Regardless of the language in which a program is written, the most devastating attacks often involve remote code execution, whereby an attacker succeeds in executing malicious code in the program's context. If attackers are allowed to upload files to a directory that is accessible from the Web and cause these files to be passed to a code interpreter (e.g. JSP/ASPX/PHP), then they can cause malicious code contained in these files to execute on the server.

The following code receives an uploaded file and assigns it to the posted object. FileUpload is of type System.Web.UI.HtmlControls.HtmlInputFile.
Example:

HttpPostedFile posted = FileUpload.PostedFile;

Even if a program stores uploaded files under a directory that isn't accessible from the Web, attackers might still be able to leverage the ability to introduce malicious content into the server environment to mount other attacks. If the program is susceptible to path manipulation, command injection, or dangerous file inclusion vulnerabilities, then an attacker might upload a file with malicious content and cause the program to read or execute it by exploiting another vulnerability.

Regardless of the language in which a program is written, the most devastating attacks often involve remote code execution, whereby an attacker succeeds in executing malicious code in the program's context. If attackers are allowed to upload files to a directory that is accessible from the Web and cause these files to be passed to the PHP interpreter, then they can cause malicious code contained in these files to execute on the server.

Example 1: The following code processes uploaded files and moves them into a directory under the Web root. Attackers may upload malicious PHP source files to this program and subsequently request them from the server, which will cause them to be executed by the PHP interpreter.

<?php
$udir = 'upload/'; // Relative path under Web root
$ufile = $udir . basename($_FILES['userfile']['name']);
if (move_uploaded_file($_FILES['userfile']['tmp_name'], $ufile)) {
    echo "Valid upload received\n";
} else {
    echo "Invalid upload rejected\n";
} ?>

Even if a program stores uploaded files under a directory that isn't accessible from the Web, attackers might still be able to leverage the ability to introduce malicious content into the server environment to mount other attacks. If the program is susceptible to path manipulation, command injection, or remote include vulnerabilities, then an attacker might upload a file with malicious content and cause the program to read or execute it by exploiting another vulnerability.

Regardless of the language in which a program is written, the most devastating attacks often involve remote code execution, whereby an attacker succeeds in executing malicious code in the program's context. If attackers are allowed to upload files to a directory that is accessible from the Web and cause these files to be passed to the Python interpreter, then they can cause malicious code contained in these files to execute on the server.

Example 1: The following code processes uploaded files and moves them into a directory under the web root. Attackers may upload malicious files to this program and subsequently request them from the server.

from django.core.files.storage import default_storage
from django.core.files.base import File
...
def handle_upload(request):
  files = request.FILES
  for f in files.values():
    path = default_storage.save('upload/', File(f))
...

Even if a program stores uploaded files under a directory that isn't accessible from the Web, attackers might still be able to leverage the ability to introduce malicious content into the server environment to mount other attacks. If the program is susceptible to path manipulation, command injection, or remote include vulnerabilities, then an attacker might upload a file with malicious content and cause the program to read or execute it by exploiting another vulnerability.

Regardless of the language in which a program is written, the most devastating attacks often involve remote code execution, whereby an attacker succeeds in executing malicious code in the program's context. If attackers are allowed to upload files to a publicly executable directory, then they can cause malicious code contained in these files to execute on the server.

Even if a program stores uploaded files under a directory that isn't publicly accessible, attackers might still be able to leverage the ability to introduce malicious content into the server environment to mount other attacks. If the program is susceptible to path manipulation, command injection, or remote include vulnerabilities, then an attacker might upload a file with malicious content and cause the program to read or execute it by exploiting another vulnerability.

Regardless of the language in which a program is written, the most devastating attacks often involve remote code execution, whereby an attacker succeeds in executing malicious code in the program's context. If attackers are allowed to upload files to a directory that is accessible from the Web and cause these files to be passed to a code interpreter (e.g. JSP/ASPX/PHP), then they can cause malicious code contained in these files to execute on the server.
Even if a program stores uploaded files under a directory that isn't accessible from the Web, attackers might still be able to leverage the ability to introduce malicious content into the server environment to mount other attacks. If the program is susceptible to path manipulation, command injection, or dangerous file inclusion vulnerabilities, then an attacker might upload a file with malicious content and cause the program to read or execute it by exploiting another vulnerability.

An <input> tag of type file indicates the program accepts file uploads.
Example:

<input type="file">