Summary When Heimdall operates in proxy mode, it constructs the "Forwarded" HTTP header after executing the matched rule pipeline by inserting the incoming request's "Host" header value directly into the header string without sanitizing commas or semicolons. This allows an attacker to inject additional parameters into the "Forwarded" header, potentially spoofing IP addresses for upstream services. Vulnerable Code File: "proxy/request_context.go" (line 201) entry := "for=" + clientIP + ";host=" + in.Host + ";proto=" + proto Go's "net/http" allows commas and semicolons in Host header values. No sanitization is applied before string concatenation. PoC Inject a spoofed IP into the Forwarded header curl -s -H "Host: evil.com,for=127.0.0.1" "http://TARGET:PORT/protected-resource" This produces the following "Forwarded" header sent to the upstream service: Forwarded: for=1.2.3.4;host=evil.com, for=127.0.0.1;proto=http Upstream services that parse the "Forwarded" header according to RFC 7239 will see two entries. If the service trusts the last or any "for=" value, the attacker successfully spoofs "127.0.0.1" as the client IP. More targeted attack: spoof to bypass IP allowlist curl -s -H "Host: legit.com;for=10.0.0.1;proto=https,for=192.168.1.1" "http://TARGET:PORT/admin-panel" Impact - IP spoofing: Upstream services behind Heimdall may trust the injected "for=" value, believing the request originates from an internal/trusted IP - Access control bypass: Applications that restrict access based on IP address by themselves, without using the corresponding heimdall capabilities (e.g., admin panels, internal APIs), can be bypassed - Affects all proxy-mode deployments where upstream services parse the "Forwarded" header.