crypto/x509: don't create certs with negative serials #47

smx509/x509.go

@@ -1284,13 +1284,17 @@ func CreateCertificate(rand io.Reader, template, parent *x509.Certificate, pub,
 		return nil, errors.New("x509: no SerialNumber given")
 	}
 
-	// RFC 5280 Section 4.1.2.2: serial number must not be longer than 20 octets
+	// RFC 5280 Section 4.1.2.2: serial number must positive and should not be longer
+	// than 20 octets.
 	//
 	// We cannot simply check for len(serialBytes) > 20, because encoding/asn1 may
 	// pad the slice in order to prevent the integer being mistaken for a negative
 	// number (DER uses the high bit of the left-most byte to indicate the sign.),
 	// so we need to double check the composition of the serial if it is exactly
 	// 20 bytes.
+	if template.SerialNumber.Sign() == -1 {
+		return nil, errors.New("x509: serial number must be positive")
+	}
 	serialBytes := template.SerialNumber.Bytes()
 	if len(serialBytes) > 20 || (len(serialBytes) == 20 && serialBytes[0]&0x80 != 0) {
 		return nil, errors.New("x509: serial number exceeds 20 octets")

smx509/x509_test.go

@@ -498,11 +498,7 @@ func TestCreateSelfSignedCertificate(t *testing.T) {
 	for _, test := range tests {
 		commonName := "test.example.com"
 		template := x509.Certificate{
-			// SerialNumber is negative to ensure that negative
-			// values are parsed. This is due to the prevalence of
-			// buggy code that produces certificates with negative
-			// serial numbers.
-			SerialNumber: big.NewInt(-1),
+			SerialNumber: big.NewInt(1),
 			Subject: pkix.Name{
 				CommonName:   commonName,
 				Organization: []string{"Σ Acme Co"},
@@ -2489,3 +2485,40 @@ func TestCreateCertificateLongSerial(t *testing.T) {
 		t.Errorf("CreateCertificate returned unexpected error: want %q, got %q", expectedErr, err)
 	}
 }
+
+var negativeSerialCert = `-----BEGIN CERTIFICATE-----
+MIIBBTCBraADAgECAgH/MAoGCCqGSM49BAMCMA0xCzAJBgNVBAMTAjopMB4XDTIy
+MDQxNDIzNTYwNFoXDTIyMDQxNTAxNTYwNFowDTELMAkGA1UEAxMCOikwWTATBgcq
+hkjOPQIBBggqhkjOPQMBBwNCAAQ9ezsIsj+q17K87z/PXE/rfGRN72P/Wyn5d6oo
+5M0ZbSatuntMvfKdX79CQxXAxN4oXk3Aov4jVSG12AcDI8ShMAoGCCqGSM49BAMC
+A0cAMEQCIBzfBU5eMPT6m5lsR6cXaJILpAaiD9YxOl4v6dT3rzEjAiBHmjnHmAss
+RqUAyJKFzqZxOlK2q4j2IYnuj5+LrLGbQA==
+-----END CERTIFICATE-----`
+
+func TestParseNegativeSerial(t *testing.T) {
+	pemBlock, _ := pem.Decode([]byte(negativeSerialCert))
+	_, err := ParseCertificate(pemBlock.Bytes)
+	if err != nil {
+		t.Fatalf("failed to parse certificate: %s", err)
+	}
+}
+
+func TestCreateNegativeSerial(t *testing.T) {
+	k, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	tmpl := &Certificate{
+		SerialNumber: big.NewInt(-1),
+		Subject: pkix.Name{
+			CommonName: ":)",
+		},
+		NotAfter:  time.Now().Add(time.Hour),
+		NotBefore: time.Now().Add(-time.Hour),
+	}
+	expectedErr := "x509: serial number must be positive"
+	_, err = CreateCertificate(rand.Reader, tmpl.asX509(), tmpl.asX509(), k.Public(), k)
+	if err == nil || err.Error() != expectedErr {
+		t.Errorf("CreateCertificate returned unexpected error: want %q, got %q", expectedErr, err)
+	}
+}