[release-branch.go1.25] crypto/x509: rework fix for CVE-2025-58187

In CL 709854 we enabled strict validation for a number of properties of
domain names (and their constraints). This caused significant breakage,
since we didn't previously disallow the creation of certificates which
contained these malformed domains.

Rollback a number of the properties we enforced, making domainNameValid
only enforce the same properties that domainToReverseLabels does. Since
this also undoes some of the DoS protections our initial fix enabled,
this change also adds caching of constraints in isValid (which perhaps
is the fix we should've initially chosen).

Updates golang#75835
Updates golang#75828
Fixes golang#75861

Change-Id: Ie6ca6b4f30e9b8a143692b64757f7bbf4671ed0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/710735
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
(cherry picked from commit 1cd7168)
Reviewed-on: https://go-review.googlesource.com/c/go/+/710677
Auto-Submit: Michael Pratt <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: rolandshoemaker

src/crypto/x509/name_constraints_test.go

@@ -1456,7 +1456,63 @@ var nameConstraintsTests = []nameConstraintsTest{
 		expectedError: "incompatible key usage",
 	},
 
-	// #77: if several EKUs are requested, satisfying any of them is sufficient.
+	// 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 constraint
+	// checking is not triggered.
+	{
+		roots: make([]constraintsSpec, 1),
+		intermediates: [][]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.
+	{
+		roots: []constraintsSpec{
+			{
+				bad: []string{"uri:"},
+			},
+		},
+		intermediates: [][]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.
+	{
+		roots: []constraintsSpec{
+			{
+				bad: []string{"uri:"},
+			},
+		},
+		intermediates: [][]constraintsSpec{
+			{
+				{},
+			},
+		},
+		leaf: leafSpec{
+			sans: []string{"email:this @ is invalid"},
+		},
+		expectedError: "cannot parse rfc822Name",
+	},
+
+	// #80: if several EKUs are requested, satisfying any of them is sufficient.
 	{
 		roots: make([]constraintsSpec, 1),
 		intermediates: [][]constraintsSpec{
@@ -1471,7 +1527,7 @@ var nameConstraintsTests = []nameConstraintsTest{
 		requestedEKUs: []ExtKeyUsage{ExtKeyUsageClientAuth, ExtKeyUsageEmailProtection},
 	},
 
-	// #78: EKUs that are not asserted in VerifyOpts are not required to be
+	// #81: EKUs that are not asserted in VerifyOpts are not required to be
 	// nested.
 	{
 		roots: make([]constraintsSpec, 1),
@@ -1490,7 +1546,7 @@ var nameConstraintsTests = []nameConstraintsTest{
 		},
 	},
 
-	// #79: a certificate without SANs and CN is accepted in a constrained chain.
+	// #82: a certificate without SANs and CN is accepted in a constrained chain.
 	{
 		roots: []constraintsSpec{
 			{
@@ -1507,7 +1563,7 @@ var nameConstraintsTests = []nameConstraintsTest{
 		},
 	},
 
-	// #80: a certificate without SANs and with a CN that does not parse as a
+	// #83: a certificate without SANs and with a CN that does not parse as a
 	// hostname is accepted in a constrained chain.
 	{
 		roots: []constraintsSpec{
@@ -1526,7 +1582,7 @@ var nameConstraintsTests = []nameConstraintsTest{
 		},
 	},
 
-	// #81: a certificate with SANs and CN is accepted in a constrained chain.
+	// #84: a certificate with SANs and CN is accepted in a constrained chain.
 	{
 		roots: []constraintsSpec{
 			{
@@ -1544,7 +1600,14 @@ var nameConstraintsTests = []nameConstraintsTest{
 		},
 	},
 
-	// #82: URIs with IPv6 addresses with zones and ports are rejected
+	// #85: .example.com is an invalid DNS name, it should not match the
+	// constraint example.com.
+	{
+		roots:         []constraintsSpec{{ok: []string{"dns:example.com"}}},
+		leaf:          leafSpec{sans: []string{"dns:.example.com"}},
+		expectedError: "cannot parse dnsName \".example.com\"",
+	},
+	// #86: URIs with IPv6 addresses with zones and ports are rejected
 	{
 		roots: []constraintsSpec{
 			{

