_file($temp_file, $upload_file)){
        if(in_array($file_ext,$ext_arr)){
             $img_path = UPLOAD_PATH . '/'. rand(10, 99).date("YmdHis").".".$file_ext;
             rename($upload_file, $img_path);
             $is_upload = true;
        }else{
            $msg = "只允许上传.jpg|.png|.gif类型文件！";
            unlink($upload_file);
        }
    }else{
        $msg = '上传出错！';
    }
}