Flash Notice: CVE-2025-29927 - Next.js Middleware Authorization Bypass

overview

CVE-2025-29927 is a critical vulnerability discovered in Next.js, a widely used React framework for developing full-stack web applications. This vulnerability allows attackers to bypass authorization checks enforced in middleware, potentially allowing unauthorized access to protected resources. The issue arises from improper handling of the x-middleware-subrequest header, which can be manipulated to avoid executing middleware checks.

Attackers can exploit this vulnerability by sending HTTP requests that include a specially crafted x-middleware-subrequest header, effectively bypassing security checks, authentication, and other measures in place. This could lead to unauthorized access to sensitive areas such as admin panels and private user data within affected applications.

Affected Products and Versions

• Next.js versions 11.1.4 through 13.5.6
• Next.js 14.x versions prior to 14.2.25
• Next.js 15.x versions prior to 15.2.3

Patched Versions

• Next.js 14.2.25
• Next.js 15.2.3

Current Threat Status

The vulnerability has been publicly disclosed, with proof-of-concept exploits now available. Although there have been no confirmed widespread exploits in the wild, the ease of exploitation combined with the popularity of Next.js categorizes this as a high-risk vulnerability. Attack techniques include:

1. Sending requests with the x-middleware-subrequest header to bypass middleware checks.
2. Gaining unauthorized access to protected routes and admin panels.
3. Bypassing content security policies implemented via middleware.

Given Next.js's widespread usage across sectors like banking and blockchain, the potential impact remains significant, particularly for organizations hosting self-managed Next.js applications reliant on middleware for authentication.

how avertium is protecting our customers

IOCs ADDED TO OUR THREAT FEEDS

Currently, there are no confirmed Indicators of Compromise (IoCs) specifically linked to the exploitation of CVE-2025-29927. Nevertheless, security researchers are actively working to identify potential IoCs associated with this vulnerability.

Potential Indicators

• HTTP Header: Requests containing the x-middleware-subrequest header, especially with values such as:
  • x-middleware-subrequest: pages/_middleware
  • x-middleware-subrequest: middleware:middleware:middleware:middleware:middleware
  • x-middleware-subrequest: src/middleware:src/middleware:src/middleware:src/middleware:src/middleware
• Network Traffic: Unusual access patterns to protected routes or admin panels without proper authentication.
• Server Logs: Successful requests logged to protected resources that bypass middleware checks.

MITRE ATT&CK AND TTPs

Initial Access

• T1190 - Exploit Public-Facing Application: Attackers could exploit publicly accessible Next.js applications to bypass authorization checks in middleware.

Execution

• T1059 - Command and Scripting Interpreter: After bypassing authentication, attackers may execute unauthorized commands on the compromised system.

Defense Evasion

• T1562 - Impair Defenses: This vulnerability allows attackers to bypass middleware authorization checks, undermining the application's defenses.

Privilege Escalation

• T1548 - Abuse Elevation Control Mechanism: Attackers may exploit this vulnerability to elevate their privileges within the application.

Lateral Movement

• T1550 - Use Alternate Authentication Material: Attackers could leverage the exploit to access resources improperly.

Discovery

• T1082 - System Information Discovery: After bypassing authorization, attackers may gather restricted system information.

Collection

• T1530 - Data from Cloud Storage Object: Attackers might collect sensitive cloud-stored data after bypassing authorization checks.

Impact

• T1499 - Endpoint Denial of Service: Exploiting this vulnerability could lead to a denial of service by overwhelming the application with unauthorized requests.

additional Recommendations + information

1. Immediate Mitigation:
   • Block the x-middleware-subrequest header at the web server level:
     • For NGINX:
     • location / {
     •           proxy_set_header x-middleware-subrequest "";
     • }
     • For Apache:
     • RequestHeader unset x-middleware-subrequest
   • Implement additional authentication checks (e.g., using NextAuth.js) and supplementary security layers for critical paths.
2. Patch and Monitor Systems:
   • Update to patched versions immediately (Next.js 15.2.3 or 14.2.25).
   • If immediate updates are not possible, utilize header-blocking workarounds and monitor logs for suspicious requests.
3. Network Security:
   • Deploy Web Application Firewall (WAF) rules to block requests with the x-middleware-subrequest header.
   • Review access controls and restrict access to protected resources.
   • Implement intrusion detection systems (IDS) to monitor for exploitation attempts.

Additional Best Practices

• Avoid relying solely on middleware for security.
• Implement multiple layers of authentication and authorization.
• Regularly test applications for security vulnerabilities and keep dependencies up to date.

ADDITIONAL SERVICE OFFERINGS

Attack Surface Management (ASM)
Avertium provides ASM services to identify and mitigate vulnerabilities like CVE-2025-29927 within an organization’s IT infrastructure by continuously scanning for exposed Next.js instances and identifying outdated versions.

Threat Detection & Response (TDR)
Avertium’s TDR service integrates security operations into an XDR-informed system to monitor for exploitation attempts and provide rapid alerts on potential bypass attempts.

Microsoft Security Solutions
Although not directly related to Next.js, Avertium can enhance protection against CVE-2025-29927 by implementing additional authentication layers and utilizing Microsoft Defender for Cloud Apps to monitor web application security.

Governance, Risk, and Compliance (GRC)
Avertium can assist organizations in managing risks associated with CVE-2025-29927 by conducting compliance audits and providing guidance on regulatory requirements related to authorization and access control.

By leveraging Avertium’s services, organizations can significantly improve their security posture against CVE-2025-29927, ensuring a comprehensive approach to cybersecurity aligned with best practices.

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