src/crypto/x509/parser.go

@@ -413,14 +413,10 @@ func parseSANExtension(der cryptobyte.String) (dnsNames, emailAddresses []string
 			if err := isIA5String(email); err != nil {
 				return errors.New("x509: SAN rfc822Name is malformed")
 			}
-			parsed, ok := parseRFC2821Mailbox(email)
-			if !ok || (ok && !domainNameValid(parsed.domain, false)) {
-				return errors.New("x509: SAN rfc822Name is malformed")
-			}
 			emailAddresses = append(emailAddresses, email)
 		case nameTypeDNS:
 			name := string(data)
-			if err := isIA5String(name); err != nil || (err == nil && !domainNameValid(name, false)) {
+			if err := isIA5String(name); err != nil {
 				return errors.New("x509: SAN dNSName is malformed")
 			}
 			dnsNames = append(dnsNames, string(name))
@@ -430,9 +426,12 @@ func parseSANExtension(der cryptobyte.String) (dnsNames, emailAddresses []string
 				return errors.New("x509: SAN uniformResourceIdentifier is malformed")
 			}
 			uri, err := url.Parse(uriStr)
-			if err != nil || (err == nil && uri.Host != "" && !domainNameValid(uri.Host, false)) {
+			if err != nil {
 				return fmt.Errorf("x509: cannot parse URI %q: %s", uriStr, err)
 			}
+			if len(uri.Host) > 0 && !domainNameValid(uri.Host, false) {
+				return fmt.Errorf("x509: cannot parse URI %q: invalid domain", uriStr)
+			}
 			uris = append(uris, uri)
 		case nameTypeIP:
 			switch len(data) {
@@ -1296,36 +1295,58 @@ func ParseRevocationList(der []byte) (*RevocationList, error) {
 	return rl, nil
 }
 
-// domainNameValid does minimal domain name validity checking. In particular it
-// enforces the following properties:
-//   - names cannot have the trailing period
-//   - names can only have a leading period if constraint is true
-//   - names must be <= 253 characters
-//   - names cannot have empty labels
-//   - names cannot labels that are longer than 63 characters
-//
-// Note that this does not enforce the LDH requirements for domain names.
+// domainNameValid is an alloc-less version of the checks that
+// domainToReverseLabels does.
 func domainNameValid(s string, constraint bool) bool {
-	if len(s) == 0 && constraint {
+	// TODO(#75835): This function omits a number of checks which we
+	// really should be doing to enforce that domain names are valid names per
+	// RFC 1034. We previously enabled these checks, but this broke a
+	// significant number of certificates we previously considered valid, and we
+	// happily create via CreateCertificate (et al). We should enable these
+	// checks, but will need to gate them behind a GODEBUG.
+	//
+	// I have left the checks we previously enabled, noted with "TODO(#75835)" so
+	// that we can easily re-enable them once we unbreak everyone.
+
+	// TODO(#75835): this should only be true for constraints.
+	if len(s) == 0 {
 		return true
 	}
-	if len(s) == 0 || (!constraint && s[0] == '.') || s[len(s)-1] == '.' || len(s) > 253 {
+
+	// Do not allow trailing period (FQDN format is not allowed in SANs or
+	// constraints).
+	if s[len(s)-1] == '.' {
 		return false
 	}
+
+	// TODO(#75835): domains must have at least one label, cannot have
+	// a leading empty label, and cannot be longer than 253 characters.
+	// if len(s) == 0 || (!constraint && s[0] == '.') || len(s) > 253 {
+	// 	return false
+	// }
+
 	lastDot := -1
 	if constraint && s[0] == '.' {
 		s = s[1:]
 	}
 
 	for i := 0; i <= len(s); i++ {
+		if i < len(s) && (s[i] < 33 || s[i] > 126) {
+			// Invalid character.
+			return false
+		}
 		if i == len(s) || s[i] == '.' {
 			labelLen := i
 			if lastDot >= 0 {
 				labelLen -= lastDot + 1
 			}
-			if labelLen == 0 || labelLen > 63 {
+			if labelLen == 0 {
 				return false
 			}
+			// TODO(#75835): labels cannot be longer than 63 characters.
+			// if labelLen > 63 {
+			// 	return false
+			// }
 			lastDot = i
 		}
 	}

