diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..aaea8ed
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+cid-fuzz.zip
diff --git a/cid.go b/cid.go
index 68f1c01..e3aab8c 100644
--- a/cid.go
+++ b/cid.go
@@ -3,6 +3,7 @@ package cid
 import (
 	"bytes"
 	"encoding/binary"
+	"errors"
 	"fmt"
 	"strings"
 
@@ -88,6 +89,23 @@ func Decode(v string) (*Cid, error) {
 	return Cast(data)
 }
 
+var (
+	ErrVarintBuffSmall = errors.New("reading varint: buffer to small")
+	ErrVarintTooBig    = errors.New("reading varint: varint bigger than 64bits" +
+		" and not supported")
+)
+
+func uvError(read int) error {
+	switch {
+	case read == 0:
+		return ErrVarintBuffSmall
+	case read < 0:
+		return ErrVarintTooBig
+	default:
+		return nil
+	}
+}
+
 func Cast(data []byte) (*Cid, error) {
 	if len(data) == 34 && data[0] == 18 && data[1] == 32 {
 		h, err := mh.Cast(data)
@@ -103,11 +121,18 @@ func Cast(data []byte) (*Cid, error) {
 	}
 
 	vers, n := binary.Uvarint(data)
+	if err := uvError(n); err != nil {
+		return nil, err
+	}
+
 	if vers != 0 && vers != 1 {
 		return nil, fmt.Errorf("invalid cid version number: %d", vers)
 	}
 
 	codec, cn := binary.Uvarint(data[n:])
+	if err := uvError(cn); err != nil {
+		return nil, err
+	}
 
 	rest := data[n+cn:]
 	h, err := mh.Cast(rest)
@@ -162,10 +187,14 @@ func (c *Cid) bytesV0() []byte {
 }
 
 func (c *Cid) bytesV1() []byte {
-	buf := make([]byte, 8+len(c.hash))
+	// two 8 bytes (max) numbers plus hash
+	buf := make([]byte, 2*binary.MaxVarintLen64+len(c.hash))
 	n := binary.PutUvarint(buf, c.version)
 	n += binary.PutUvarint(buf[n:], c.codec)
-	copy(buf[n:], c.hash)
+	cn := copy(buf[n:], c.hash)
+	if cn != len(c.hash) {
+		panic("copy hash length is inconsistent")
+	}
 
 	return buf[:n+len(c.hash)]
 }
@@ -240,7 +269,7 @@ func (p Prefix) Sum(data []byte) (*Cid, error) {
 }
 
 func (p Prefix) Bytes() []byte {
-	buf := make([]byte, 16)
+	buf := make([]byte, 4*binary.MaxVarintLen64)
 	n := binary.PutUvarint(buf, p.Version)
 	n += binary.PutUvarint(buf[n:], p.Codec)
 	n += binary.PutUvarint(buf[n:], uint64(p.MhType))
diff --git a/cid_fuzz.go b/cid_fuzz.go
new file mode 100644
index 0000000..357e907
--- /dev/null
+++ b/cid_fuzz.go
@@ -0,0 +1,37 @@
+// +build gofuzz
+
+package cid
+
+func Fuzz(data []byte) int {
+	cid, err := Cast(data)
+
+	if err != nil {
+		return 0
+	}
+
+	_ = cid.Bytes()
+	_ = cid.String()
+	p := cid.Prefix()
+	_ = p.Bytes()
+
+	if !cid.Equals(cid) {
+		panic("inequality")
+	}
+
+	// json loop
+	json, err := cid.MarshalJSON()
+	if err != nil {
+		panic(err.Error())
+	}
+	cid2 := &Cid{}
+	err = cid2.UnmarshalJSON(json)
+	if err != nil {
+		panic(err.Error())
+	}
+
+	if !cid.Equals(cid2) {
+		panic("json loop not equal")
+	}
+
+	return 1
+}
diff --git a/cid_test.go b/cid_test.go
index 0c51bdb..c03e281 100644
--- a/cid_test.go
+++ b/cid_test.go
@@ -119,6 +119,15 @@ func TestPrefixRoundtrip(t *testing.T) {
 	}
 }
 
+func Test16BytesVarint(t *testing.T) {
+	data := []byte("this is some test content")
+	hash, _ := mh.Sum(data, mh.SHA2_256, -1)
+	c := NewCidV1(CBOR, hash)
+
+	c.codec = 1 << 63
+	_ = c.Bytes()
+}
+
 func TestFuzzCid(t *testing.T) {
 	buf := make([]byte, 128)
 	for i := 0; i < 200; i++ {
diff --git a/fuzz-data/corpus/cid0 b/fuzz-data/corpus/cid0
new file mode 100644
index 0000000..56fd786
--- /dev/null
+++ b/fuzz-data/corpus/cid0
@@ -0,0 +1 @@
+ ëgáD1üüÊe�-D˜/¹q3ø~å(Ä7`8–�‡n
\ No newline at end of file
diff --git a/fuzz-data/corpus/cid1 b/fuzz-data/corpus/cid1
new file mode 100644
index 0000000..e0420b6
--- /dev/null
+++ b/fuzz-data/corpus/cid1
@@ -0,0 +1 @@
+
q -[·Ã¯¾hÀ[Í�‰
Ê(ΰ[õ)êùï‹D²¹
\ No newline at end of file