CVE-2025-66570: cpp-httplib Untrusted HTTP Header Handling: Internal Header Shadowing (REMOTE*/LOCAL*)

cpp-httplib is a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to 0.27.0, a vulnerability allows attacker-controlled HTTP headers to influence server-visible metadata, logging, and authorization decisions. An attacker can inject headers named REMOTEADDR, REMOTEPORT, LOCALADDR, LOCALPORT that are parsed into the request header multimap via readheaders() in httplib.h (headers.emplace), then the server later appends its own internal metadata using the same header names in Server::processrequest without erasing duplicates. Because Request::getheadervalue returns the first entry for a header key (id == 0) and the client-supplied headers are parsed before server-inserted headers, downstream code that uses these header names may inadvertently use attacker-controlled values. Affected files/locations: cpp-httplib/httplib.h (readheaders, Server::processrequest, Request::getheadervalue, getheadervalueu64) and cpp-httplib/docker/main.cc (getclientip, nginxaccesslogger, nginxerrorlogger). Attack surface: attacker-controlled HTTP headers in incoming requests flow into the Request.headers multimap and into logging code that reads forwarded headers, enabling IP spoofing, log poisoning, and authorization bypass via header shadowing. This vulnerability is fixed in 0.27.0.