The National Vulnerability Database (NVD) has assigned identifier CVE-2022-27261 to an arbitrary file write vulnerability discovered in Express-FileUpload v1.3.1. This vulnerability allows attackers to upload multiple files with the same name, leading to an overwrite of files in the web application server. In this post, we'll explore the vulnerability in detail, examine a code snippet showcasing the issue, discuss the potential impacts, and suggest mitigations.

Vulnerability Details

Express-FileUpload is a popular middleware for handling file uploads in Node.js applications built on the Express framework. However, a flaw in the version 1.3.1 allows attackers to upload files with the same names repeatedly and cause an unintended overwrite of existing files. This arbitrary file write vulnerability poses a significant security risk to web applications using the affected version of Express-FileUpload middleware.

The following code snippet demonstrates the vulnerability in action

// Express app configuration
const express = require('express');
const fileUpload = require('express-fileupload');
const app = express();


// Endpoint for file uploads'/upload', (req, res) => {
  if (req.files) {
    let sampleFile = req.files.sample;'./uploads/' +, (err) => {
      if (err) {
        return res.status(500).send(err);
      res.send('File uploaded!');
  } else {
    res.status(400).send('No file uploaded.');

// Start the app
const port = 300;
app.listen(port, () => {
  console.log(App is listening on port ${port});

In this example, an endpoint is created to handle file uploads. During the upload process, the application saves files to the 'uploads' directory, using the user-provided file name.

However, when multiple files share the same name, the current implementation will overwrite the previously uploaded file without any check or warning. This results in the arbitrary file write vulnerability, which could have severe potential impacts on the affected server.

Exploit and Impact

An attacker exploiting this vulnerability could upload multiple files with the same name and execute a range of attacks. Here are a few examples:

1. Integrity attacks: By overwriting existing files on the server, attackers can manipulate its content and compromise the integrity of the application.

2. Information disclosure: Attackers might overwrite existing files to gain unauthorized access to sensitive data on the server.

3. Denial of Service (DoS): Overwriting critical system files could lead to performance issues or even a complete shutdown of the server.


At the time of writing, Express-FileUpload has not released a patch to address CVE-2022-27261. Hence, developers need to implement their mitigations to protect their applications. These might include:

1. Updating the application logic to prevent file overwrites: Implement a check that verifies if a file with the same name already exists before saving it to the server. If a duplicate is found, either reject the upload or create a new unique file name for the uploaded file.

2. Improving access controls: Add proper authentication and authorization mechanisms to the file upload endpoints, reducing the chance of unauthorized individuals exploiting the vulnerability.

3. Regularly monitor and audit file uploads: Keep track of uploaded files and log any suspicious activity. Investigate and respond to alerts quickly.


CVE-2022-27261 represents a significant security risk to web applications using the unpatched version of Express-FileUpload middleware. Developers should take immediate action to mitigate the vulnerability and protect their applications from potential attacks. While waiting for an official patch from the library developers, you can implement the mitigation strategies outlined above to safeguard your applications and servers.


Published on: 04/12/2022 17:15:00 UTC
Last modified on: 04/19/2022 19:35:00 UTC