Validate X-Forwarded-Prefix value for dashboard redirect

Co-authored-by: Kevin Pollet <pollet.kevin@gmail.com>

pkg/api/dashboard/dashboard.go

@@ -79,7 +79,13 @@ func Append(router *mux.Router, basePath string, customAssets fs.FS) error {
 	router.Methods(http.MethodGet).
 		Path(basePath).
 		HandlerFunc(func(resp http.ResponseWriter, req *http.Request) {
-			prefix := strings.TrimSuffix(req.Header.Get("X-Forwarded-Prefix"), "/")
+			xfPrefix := req.Header.Get("X-Forwarded-Prefix")
+			if strings.Contains(xfPrefix, "//") {
+				log.Error().Msgf("X-Forwarded-Prefix contains an invalid value: %s, defaulting to empty prefix", xfPrefix)
+				xfPrefix = ""
+			}
+
+			prefix := strings.TrimSuffix(xfPrefix, "/")
 			http.Redirect(resp, req, prefix+dashboardPath, http.StatusFound)
 		})
 

pkg/api/dashboard/dashboard_test.go

@@ -9,7 +9,9 @@ import (
 	"testing/fstest"
 	"time"
 
+	"github.com/gorilla/mux"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func Test_ContentSecurityPolicy(t *testing.T) {
@@ -60,6 +62,52 @@ func Test_ContentSecurityPolicy(t *testing.T) {
 	}
 }
 
+func Test_XForwardedPrefix(t *testing.T) {
+	testCases := []struct {
+		desc     string
+		prefix   string
+		expected string
+	}{
+		{
+			desc:     "location in X-Forwarded-Prefix",
+			prefix:   "//foobar/test",
+			expected: "/dashboard/",
+		},
+		{
+			desc:     "scheme in X-Forwarded-Prefix",
+			prefix:   "http://foobar",
+			expected: "/dashboard/",
+		},
+		{
+			desc:     "path in X-Forwarded-Prefix",
+			prefix:   "foobar",
+			expected: "/foobar/dashboard/",
+		},
+	}
+
+	router := mux.NewRouter()
+	err := Append(router, "/", fstest.MapFS{"index.html": &fstest.MapFile{
+		Mode:    0o755,
+		ModTime: time.Now(),
+	}})
+	require.NoError(t, err)
+
+	for _, test := range testCases {
+		t.Run(test.desc, func(t *testing.T) {
+			t.Parallel()
+
+			req := httptest.NewRequest(http.MethodGet, "/", http.NoBody)
+			req.Header.Set("X-Forwarded-Prefix", test.prefix)
+			rw := httptest.NewRecorder()
+
+			router.ServeHTTP(rw, req)
+
+			assert.Equal(t, http.StatusFound, rw.Code)
+			assert.Equal(t, test.expected, rw.Result().Header.Get("Location"))
+		})
+	}
+}
+
 type errorFS struct{}
 
 func (e errorFS) Open(name string) (fs.File, error) {