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Upload Insecure Files

Uploaded files may pose a significant risk if not handled correctly. A remote attacker could send a multipart/form-data POST request with a specially-crafted filename or mime type and execute arbitrary code.

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Summary

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Tools

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Methodology

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Defaults Extensions

Here is a list of the default extensions for web shell pages in the selected languages (PHP, ASP, JSP).

  • PHP Server

  • ASP Server

  • JSP : .jsp, .jspx, .jsw, .jsv, .jspf, .wss, .do, .actions

  • Perl: .pl, .pm, .cgi, .lib

  • Coldfusion: .cfm, .cfml, .cfc, .dbm

  • Node.js: .js, .json, .node

Other extensions that can be abused to trigger other vulnerabilities.

  • .svg: XXE, XSS, SSRF

  • .gif: XSS

  • .csv: CSV Injection

  • .xml: XXE

  • .avi: LFI, SSRF

  • .js : XSS, Open Redirect

  • .zip: RCE, DOS, LFI Gadget

  • .html : XSS, Open Redirect

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Upload Tricks

Extensions:

  • Use double extensions : .jpg.php, .png.php5

  • Use reverse double extension (useful to exploit Apache misconfigurations where anything with extension .php, but not necessarily ending in .php will execute code): .php.jpg

  • Random uppercase and lowercase : .pHp, .pHP5, .PhAr

  • Null byte (works well against pathinfo())

    • .php%00.gif

    • .php\x00.gif

    • .php%00.png

    • .php\x00.png

    • .php%00.jpg

    • .php\x00.jpg

  • Special characters

    • Multiple dots : file.php...... , on Windows when a file is created with dots at the end those will be removed.

    • Whitespace and new line characters

      • file.php%20

      • file.php%0d%0a.jpg

      • file.php%0a

    • Right to Left Override (RTLO): name.%E2%80%AEphp.jpg will became name.gpj.php.

    • Slash: file.php/, file.php.\, file.j\sp, file.j/sp

    • Multiple special characters: file.jsp/././././.

  • On Windows OS, include, require and require_once functions will convert "foo.php" followed by one or more of the chars \x20 ( ), \x22 ("), \x2E (.), \x3C (<), \x3E (>) back to "foo.php".

  • On Windows OS, fopen function will convert "foo.php" followed by one or more of the chars \x2E (.), \x2F (/), \x5C () back to "foo.php".

  • On Windows OS, move_uploaded_file function will convert "foo.php" followed by one or more of the chars \x2E (.), \x2F (/), \x5C () back to "foo.php".

  • On Windows OS, when running PHP on IIS some characters are automatically converted to other characters when it is going to save a file (e.g. web<< becomes web** and can replace web.config).

    • \x3E (>) is converted to \x3F (?)

    • \x3C (<) is converted to \x2A (*)

    • \x22 (") is converted to \x2E (.), to use this trick in a file upload request the "Content-Disposition" header should use single quotes (e.g. filename='web"config').

File Identification:

MIME type, a MIME type (Multipurpose Internet Mail Extensions type) is a standardized identifier that tells browsers, servers, and applications what kind of file or data is being handled. It consists of a type and a subtype, separated by a slash. Change Content-Type : application/x-php or Content-Type : application/octet-stream to Content-Type : image/gif to disguise the content as an image.

Magic Bytesarrow-up-right - Sometimes applications identify file types based on their first signature bytes. Adding/replacing them in a file might trick the application.

  • 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 Encapsulation:

Using NTFS alternate data stream (ADS) in Windows. In this case, a colon character ":" will be inserted after a forbidden extension and before a permitted one. As a result, an empty file with the forbidden extension will be created on the server (e.g. "file.asax:.jpg"). This file might be edited later using other techniques such as using its short filename. The "::$data" pattern can also be used to create non-empty files. Therefore, adding a dot character after this pattern might also be useful to bypass further restrictions (.e.g. "file.asp::$data.")

Other Techniques:

PHP web shells don't always have the <?php tag, here are some alternatives:

  • Using a PHP script tag <script language="php">

  • The <?= is shorthand syntax in PHP for outputting values. It is equivalent to using <?php echo.

