diff --git a/middleware/auth.go b/middleware/auth.go
index 0564a40..55ad6b7 100644
--- a/middleware/auth.go
+++ b/middleware/auth.go
@@ -15,6 +15,9 @@ func BearerAuth(tokenFunc func(ctx context.Context) (string, error)) Middleware
 			if err != nil {
 				return nil, err
 			}
+			// RoundTrippers must not mutate the caller's request; clone before
+			// setting headers (req.Clone is shallow + a header copy).
+			req = req.Clone(req.Context())
 			req.Header.Set("Authorization", "Bearer "+token)
 			return next.RoundTrip(req)
 		})
@@ -26,6 +29,7 @@ func BearerAuth(tokenFunc func(ctx context.Context) (string, error)) Middleware
 func BasicAuth(username, password string) Middleware {
 	return func(next http.RoundTripper) http.RoundTripper {
 		return RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
+			req = req.Clone(req.Context())
 			req.SetBasicAuth(username, password)
 			return next.RoundTrip(req)
 		})
diff --git a/middleware/headers.go b/middleware/headers.go
index 895f890..00b6567 100644
--- a/middleware/headers.go
+++ b/middleware/headers.go
@@ -7,10 +7,17 @@ import "net/http"
 func DefaultHeaders(headers http.Header) Middleware {
 	return func(next http.RoundTripper) http.RoundTripper {
 		return RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
+			// Clone lazily on the first header we actually add, so that
+			// RoundTrippers never mutate the caller's request.
+			cloned := false
 			for key, values := range headers {
 				if req.Header.Get(key) != "" {
 					continue
 				}
+				if !cloned {
+					req = req.Clone(req.Context())
+					cloned = true
+				}
 				for _, v := range values {
 					req.Header.Add(key, v)
 				}
diff --git a/server/middleware.go b/server/middleware.go
index 9b7b79f..78300aa 100644
--- a/server/middleware.go
+++ b/server/middleware.go
@@ -25,8 +25,8 @@ func Chain(mws ...Middleware) Middleware {
 // underlying ResponseWriter's optional interfaces (Flusher, Hijacker, etc.).
 type statusWriter struct {
 	http.ResponseWriter
-	status      int
-	written     bool
+	status  int
+	written bool
 }
 
 // WriteHeader captures the status code and delegates to the underlying writer.
diff --git a/server/middleware_requestid.go b/server/middleware_requestid.go
index 5cfdb26..4a005b0 100644
--- a/server/middleware_requestid.go
+++ b/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 {
diff --git a/server/middleware_test.go b/server/middleware_test.go
index 0419885..c9d61bd 100644
--- a/server/middleware_test.go
+++ b/server/middleware_test.go
@@ -214,6 +214,36 @@ func TestRequestID(t *testing.T) {
 			t.Fatalf("expected empty, got %q", id)
 		}
 	})
+
+	t.Run("rejects unsafe incoming ID", func(t *testing.T) {
+		cases := map[string]string{
+			"header injection": "abc\r\nX-Injected: 1",
+			"contains space":   "has space",
+			"too long":         strings.Repeat("a", 200),
+		}
+		for name, badID := range cases {
+			t.Run(name, func(t *testing.T) {
+				var gotID string
+				handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+					gotID = server.RequestIDFromContext(r.Context())
+					w.WriteHeader(http.StatusOK)
+				})
+				mw := server.RequestID()(handler)
+
+				w := httptest.NewRecorder()
+				req := httptest.NewRequest(http.MethodGet, "/", nil)
+				req.Header.Set("X-Request-Id", badID)
+				mw.ServeHTTP(w, req)
+
+				if gotID == badID {
+					t.Fatalf("unsafe incoming ID was propagated verbatim: %q", gotID)
+				}
+				if len(gotID) != 36 {
+					t.Fatalf("expected a freshly generated UUID, got %q (len %d)", gotID, len(gotID))
+				}
+			})
+		}
+	})
 }
 
 func TestRequestID_UUIDFormat(t *testing.T) {