Critical Next.js Vulnerability: Authorization Bypass in Middleware

What Happened

On March 21, 2025, a critical vulnerability (GHSA-f82v-jwr5-mffw/CVE-2025-29927) was disclosed in Next.js. This vulnerability allows attackers to bypass authorization checks implemented in middleware by manipulating a specific header. With a CVSS score of 9.1, this is classified as a critical security issue.

The vulnerability affects the following Next.js versions:

• Next.js >= 13.0.0, < 13.5.9

• Next.js >= 14.0.0, < 14.2.25

• Next.js >= 15.0.0, < 15.2.3

• Next.js >= 11.1.4, < 12.3.5

What Organizations Should Do

1. Update to patched versions immediately

• For Next.js 15.x: Update to version 15.2.3 or later

• For Next.js 14.x: Update to version 14.2.25 or later

• For Next.js 13.x: Update to version 13.5.9 or later

• For Next.js 12.x: Update to version 12.3.5 or later

2. If you can't update immediately:

• Implement network-level filtering to block the x-middleware-subrequest header

• Increase monitoring for unusual access patterns

• Prioritize updating based on external exposure and data sensitivity

3. Verify and validate:

• Confirm successful patch application

• Test that authorization controls still function properly

Using Jit.io to Respond

Jit.io's ASPM platform provides immediate support for organizations managing this vulnerability:

Detection & Discovery:

• Our SCA capabilities automatically detect vulnerable Next.js versions across your repositories

• SBOM scanning identifies all instances where Next.js is used, including transitive dependencies

• Our platform can quickly search for Next.js usage patterns across your codebase

Prioritization & Response:

• Jit.io contextually prioritizes affected applications based on exposure and sensitivity

• One-click activation of additional security controls to mitigate risk

• Automated PR creation for dependency updates to patched versions

Monitoring & Validation:

• Continuous monitoring confirms successful remediation

• Change-based scanning ensures patches are applied and remain in place

• Integration with development workflows ensures vulnerabilities are addressed during the development cycle

Deep Dive on the Issue

Vulnerability Mechanics

The vulnerability exists in the Next.js middleware implementation. Middleware in Next.js is a powerful feature used for authentication, authorization, and other security controls that execute before reaching the actual route handler.

The issue specifically involves the x-middleware-subrequest header, which is used internally by Next.js. When this header is manipulated by an attacker in a specific way, it causes the middleware security checks to be bypassed entirely, allowing unauthorized access to protected routes and resources.

Applications using middleware for critical security functions like:

• Authentication verification

• Role-based access control

• API protection

• Request validation

• Custom security headers

are particularly at risk if they haven't implemented additional security layers beyond the middleware.

Attack Surface & Impact

The vulnerability presents a significant risk because:

1. Middleware-based authorization is a common pattern in Next.js applications

2. The vulnerability requires minimal technical knowledge to exploit

3. There are no obvious signs of exploitation in standard logs

4. It potentially provides direct access to protected resources

Applications with the following characteristics are most at risk:

• External/internet-facing deployments

• Applications handling sensitive data

• Systems with middleware as the primary security control

• Applications with administrative interfaces protected by middleware

Mitigations if You Can't Patch

If immediate patching is not feasible, implement the following mitigations:

Network-Level Filtering

Block any external requests containing the x-middleware-subrequest header:

For Nginx:

# Add to server or location block
if ($http_x_middleware_subrequest) {
     return 403 "Forbidden";
}

For Apache:

# In .htaccess or httpd.conf
<IfModule mod_rewrite.c>
   RewriteEngine On
   RewriteCond %{HTTP:x-middleware-subrequest} .+
   RewriteRule .* - [F,L]
</IfModule>

For AWS WAF:

{
  "Name": "Block-Middleware-Bypass",
  "Priority": 10,
  "Action": {
    "Block": {}
  },
  "Statement": {
    "ByteMatchStatement": {
      "SearchString": "x-middleware-subrequest",
      "FieldToMatch": {
        "Headers": {
          "MatchPattern": {
            "All": {}
          }
        }
      },
      "TextTransformations": [
        {
          "Priority": 0,
          "Type": "LOWERCASE"
        }
      ],
      "PositionalConstraint": "CONTAINS"
    }},
  "VisibilityConfig": {
     "SampledRequestsEnabled": true,
     "CloudWatchMetricsEnabled": true,
     "MetricName": "NextJsMiddlewareBypassAttempt"
  }
}

Additional Defense-in-Depth Measures

1. Implement secondary authorization checks: Add authorization logic in API routes and page components, not just middleware
2. Use multi-factor security controls: Combine session validation with other verification mechanism
3. Increase monitoring: Watch for unusual access patterns or unauthorized resource access
4. Tighten network access controls: Restrict access to sensitive routes by IP or network where possible

Special Notes

• Next.js deployments hosted on Vercel are automatically protected via their firewall implementation

• This mitigation should be viewed as temporary while preparing for a proper update

For organizations looking for assistance in managing this vulnerability, Jit.io's security team is available to help. Visit jit.io to learn more about our ASPM platform and how it can strengthen your security posture against similar vulnerabilities.