Validate prefix length bounds in DecodePrefix

shared/netiputil/compact.go

@@ -26,14 +26,22 @@ func DecodePrefix(b []byte) (netip.Prefix, error) {
 	case 5:
 		var ip4 [4]byte
 		copy(ip4[:], b)
-		return netip.PrefixFrom(netip.AddrFrom4(ip4), int(b[len(b)-1])), nil
+		bits := int(b[len(b)-1])
+		if bits > 32 {
+			return netip.Prefix{}, fmt.Errorf("invalid IPv4 prefix length %d (max 32)", bits)
+		}
+		return netip.PrefixFrom(netip.AddrFrom4(ip4), bits), nil
 	case 17:
 		var ip6 [16]byte
 		copy(ip6[:], b)
 		addr := netip.AddrFrom16(ip6).Unmap()
 		bits := int(b[len(b)-1])
-		if addr.Is4() && bits > 32 {
-			bits = 32
+		if addr.Is4() {
+			if bits > 32 {
+				bits = 32
+			}
+		} else if bits > 128 {
+			return netip.Prefix{}, fmt.Errorf("invalid IPv6 prefix length %d (max 128)", bits)
 		}
 		return netip.PrefixFrom(addr, bits), nil
 	default:

shared/netiputil/compact_test.go

@@ -110,6 +110,22 @@ func TestDecodePrefixInvalidLength(t *testing.T) {
 	assert.Error(t, err)
 }
 
+func TestDecodePrefixInvalidBits(t *testing.T) {
+	// v4 with bits > 32
+	b := []byte{10, 0, 0, 1, 33}
+	_, err := DecodePrefix(b)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "invalid IPv4 prefix length 33")
+
+	// v6 with bits > 128
+	b = make([]byte, 17)
+	b[0] = 0xfd
+	b[16] = 129
+	_, err = DecodePrefix(b)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "invalid IPv6 prefix length 129")
+}
+
 func TestDecodePrefixUnmapsV6Input(t *testing.T) {
 	// If someone encodes a v4-mapped v6 as 17 bytes, decode should unmap it
 	// and clamp the prefix length to 32 for v4