CORSLeak: Abusing IAP for a Stealthy Data Exfiltration Attack

Mitiga Research demonstrates a cloud misconfiguration attack where an attacker with roles/appengine.deployer in a GCP project can encode sensitive data into App Engine CORS response headers and expose it externally via unauthenticated IAP-forwarded OPTIONS preflight requests. The report outlines the attack flow, prerequisites, limitations (8 KB header limit, deploy quotas), responsible disclosure timeline with Google, and mitigation steps including disabling IAP CORS preflight forwarding, restricting deploy permissions, and monitoring deploy activity.