README.md

@@ -132,7 +132,8 @@ Where:
   or `85` when the URL has no `q`; a request whose `q` is anything else is
   refused with 400
 - `fit` — the URL's `fit` query parameter (cover, contain, fill, inside,
-  outside), or `cover` when the URL has no `fit`
+  outside), or `cover` when the URL has no `fit`; a request whose `fit` is
+  anything else, an empty `fit=` included, is refused with 400
 
 **Example:** resize `https://cdn.example.com/photos/cat.jpg` to 800x600
 WebP with expiration 1704067200, default quality and fit:

TODO.md

@@ -30,6 +30,12 @@ exhaustion
 
 # Completed Steps
 
+- 2026-09-28 refuse an empty `fit` on `/v1/image/` (closes #139): a
+  `fit` in the URL with an empty value (`fit=`) is a 400 naming `fit`,
+  instead of being served as `cover` and verified against a signature
+  made for `cover`; only a `fit` missing from the URL is still `cover`;
+  any other value still goes through the existing fit-mode check;
+  `README.md` says so where it documents `fit`.
 - 2026-09-28 refuse an invalid `q` on `/v1/image/` (closes #134): a `q`
   that is not a whole number from 1 to 100, an empty `q` included, is a
   400 naming `q` and the value, instead of being served at the default

internal/handlers/handlers_internal_test.go

@@ -356,3 +356,41 @@ func TestHandleImage_InvalidQuery_Returns400(t *testing.T) {
 		})
 	}
 }
+
+// TestHandleImage_EmptyFit_Returns400 verifies that the plain image route
+// answers a fit that is in the URL but empty with 400 naming fit, instead of
+// serving the image as cover. Only a fit missing from the URL means cover.
+func TestHandleImage_EmptyFit_Returns400(t *testing.T) {
+	t.Parallel()
+
+	fix := setupTestHandler(t)
+
+	r := chi.NewRouter()
+	r.Get("/v1/image/*", fix.handler.HandleImage())
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet,
+		"/v1/image/"+fix.goodHost+"/images/photo.jpg/50x50.jpeg?fit=", nil)
+	rec := httptest.NewRecorder()
+
+	r.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusBadRequest {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
+	}
+
+	t.Logf("GET %s: %d %s", req.URL, rec.Code, rec.Body)
+
+	var body struct {
+		Error string `json:"error"`
+	}
+
+	err := json.NewDecoder(rec.Body).Decode(&body)
+	if err != nil {
+		t.Fatalf("decoding response body: %v", err)
+	}
+
+	wantError := `invalid fit: not a fit mode, got ""`
+	if body.Error != wantError {
+		t.Errorf("error = %q, want %q", body.Error, wantError)
+	}
+}

internal/handlers/image.go

@@ -144,8 +144,14 @@ func (s *Handlers) parseImageRequest(
 		return nil, false
 	}
 
-	if fit := query.Get("fit"); fit != "" {
-		req.FitMode = imgcache.FitMode(fit)
+	// Only a fit missing from the URL is cover. A fit in the URL that is not a
+	// fit mode is refused by the fit-mode check below; that check would take an
+	// empty fit as missing, so an empty one is refused here.
+	req.FitMode = imgcache.FitMode(query.Get("fit"))
+	if query.Has("fit") && req.FitMode == "" {
+		s.respondError(w, `invalid fit: not a fit mode, got ""`, http.StatusBadRequest)
+
+		return nil, false
 	}
 
 	// Default fit mode if not set