diff --git a/internal/encoding/messageset/messageset.go b/internal/encoding/messageset/messageset.go
index 55526cf..4bd0e4e 100644
--- a/internal/encoding/messageset/messageset.go
+++ b/internal/encoding/messageset/messageset.go
@@ -6,6 +6,8 @@
 package messageset
 
 import (
+	"math"
+
 	"google.golang.org/protobuf/internal/encoding/wire"
 	"google.golang.org/protobuf/internal/errors"
 	pref "google.golang.org/protobuf/reflect/protoreflect"
@@ -146,6 +148,9 @@
 				return 0, nil, 0, wire.ParseError(n)
 			}
 			b = b[n:]
+			if v < 1 || v > math.MaxInt32 {
+				return 0, nil, 0, errors.New("invalid type_id in message set")
+			}
 			typeid = wire.Number(v)
 		case num == FieldMessage && wtyp == wire.BytesType:
 			m, n := wire.ConsumeBytes(b)
@@ -178,6 +183,13 @@
 				}
 			}
 			b = b[n:]
+		default:
+			// We have no place to put it, so we just ignore unknown fields.
+			n := wire.ConsumeFieldValue(num, wtyp, b)
+			if n < 0 {
+				return 0, nil, 0, wire.ParseError(n)
+			}
+			b = b[n:]
 		}
 	}
 }
diff --git a/proto/messageset_test.go b/proto/messageset_test.go
index ff41342..1eedcc9 100644
--- a/proto/messageset_test.go
+++ b/proto/messageset_test.go
@@ -17,6 +17,7 @@
 func init() {
 	if flags.ProtoLegacy {
 		testValidMessages = append(testValidMessages, messageSetTestProtos...)
+		testInvalidMessages = append(testInvalidMessages, messageSetInvalidTestProtos...)
 	}
 }
 
@@ -218,6 +219,27 @@
 		}.Marshal(),
 	},
 	{
+		desc: "MessageSet with unknown field",
+		decodeTo: []proto.Message{func() proto.Message {
+			m := &messagesetpb.MessageSetContainer{MessageSet: &messagesetpb.MessageSet{}}
+			proto.SetExtension(m.MessageSet, msetextpb.E_Ext1_MessageSetExtension, &msetextpb.Ext1{
+				Ext1Field1: proto.Int32(10),
+			})
+			return m
+		}()},
+		wire: pack.Message{
+			pack.Tag{1, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.StartGroupType},
+				pack.Tag{2, pack.VarintType}, pack.Varint(1000),
+				pack.Tag{3, pack.BytesType}, pack.LengthPrefix(pack.Message{
+					pack.Tag{1, pack.VarintType}, pack.Varint(10),
+				}),
+				pack.Tag{4, pack.VarintType}, pack.Varint(0),
+				pack.Tag{1, pack.EndGroupType},
+			}),
+		}.Marshal(),
+	},
+	{
 		desc:          "MessageSet with required field set",
 		checkFastInit: true,
 		decodeTo: []proto.Message{func() proto.Message {
@@ -257,3 +279,34 @@
 		}.Marshal(),
 	},
 }
+
+var messageSetInvalidTestProtos = []testProto{
+	{
+		desc: "MessageSet with type id 0",
+		decodeTo: []proto.Message{
+			(*messagesetpb.MessageSetContainer)(nil),
+		},
+		wire: pack.Message{
+			pack.Tag{1, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.StartGroupType},
+				pack.Tag{2, pack.VarintType}, pack.Uvarint(0),
+				pack.Tag{3, pack.BytesType}, pack.LengthPrefix(pack.Message{}),
+				pack.Tag{1, pack.EndGroupType},
+			}),
+		}.Marshal(),
+	},
+	{
+		desc: "MessageSet with type id overflowing int32",
+		decodeTo: []proto.Message{
+			(*messagesetpb.MessageSetContainer)(nil),
+		},
+		wire: pack.Message{
+			pack.Tag{1, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.StartGroupType},
+				pack.Tag{2, pack.VarintType}, pack.Uvarint(0x80000000),
+				pack.Tag{3, pack.BytesType}, pack.LengthPrefix(pack.Message{}),
+				pack.Tag{1, pack.EndGroupType},
+			}),
+		}.Marshal(),
+	},
+}