crypto/x509: parse invalid DNS names and email addresses.

Go 1.10 requires that SANs in certificates are valid. However, a
non-trivial number of (generally non-WebPKI) certificates have invalid
strings in dnsName fields and some have even put those dnsName SANs in
CA certificates.

This change defers validity checking until name constraints are checked.

Fixes #23995, #23711.

Change-Id: I2e0ebb0898c047874a3547226b71e3029333b7f1
Reviewed-on: https://go-review.googlesource.com/96378
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/x509/name_constraints_test.go

@@ -11,6 +11,7 @@ import (
 	"crypto/rand"
 	"crypto/x509/pkix"
 	"encoding/asn1"
+	"encoding/hex"
 	"encoding/pem"
 	"fmt"
 	"io/ioutil"
@@ -1482,6 +1483,64 @@ var nameConstraintsTests = []nameConstraintsTest{
 		},
 		requestedEKUs: []ExtKeyUsage{ExtKeyUsageServerAuth},
 	},
+
+	// An invalid DNS SAN should be detected only at validation time so
+	// that we can process CA certificates in the wild that have invalid SANs.
+	// See https://github.com/golang/go/issues/23995
+
+	// #77: an invalid DNS or mail SAN will not be detected if name constaint
+	// checking is not triggered.
+	nameConstraintsTest{
+		roots: []constraintsSpec{
+			constraintsSpec{},
+		},
+		intermediates: [][]constraintsSpec{
+			[]constraintsSpec{
+				constraintsSpec{},
+			},
+		},
+		leaf: leafSpec{
+			sans: []string{"dns:this is invalid", "email:this @ is invalid"},
+		},
+	},
+
+	// #78: an invalid DNS SAN will be detected if any name constraint checking
+	// is triggered.
+	nameConstraintsTest{
+		roots: []constraintsSpec{
+			constraintsSpec{
+				bad: []string{"uri:"},
+			},
+		},
+		intermediates: [][]constraintsSpec{
+			[]constraintsSpec{
+				constraintsSpec{},
+			},
+		},
+		leaf: leafSpec{
+			sans: []string{"dns:this is invalid"},
+		},
+		expectedError: "cannot parse dnsName",
+	},
+
+	// #79: an invalid email SAN will be detected if any name constraint
+	// checking is triggered.
+	nameConstraintsTest{
+		roots: []constraintsSpec{
+			constraintsSpec{
+				bad: []string{"uri:"},
+			},
+		},
+		intermediates: [][]constraintsSpec{
+			[]constraintsSpec{
+				constraintsSpec{},
+			},
+		},
+		leaf: leafSpec{
+			sans: []string{"email:this @ is invalid"},
+		},
+		expectedError: "cannot parse rfc822Name",
+	},
 }
 
 func makeConstraintsCACert(constraints constraintsSpec, name string, key *ecdsa.PrivateKey, parent *Certificate, parentKey *ecdsa.PrivateKey) (*Certificate, error) {
@@ -1550,6 +1609,13 @@ func makeConstraintsLeafCert(leaf leafSpec, key *ecdsa.PrivateKey, parent *Certi
 			}
 			template.IPAddresses = append(template.IPAddresses, ip)
 
+		case strings.HasPrefix(name, "invalidip:"):
+			ipBytes, err := hex.DecodeString(name[10:])
+			if err != nil {
+				return nil, fmt.Errorf("cannot parse invalid IP: %s", err)
+			}
+			template.IPAddresses = append(template.IPAddresses, net.IP(ipBytes))
+
 		case strings.HasPrefix(name, "email:"):
 			template.EmailAddresses = append(template.EmailAddresses, name[6:])
 
@@ -2011,12 +2077,13 @@ func TestBadNamesInConstraints(t *testing.T) {
 }
 
 func TestBadNamesInSANs(t *testing.T) {
-	// Bad names in SANs should not parse.
+	// Bad names in URI and IP SANs should not parse. Bad DNS and email SANs
+	// will parse and are tested in name constraint tests at the top of this
+	// file.
 	badNames := []string{
-		"dns:foo.com.",
-		"email:abc@foo.com.",
-		"email:foo.com.",
 		"uri:https://example.com./dsf",
+		"invalidip:0102",
+		"invalidip:0102030405",
 	}
 
 	priv, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)

src/crypto/x509/verify.go

@@ -640,8 +640,7 @@ func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *V
 				name := string(data)
 				mailbox, ok := parseRFC2821Mailbox(name)
 				if !ok {
-					// This certificate should not have parsed.
-					return errors.New("x509: internal error: rfc822Name SAN failed to parse")
+					return fmt.Errorf("x509: cannot parse rfc822Name %q", mailbox)
 				}
 
 				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "email address", name, mailbox,
@@ -653,6 +652,10 @@ func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *V
 
 			case nameTypeDNS:
 				name := string(data)
+				if _, ok := domainToReverseLabels(name); !ok {
+					return fmt.Errorf("x509: cannot parse dnsName %q", name)
+				}
+
 				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "DNS name", name, name,
 					func(parsedName, constraint interface{}) (bool, error) {
 						return matchDomainConstraint(parsedName.(string), constraint.(string))

src/crypto/x509/x509.go

@@ -706,7 +706,9 @@ type Certificate struct {
 	OCSPServer            []string
 	IssuingCertificateURL []string
 
-	// Subject Alternate Name values
+	// Subject Alternate Name values. (Note that these values may not be valid
+	// if invalid values were contained within a parsed certificate. For
+	// example, an element of DNSNames may not be a valid DNS domain name.)
 	DNSNames       []string
 	EmailAddresses []string
 	IPAddresses    []net.IP
@@ -1126,17 +1128,9 @@ func parseSANExtension(value []byte) (dnsNames, emailAddresses []string, ipAddre
 	err = forEachSAN(value, func(tag int, data []byte) error {
 		switch tag {
 		case nameTypeEmail:
-			mailbox := string(data)
-			if _, ok := parseRFC2821Mailbox(mailbox); !ok {
-				return fmt.Errorf("x509: cannot parse rfc822Name %q", mailbox)
-			}
-			emailAddresses = append(emailAddresses, mailbox)
+			emailAddresses = append(emailAddresses, string(data))
 		case nameTypeDNS:
-			domain := string(data)
-			if _, ok := domainToReverseLabels(domain); !ok {
-				return fmt.Errorf("x509: cannot parse dnsName %q", string(data))
-			}
-			dnsNames = append(dnsNames, domain)
+			dnsNames = append(dnsNames, string(data))
 		case nameTypeURI:
 			uri, err := url.Parse(string(data))
 			if err != nil {
@@ -1153,7 +1147,7 @@ func parseSANExtension(value []byte) (dnsNames, emailAddresses []string, ipAddre
 			case net.IPv4len, net.IPv6len:
 				ipAddresses = append(ipAddresses, data)
 			default:
-				return errors.New("x509: certificate contained IP address of length " + strconv.Itoa(len(data)))
+				return errors.New("x509: cannot parse IP address of length " + strconv.Itoa(len(data)))
 			}
 		}