Summary
CVE-2025-29927 is a critical Next.js middleware flaw allowing attackers to bypass authorization by sending a crafted header. This lets unauthorized users access protected routes and sensitive data, risking data breaches and full app takeover. The issue affects versions 11.1.4 to 15.2.2 and is fixed in versions 12.3.5, 13.5.9, 14.2.25, and 15.2.3. Immediate patching and additional security measures are vital to prevent serious operational and reputational damage.
Urgent Actions Required
- Update Next.js to versions 12.3.5, 13.5.9, 14.2.25, or 15.2.3 immediately.
- Block or strip the x-middleware-subrequest header at the proxy or CDN if patching isn’t possible yet.
- Monitor for requests containing the x-middleware-subrequest header from external sources.
Which Systems Are Vulnerable to CVE-2025-29927?
Technical Overview
- Vulnerability Type: Authorization Bypass via Middleware Header Spoofing (x-middleware-subrequest)
- Affected Software/Versions:
- Next.js 11.1.4 – 13.5.8
- Next.js 14.0.0 – 14.2.24
- Next.js 15.0.0 – 15.2.2
- Attack Vector: Network (HTTP/HTTPS)
- CVSS Score: 9.1
- Exploitability Score:
- Attack Vector: Network
- Attack Complexity: Low
- Privileges Required: None
- User Interaction: None
- Patch Availability: Yes, available2
How Does the CVE-2025-29927 Exploit Work?
The attack typically follows these steps:
What Causes CVE-2025-29927?
Vulnerability Root Cause:
This vulnerability stems from improper validation of HTTP headers in Next.js middleware. Specifically, the application blindly trusts the x-middleware-subrequest header, allowing attackers to spoof this header and bypass authentication controls. This flaw in the header handling logic enables unauthorized access by tricking the server into treating malicious requests as legitimate, leading to an authentication bypass without any user credentials or interaction.
How Can You Mitigate CVE-2025-29927?
If immediate patching is delayed or not possible:
- Strip or block the x-middleware-subrequest header at your CDN, proxy, or load balancer (e.g., NGINX, Cloudflare).
- Deploy WAF or edge rules to reject requests containing the x-middleware-subrequest header.
- Modify middleware logic to deny access by default if this header is present or improperly structured.
- Implement in-route authorization checks to verify access even if middleware is skipped.
- Run tests using crafted headers to ensure middleware protections are not bypassed.
Which Assets and Systems Are at Risk?
Asset Types Affected:
- Web Applications – Apps built with vulnerable versions of Next.js (11.1.4 to 15.2.2)
- API Services – Especially those relying on middleware for authentication or access control
Business-Critical Systems at Risk:
- Customer Portals – If middleware controls access to dashboards, profiles, or billing areas
- Admin Panels – Risk of unauthorized access to sensitive configuration or management interfaces
- Multi-tenant SaaS platforms – Risk of cross-tenant data exposure if authorization is middleware-based
- Internal tools or dashboards – If protected via middleware-based access logic
Exposure Level:
- Internet-facing applications – Especially those using Next.js middleware for the session or role enforcement
- Internal web apps – If accessed over an intranet or VPN and not patched, still vulnerable
- CI/CD or staging environments – Often overlooked but can leak credentials or tokens if exploited
Will Patching CVE-2025-29927 Cause Downtime?
- Patch application impact: Low. To fix the issue, update to Next.js versions 12.3.5, 13.5.9, 14.2.25, or 15.2.3. Most apps just need a rebuild and redeploy, which causes little or no downtime if CI/CD is used. If your app has complex middleware, test the update first to avoid issues.
- Mitigation (if immediate patching is not possible): Partial risk reduction is possible by stripping or blocking the x-middleware-subrequest header at the CDN, reverse proxy (e.g., NGINX), or application layer. This is not a complete mitigation—middleware-based authentication may still be bypassed via crafted internal headers. All internet-facing apps remain exposed until a full patch is applied.
How Can You Detect CVE-2025-29927 Exploitation?
Exploitation Signatures:
Look for crafted requests containing the x-middleware-subrequest header with repeated or known middleware paths like middleware, src/middleware, or pages/_middleware. Headers with 5 or more repeated entries may indicate an active bypass attempt.
