#File Upload Vulnerabilities

File upload vulnerabilities are when a

web server allows users to upload files to its filesystem without sufficiently validating things like their name, type, contents, or size

Failing

to properly

enforce restrictions

on these could mean that even a basic image upload function can be used to upload arbitrary and potentially dangerous files instead. This could even include

server-side script

files that enable

remote code execution

The impact of file upload vulnerabilities generally depends on

two key factors

>
Which aspect of the file the website
```
fails to validate
```
properly, whether that be its size, type, contents, and so on.
>
What
```
restrictions are imposed
```
on the file once it has been successfully uploaded.

Examples -

>
If the filename isn't validated properly, this could allow an attacker to overwrite critical files simply by uploading a file with the same name.
>
If the server is also vulnerable to directory traversal, this could mean attackers are even able to upload files to unanticipated locations.
>
Failing to make sure that the size of the file falls within expected thresholds could also enable a form of denial-of-service (DoS) attack, whereby the attacker fills the available disk space.

Exploiting unrestricted file uploads to deploy a web shell

From a security perspective, the worst possible scenario is when a website allows you to upload server-side scripts, such as PHP, Java, or Python files, and is also configured to execute them as code. This makes it trivial to create your own web shell on the server.

Web shell A web shell is a malicious script that enables an attacker to execute arbitrary commands on a remote web server simply by sending HTTP requests to the right endpoint.

If you're able to successfully upload a web shell, you effectively have full control over the server. This means you can read and write arbitrary files, exfiltrate sensitive data, even use the server to pivot attacks against both internal infrastructure and other servers outside the network. For example, the following PHP one-liner could be used to read arbitrary files from the server's filesystem:

php

<?php echo file_get_contents('/path/to/target/file'); ?>

php

<?php echo system($_GET['command']); ?>

This script enables you to pass an arbitrary system command via a query parameter as follows:

GET /example/exploit.php?command=id HTTP/1.1

###Exploiting flawed validation of file uploads

In the wild, it's unlikely that you'll find a website that has

no protection

against

file upload attacks

. But just because defenses are in place, that doesn't mean that they're

robust

. You can sometimes still exploit flaws in these mechanisms to obtain a web shell for

remote code execution

Flawed file type validation

When submitting

HTML forms

, the browser typically sends the provided data in a

POST request

with the

content type application/x-www-form-url-encoded

. This is

fine for sending simple text

like your name or address. However, it

isn't suitable for sending large amounts of binary data, such as an entire image file or a PDF document

. In this case, the content type

multipart/form-data

is preferred.

One way that

websites may attempt to validate file uploads

is to check that this input-specific

Content-Type header

matches an expected

MIME type

. If the server is only expecting image files, for example, it may only allow types like

image/jpeg and image/png

. Problems can arise when the value of this header is implicitly trusted by the server. If no further validation is performed to check whether the contents of the file actually match the supposed MIME type, this defense can be easily bypassed using tools like Burp Repeater.

Preventing file execution in user-accessible directories

While it's clearly better to

prevent dangerous file types being uploaded

in the first place, the second line of defense is to

stop the server from executing any scripts

that do slip through the net.

Insufficient blacklisting of dangerous file types

One of the more

obvious ways of preventing users

from uploading malicious scripts is to

blacklist

potentially

dangerous file extensions

.php

. The practice of blacklisting is inherently flawed as it's difficult to explicitly block every possible file extension that could be used to execute code. Such blacklists can sometimes be

bypassed

by using

lesser known, alternative file extensions

that may still be executable, such as

.php5, .shtml

, and so on.

Overriding the server configuration

As we discussed in the previous section, servers typically won't execute files unless they have been configured to do so. For example, before an Apache server will execute PHP files requested by a client, developers might have to add the following directives to their

/etc/apache2/apache2.conf

file:

LoadModule php_module /usr/lib/apache2/modules/libphp.so
    AddType application/x-httpd-php .php

Many 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.

Similarly, developers can make directory-specific configuration on IIS servers using a web.config file. This might include directives such as the following, which in this case allows JSON files to be served to users:

<staticContent>
    <mimeMap fileExtension=".json" mimeType="application/json" />
</staticContent>

Web servers use these kinds of configuration files when present, but you're not normally allowed to access them using HTTP requests. However, you may occasionally find servers that fail to stop you from uploading your own malicious configuration file. In this case, even if the file extension you need is blacklisted, you may be able to trick the server into mapping an arbitrary, custom file extension to an executable MIME type.

Obfuscating file extensions

Even the most exhaustive blacklists can potentially be bypassed using classic obfuscation techniques. Let's say the validation code is case sensitive and fails to recognize that

exploit.pHp

is in fact a

.php

file. If the code that subsequently maps the file extension to a MIME type is not case sensitive, this discrepancy allows you to sneak malicious PHP files past validation that may eventually be executed by the server.

Some techniques to achieve this are as follows -

You can also achieve similar results using the following techniques:

>
```
Provide multiple extensions
```
. Depending on the algorithm used to parse the filename, the following file may be interpreted as either a PHP file or JPG image:
```
exploit.php.jpg
```
>
```
Add trailing characters
```
. Some components will strip or ignore trailing whitespaces, dots, and suchlike:
```
exploit.php.
```
>
```
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
>
```
Add semicolons or URL-encoded null byte characters
```
before the file extension. If validation is written in a high-level language like PHP or Java, but the server processes the file using lower-level functions in C/C++, for example, this can cause discrepancies in what is treated as the end of the filename:
```
exploit.asp;.jpg
```
or
```
exploit.asp%00.jpg
```
>
```
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.

