Honor RoundTripper contract in middleware; validate incoming X-Request-Id

BearerAuth, BasicAuth and DefaultHeaders mutated the caller's request, which
violates the RoundTripper contract and risks races on shared/retried requests;
clone before writing headers (matching RequestID). Validate the incoming
X-Request-Id (length and character set) before propagating it to logs and the
response header, preventing log forging and header splitting from a
client-controlled value.

server/middleware_requestid.go

@@ -9,9 +9,14 @@ import (
 	"git.codelab.vc/pkg/httpx/internal/requestid"
 )
 
+// maxRequestIDLen bounds the length of a client-supplied request ID that we
+// are willing to propagate.
+const maxRequestIDLen = 128
+
 // RequestID returns a middleware that assigns a unique request ID to each
-// request. If the incoming request already has an X-Request-Id header, that
-// value is used. Otherwise a new UUID v4 is generated via crypto/rand.
+// request. If the incoming request carries a valid X-Request-Id header, that
+// value is reused; otherwise (or if the supplied value is empty, too long, or
+// contains unsafe characters) a new UUID v4 is generated via crypto/rand.
 //
 // The request ID is stored in the request context (retrieve with
 // RequestIDFromContext) and set on the response X-Request-Id header.
@@ -19,7 +24,7 @@ func RequestID() Middleware {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			id := r.Header.Get("X-Request-Id")
-			if id == "" {
+			if !validRequestID(id) {
 				id = newUUID()
 			}
 
@@ -30,6 +35,27 @@ func RequestID() Middleware {
 	}
 }
 
+// validRequestID reports whether a client-supplied request ID is safe to
+// propagate: non-empty, within a sane length, and restricted to characters
+// that cannot forge log lines or split response headers.
+func validRequestID(id string) bool {
+	if id == "" || len(id) > maxRequestIDLen {
+		return false
+	}
+	for i := 0; i < len(id); i++ {
+		c := id[i]
+		switch {
+		case c >= 'a' && c <= 'z',
+			c >= 'A' && c <= 'Z',
+			c >= '0' && c <= '9',
+			c == '-', c == '_', c == '.':
+		default:
+			return false
+		}
+	}
+	return true
+}
+
 // RequestIDFromContext returns the request ID from the context, or an empty
 // string if none is set.
 func RequestIDFromContext(ctx context.Context) string {