Understanding Referrer Information Risks

Referrer headers can inadvertently leak sensitive information through URLs containing session tokens, user IDs, search queries, or internal paths. When users click external links or load third-party resources, browsers traditionally send the full URL of the originating page, potentially exposing private data to external parties. This information leakage can compromise user privacy, reveal internal application structure, and even facilitate targeted attacks.

Consider URLs like:

https://app.example.com/admin/users?search=john&token=abc123
https://medical.example.com/patient/12345/results
https://intranet.example.com/confidential/project-phoenix

Without proper referrer policies, these sensitive URLs could be transmitted to external analytics services, advertising networks, or malicious sites.