MITRE ATT&CK Mapping:
Indicators of Compromise (IOCs/IOAs):
- x-middleware-subrequest header values exceeding depth threshold (e.g., middleware:middleware:middleware:middleware:middleware)
- Access to protected URLs (e.g., /admin, /dashboard) without associated auth tokens
- HTTP 200 responses where redirects or denials were expected
Behavioral Indicators:
- Authentication middleware is not triggered during access to restricted routes
- Missing redirect headers like x-middleware-redirect or x-middleware-rewrite in response
- Anomalous access patterns to internal APIs without session context
Alerting Strategy:
- Priority: Critical
- Trigger alerts for:
- Incoming requests with x-middleware-subrequest header containing known or repeated middleware paths
- Auth-restricted paths accessed without login tokens or auth headers
- Deviations from expected redirect behavior (e.g., missing 307/302 responses where required)
Remediation & Response
Patch/Upgrade Instructions:
- Next.js Patch Advisory2
Mitigation Steps if No Patch:
- Remove or block the x-middleware-subrequest header at your proxy (e.g., NGINX, Apache).
- Block requests containing the header using a WAF or CDN rule.
- Avoid trusting this header in your app logic.
- Add checks in page/API routes to verify user permissions directly.
- Log and monitor any requests that include this header to catch possible attacks.
Remediation Timeline:
- Immediate (0–2 hrs): Implement header filtering at edge/proxy.
- Within 8 hrs: Patch all critical Next.js instances.
- Within 24 hrs: Verify no vulnerable versions remain in CI/CD pipelines or production.
Rollback Plan:
- If the patch causes unintended issues, revert to the previous stable version and restore header-filtering rules.
- Document rollback steps in your change-management system (e.g., date/time, responsible engineer, version details).
Incident Response Considerations:
- Quickly isolate affected applications or servers to prevent unauthorized internal API access via spoofed headers.
- Collect forensic data such as reverse proxy logs, CDN logs, and middleware request traces that include the x-middleware-subrequest header.
- Conduct a thorough investigation to determine whether internal-only routes were accessed and identify any compromised services or user data.
- After applying the patch, enhance logging and implement request validation to detect potential header spoofing attempts and verify patch coverage.
Compliance & Governance Notes
Standards Impacted:
- ISO 27001: A.12.6.1 – Management of technical vulnerabilities.
- NIST 800-53: SI-2 – Flaw Remediation.
- PCI-DSS: Patching within 30 days; failure to do so may trigger non-compliance per v4.0 “30-day patch window.”
Audit Trail Requirement:
- Log every instance of x-middleware-subrequest header in access logs (include timestamp, source IP, URI).
- Record patch deployment details: date, time, engineer, target hosts, and version applied.
- Maintain a revision-controlled change log for “Next.js upgrade” in your change-management system.
- Provide weekly status reports to compliance officers until fully patched.
Policy Alignment:
- Update Header Validation Policy to disallow unvalidated x-middleware-subrequest.
- Amend the Vulnerability Management Policy to require monthly Next.js version scans (CI/CD integration).
- Add a Two-Factor Auth Enforcement policy for all protected routes outside of middleware.
- Revise the Incident Response Plan to include steps for “Next.js middleware bypass” scenarios (detection → isolation → remediation).
Where Can I Find More Information on CVE-2025-29927?
CVSS Breakdown Table
| Metric | Value | Description |
|---|---|---|
| Base Score | 9.1 | High severity vulnerability indicating critical impact and exploitability |
| Attack Vector | Network | Exploitable remotely over HTTP/HTTPS without local access |
| Attack Complexity | Low | Attack does not require special conditions; straightforward exploitation |
| Privileges Required | None | No authentication or elevated privileges needed to exploit |
| User Interaction | None | No user action required for exploitation (no clicks or input necessary) |
| Scope | Unchanged | Exploitation impacts only the vulnerable component, not extended to other system components |
| Confidentiality Impact | High | Successful exploit can lead to full disclosure of sensitive data |
| Integrity Impact | High | Exploit can allow unauthorized modification of data or bypass security controls |
| Availability Impact | Low | Minimal impact on system availability, denial of service is limited or unlikely |
