Possible Next.js Middleware Auth Bypass via X-Middleware-Subrequest Header (CVE-2025-29927)

ATT&CK techniques

Initial Access → Defense Evasion

Recon
Resource Dev
Initial Access
- T1078Valid Accounts
- T1190Exploit Public-Facing Application
Execution
Persistence
- T1078Valid Accounts
Priv Esc
- T1078Valid Accounts
Defense Evasion
- T1078Valid Accounts
Cred Access
Discovery
Lateral Movement
Collection
C2
Exfiltration
Impact

What it detects

This rule detects HTTP requests carrying the X-Middleware-Subrequest header, which Next.js internally uses to prevent middleware recursion and which attackers forge to bypass middleware-based authentication in CVE-2025-29927 as analyzed by Assetnote. A client-supplied value for this header causes the framework to skip auth middleware and return protected content. Because legitimate external clients never send this internal header, its presence strongly indicates an exploitation attempt.

Reporting behind it

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Possible Next.js Middleware Auth Bypass via X-Middleware-Subrequest Header (CVE-2025-29927)

Pivot detection · T1190 · 9 related

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.

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