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Filename Vulnerabilities

Sometimes the vulnerability is not the upload but how the file is handled after. You might want to upload files with payloads in the filename.

  • Time-Based SQLi Payloads: e.g. poc.js'(select*from(select(sleep(20)))a)+'.extension

  • LFI/Path Traversal Payloads: e.g. image.png../../../../../../../etc/passwd

  • XSS Payloads e.g. '"><img src=x onerror=alert(document.domain)>.extension

  • File Traversal e.g. ../../../tmp/lol.png

  • Command Injection e.g. ; sleep 10;

Also you upload:

  • HTML/SVG files to trigger an XSS

  • EICAR file to check the presence of an antivirus

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Picture Compression

Create valid pictures hosting PHP code. Upload the picture and use a Local File Inclusion to execute the code. The shell can be called with the following command : curl 'http://localhost/test.php?0=system' --data "1='ls'".

  • Picture Metadata, hide the payload inside a comment tag in the metadata.

  • Picture Resize, hide the payload within the compression algorithm in order to bypass a resize. Also defeating getimagesize() and imagecreatefromgif().

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Picture Metadata

Create a custom picture and insert exif tag with exiftool. A list of multiple exif tags can be found at exiv2.orgarrow-up-right

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Configuration Files

If you are trying to upload files to a :

Configuration files examples

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Apache: .htaccess

The AddType directive in an .htaccess file is used to specify the MIME (Multipurpose Internet Mail Extensions) type for different file extensions on an Apache HTTP Server. This directive helps the server understand how to handle different types of files and what content type to associate with them when serving them to clients (such as web browsers).

Here is the basic syntax of the AddType directive:

Exploit AddType directive by uploading an .htaccess file with the following content.

Then upload any file with .rce extension.

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WSGI: uwsgi.ini

uWSGI configuration files can include “magic” variables, placeholders and operators defined with a precise syntax. The ‘@’ operator in particular is used in the form of @(filename) to include the contents of a file. Many uWSGI schemes are supported, including “exec” - useful to read from a process’s standard output. These operators can be weaponized for Remote Command Execution or Arbitrary File Write/Read when a .ini configuration file is parsed:

Example of a malicious uwsgi.ini file:

When the configuration file will be parsed (e.g. restart, crash or autoreload) payload will be executed.

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Dependency Manager

Alternatively you may be able to upload a JSON file with a custom scripts, try to overwrite a dependency manager configuration file.

  • package.json

  • composer.json

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CVE - ImageMagick

If the backend is using ImageMagick to resize/convert user images, you can try to exploit well-known vulnerabilities such as ImageTragik.

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CVE-2016–3714 - ImageTragik

Upload this content with an image extension to exploit the vulnerability (ImageMagick , 7.0.1-1)

  • ImageTragik - example #1

  • ImageTragik - example #3

The vulnerability can be triggered by using the convert command.

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CVE-2022-44268

CVE-2022-44268 is an information disclosure vulnerability identified in ImageMagick. An attacker can exploit this by crafting a malicious image file that, when processed by ImageMagick, can disclose information from the local filesystem of the server running the vulnerable version of the software.

  • Generate the payload

  • Trigger the exploit by uploading the file. The backend might use something like convert pngout.png pngconverted.png

  • Download the converted picture and inspect its content with: identify -verbose pngconverted.png

  • Convert the exfiltrated data: python3 -c 'print(bytes.fromhex("HEX_FROM_FILE").decode("utf-8"))'

More payloads in the folder Picture ImageMagick/.

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CVE - FFMpeg HLS

FFmpeg is an open source software used for processing audio and video formats. You can use a malicious HLS playlist inside an AVI video to read arbitrary files.

  1. ./gen_xbin_avi.py file://<filename> file_read.avi

  2. Upload file_read.avi to some website that processes videofiles

  3. On server side, done by the videoservice: ffmpeg -i file_read.avi output.mp4

  4. Click "Play" in the videoservice.

  5. If you are lucky, you'll the content of <filename> from the server.

The script creates an AVI that contains an HLS playlist inside GAB2. The playlist generated by this script looks like this:

More payloads in the folder CVE FFmpeg HLS/.

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Labs

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References

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