When Proxies Become the Attack Vectors in Web Architectures

Many modern web applications rely on a flawed assumption: backends can blindly trust security-critical headers from upstream reverse proxies. This assumption breaks down because HTTP RFC flexibility allows different servers to interpret the same header field in fundamentally different ways, creating exploitable gaps that attackers are increasingly targeting. Two recent CVEs I discovered expose this systemic problem and demonstrate why these are not isolated bugs, but symptoms of a much broader architectural flaw. When CVE-2025-48865 in Fabio and CVE-2025-64484 in OAuth2-proxy both enable identical attack patterns across completely different technologies, it reveals that our industry has fundamentally misunderstood where the real security boundaries lie. TL;DR: Two newly discovered CVEs (CVE-2025-48865 in Fabio, CVE-2025-64484 in OAuth2-proxy) expose a systemic vulnerability in how reverse proxies handle header processing. By exploiting hop-by-hop header stripping and underscore-hyphen