diff --git a/encoding/protojson/decode.go b/encoding/protojson/decode.go
index e8552f00..cb4f7fbc 100644
--- a/encoding/protojson/decode.go
+++ b/encoding/protojson/decode.go
@@ -184,6 +184,9 @@ func (o UnmarshalOptions) unmarshalFields(m pref.Message, skipTypeURL bool) erro
 			}
 			if extType != nil {
 				fd = extType.TypeDescriptor()
+				if !messageDesc.ExtensionRanges().Has(fd.Number()) || fd.ContainingMessage().FullName() != messageDesc.FullName() {
+					return errors.New("message %v cannot be extended by %v", messageDesc.FullName(), fd.FullName())
+				}
 			}
 		} else {
 			// The name can either be the JSON name or the proto field name.
diff --git a/encoding/prototext/decode.go b/encoding/prototext/decode.go
index 1bf25c27..99ef5dcd 100644
--- a/encoding/prototext/decode.go
+++ b/encoding/prototext/decode.go
@@ -128,6 +128,9 @@ func (o UnmarshalOptions) unmarshalMessage(tmsg [][2]text.Value, m pref.Message)
 			}
 			if xt != nil {
 				fd = xt.TypeDescriptor()
+				if !messageDesc.ExtensionRanges().Has(fd.Number()) || fd.ContainingMessage().FullName() != messageDesc.FullName() {
+					return errors.New("message %v cannot be extended by %v", messageDesc.FullName(), fd.FullName())
+				}
 			}
 		}