Additionally, if this transformation isn't applied recursively, you can position the prohibited string in such a way that removing it still leaves behind a valid file extension. For example, consider what happens if you strip .php from the following filename:

exploit.p.phphp

This is just a small selection of the many ways it's possible to obfuscate file extensions.

###Flawed validation of the file's contents

Instead of implicitly trusting the

Content-Type

specified in a request, more secure servers try to verify that the contents of the file actually match what is expected.

In the case of an image upload function, the server might try to verify certain intrinsic properties of an image, such as its dimensions. If you try uploading a PHP script, for example, it won't have any dimensions at all. Therefore, the server can deduce that it can't possibly be an image, and reject the upload accordingly.

Similarly, certain file types may always contain a specific sequence of bytes in their

header or footer

(Magic Number). These can be used like a

fingerprint or signature

to determine whether the contents match the expected type. For example,

JPEG

files always begin with the bytes

FF D8 FF

>We can create Polygot PHP files which are nothing but JPG files with PHP code as their metadata. Rename the file as *.php and upload as application/x-php.

###Exploiting file upload race conditions

Modern frameworks are more battle-hardened against these kinds of attacks. They generally don't upload files directly to their intended destination on the filesystem. Instead, they take precautions like uploading to a

temporary, sandboxed directory

first and randomizing the name to avoid overwriting existing files. They then perform validation on this temporary file and only transfer it to its destination once it is deemed safe to do so.

That said, developers sometimes implement their own processing of file uploads independently of any framework. Not only is this fairly complex to do well, it can also introduce dangerous

race conditions

that enable an attacker to completely bypass even the most robust validation.

For example, some websites upload the file directly to the main filesystem and then remove it again if it doesn't pass validation. This kind of behavior is typical in websites that rely on anti-virus software and the like to check for malware. This may only take a few milliseconds, but for the short time that the file exists on the server, the attacker can potentially still execute it.

These vulnerabilities are often extremely subtle, making them difficult to detect during blackbox testing unless you can find a way to leak the relevant source code.

Race conditions in URL-based file uploads

Similar race conditions can occur in functions that allow you to upload a file by providing a URL. In this case, the server has to fetch the file over the internet and create a local copy before it can perform any validation.

As the file is loaded using HTTP, developers are unable to use their framework's built-in mechanisms for securely validating files. Instead, they may manually create their own processes for temporarily storing and validating the file, which may not be quite as secure.

For example, if the file is loaded into a temporary directory with a randomized name, in theory, it should be impossible for an attacker to exploit any race conditions. If they don't know the name of the directory, they will be unable to request the file in order to trigger its execution. On the other hand, if the randomized directory name is generated using pseudo-random functions like PHP's

uniqid()

, it can potentially be

brute-forced

To make attacks like this easier, you can try to extend the amount of time taken to process the file, thereby lengthening the window for brute-forcing the directory name. One way of doing this is by uploading a larger file. If it is processed in chunks, you can potentially take advantage of this by creating a

malicious file with the payload at the start, followed by a large number of arbitrary padding bytes

###Exploiting file upload vulnerabilities without remote code execution

In the examples we've looked at so far, we've been able to upload

server-side

scripts for

remote code execution

. This is the most serious consequence of an insecure file upload function, but these vulnerabilities can still be exploited in other ways.

Uploading malicious client-side scripts

Although you might not be able to execute scripts on the server, you may still be able to upload scripts for client-side attacks. For example, if you can upload HTML files or SVG images, you can potentially use

<script>

tags to create

stored XSS payloads

If the uploaded file then appears on a page that is visited by other users, their browser will execute the script when it tries to render the page. Note that due to

same-origin policy

restrictions, these kinds of attacks will only work if the uploaded file is served from the

same origin to which you upload

it.

Exploiting vulnerabilities in the parsing of uploaded files

If the uploaded file seems to be both stored and served securely, the last resort is to try exploiting vulnerabilities specific to the parsing or processing of different file formats. For example, you know that the server parses XML-based files, such as Microsoft Office

.doc

.xls

files, this may be a potential vector for

XXE injection attacks

###Uploading files using PUT

It's worth noting that some web servers may be configured to support PUT requests. If appropriate defenses aren't in place, this can provide an alternative means of uploading malicious files, even when an upload function isn't available via the web interface.

PUT /images/exploit.php HTTP/1.1
Host: vulnerable-website.com
Content-Type: application/x-httpd-php
Content-Length: 49

<?php echo file_get_contents('/path/to/file'); ?>

Tip: You can try sending OPTIONS requests to different endpoints to test for any that advertise support for the PUT method.

###How to prevent file upload vulnerabilities

Allowing users to upload files is commonplace and doesn't have to be dangerous as long as you take the right precautions. In general, the most effective way to protect your own websites from these vulnerabilities is to implement all of the following practices:

>Check the file extension against a whitelist of permitted extensions rather than a blacklist of prohibited ones. It's much easier to guess which extensions you might want to allow than it is to guess which ones an attacker might try to upload.
>Make sure the filename doesn't contain any substrings that may be interpreted as a directory or a traversal sequence (../).
>Rename uploaded files to avoid collisions that may cause existing files to be overwritten.
>Do not upload files to the server's permanent filesystem until they have been fully validated.
>As much as possible, use an established framework for preprocessing file uploads rather than attempting to write your own validation mechanisms.