CVE-2025-29927: Critical Next.js Security Flaw Exposed

Hello there! Your guy Zokomon is back with another important cybersecurity alert. Today, we’re talking about CVE-2025-29927, a serious vulnerability found in Next.js. This flaw lets attackers bypass authorization checks in middleware, potentially leading to unauthorized access. If you’re a developer or security researcher, you need to know how this works and how to fix it. Let’s dive in! 🚀

What is CVE-2025-29927? 🛑

Next.js uses middleware to run code before processing requests. This helps with authentication, redirections, and security checks. However, CVE-2025-29927 allows hackers to bypass middleware protections by manipulating the internal header x-middleware-subrequest.

Why is this a big deal?

✅ Attackers can skip authorization checks and access restricted parts of a website. ✅ Websites using Next.js middleware for security are at risk. ✅ Unauthorized users might steal data or perform actions they shouldn’t.

Who is Affected by CVE-2025-29927 ? 🎯

This vulnerability affects self-hosted Next.js applications running next start with standalone output. If your website uses middleware for security checks, you could be at risk.

🔹 Not Affected:

• Websites hosted on Vercel or Netlify.
• Applications deployed as static exports.

🔹 At Risk:

• Self-hosted Next.js apps using middleware for authentication.
• Apps running on custom servers with next start.

How to Fix CVE-2025-29927? 🛠️

To protect your website, follow these steps:

✅ Upgrade Next.js – Update to the latest patched versions:

• Next.js 15.x → 15.2.3
• Next.js 14.x → 14.2.25
• Next.js 13.x → 13.5.9
• Next.js 12.x → 12.3.5

✅ Block Dangerous Headers – Configure your server to block external requests containing x-middleware-subrequest.

✅ Double-check Security – Don’t rely only on middleware for security. Add extra validation in your app’s core logic.

How Was This Discovered? 🔍

Security researchers Rachid A. and Yasser Allam found this vulnerability while analyzing Next.js middleware. Their research showed how attackers could exploit the x-middleware-subrequest header to bypass security.

🔹 They tested real-world scenarios where middleware was the only security layer. 🔹 They found that attackers could gain access to restricted areas without authentication. 🔹 This discovery led to immediate action from the Next.js team to patch the issue.

Lessons for Developers 💡

This case highlights why security should never depend on a single layer. If you’re a developer, keep these points in mind:

🔹 Always validate requests beyond middleware. 🔹 Regularly update frameworks to patch vulnerabilities. 🔹 Monitor security advisories for new threats.

Final Thoughts 🎤

CVE-2025-29927 is a serious flaw that affects many self-hosted Next.js applications. The good news? A fix is available. Update your Next.js version now, review your middleware security, and stay informed about new threats. Cybersecurity is a continuous process—stay ahead of the game! 🔥

🔹 What do you think? Have you updated your Next.js app yet? Drop your thoughts in the comments! 👇

And don’t miss my previous blog, where we discussed ” Brute-Force Attack on Bumble 500$ Bug “

Stay safe and hack smart! ❤️