src/crypto/x509/parser_test.go

@@ -5,6 +5,9 @@
 package x509
 
 import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
 	"encoding/asn1"
 	"encoding/pem"
 	"os"
@@ -260,7 +263,31 @@ func TestDomainNameValid(t *testing.T) {
 		constraint bool
 		valid      bool
 	}{
-		{"empty name, name", "", false, false},
+		// TODO(#75835): these tests are for stricter name validation, which we
+		// had to disable. Once we reenable these strict checks, behind a
+		// GODEBUG, we should add them back in.
+		// {"empty name, name", "", false, false},
+		// {"254 char label, name", strings.Repeat("a.a", 84) + "aaa", false, false},
+		// {"254 char label, constraint", strings.Repeat("a.a", 84) + "aaa", true, false},
+		// {"253 char label, name", strings.Repeat("a.a", 84) + "aa", false, false},
+		// {"253 char label, constraint", strings.Repeat("a.a", 84) + "aa", true, false},
+		// {"64 char single label, name", strings.Repeat("a", 64), false, false},
+		// {"64 char single label, constraint", strings.Repeat("a", 64), true, false},
+		// {"64 char label, name", "a." + strings.Repeat("a", 64), false, false},
+		// {"64 char label, constraint", "a." + strings.Repeat("a", 64), true, false},
+
+		// TODO(#75835): these are the inverse of the tests above, they should be removed
+		// once the strict checking is enabled.
+		{"254 char label, name", strings.Repeat("a.a", 84) + "aaa", false, true},
+		{"254 char label, constraint", strings.Repeat("a.a", 84) + "aaa", true, true},
+		{"253 char label, name", strings.Repeat("a.a", 84) + "aa", false, true},
+		{"253 char label, constraint", strings.Repeat("a.a", 84) + "aa", true, true},
+		{"64 char single label, name", strings.Repeat("a", 64), false, true},
+		{"64 char single label, constraint", strings.Repeat("a", 64), true, true},
+		{"64 char label, name", "a." + strings.Repeat("a", 64), false, true},
+		{"64 char label, constraint", "a." + strings.Repeat("a", 64), true, true},
+
+		// Check we properly enforce properties of domain names.
 		{"empty name, constraint", "", true, true},
 		{"empty label, name", "a..a", false, false},
 		{"empty label, constraint", "a..a", true, false},
@@ -274,23 +301,60 @@ func TestDomainNameValid(t *testing.T) {
 		{"trailing period, constraint", "a.", true, false},
 		{"bare label, name", "a", false, true},
 		{"bare label, constraint", "a", true, true},
-		{"254 char label, name", strings.Repeat("a.a", 84) + "aaa", false, false},
-		{"254 char label, constraint", strings.Repeat("a.a", 84) + "aaa", true, false},
-		{"253 char label, name", strings.Repeat("a.a", 84) + "aa", false, false},
-		{"253 char label, constraint", strings.Repeat("a.a", 84) + "aa", true, false},
-		{"64 char single label, name", strings.Repeat("a", 64), false, false},
-		{"64 char single label, constraint", strings.Repeat("a", 64), true, false},
 		{"63 char single label, name", strings.Repeat("a", 63), false, true},
 		{"63 char single label, constraint", strings.Repeat("a", 63), true, true},
-		{"64 char label, name", "a." + strings.Repeat("a", 64), false, false},
-		{"64 char label, constraint", "a." + strings.Repeat("a", 64), true, false},
 		{"63 char label, name", "a." + strings.Repeat("a", 63), false, true},
 		{"63 char label, constraint", "a." + strings.Repeat("a", 63), true, true},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
-			if tc.valid != domainNameValid(tc.dnsName, tc.constraint) {
+			valid := domainNameValid(tc.dnsName, tc.constraint)
+			if tc.valid != valid {
 				t.Errorf("domainNameValid(%q, %t) = %v; want %v", tc.dnsName, tc.constraint, !tc.valid, tc.valid)
 			}
+			// Also check that we enforce the same properties as domainToReverseLabels
+			trimmedName := tc.dnsName
+			if tc.constraint && len(trimmedName) > 1 && trimmedName[0] == '.' {
+				trimmedName = trimmedName[1:]
+			}
+			_, revValid := domainToReverseLabels(trimmedName)
+			if valid != revValid {
+				t.Errorf("domainNameValid(%q, %t) = %t != domainToReverseLabels(%q) = %t", tc.dnsName, tc.constraint, valid, trimmedName, revValid)
+			}
 		})
 	}
 }
+
+func TestRoundtripWeirdSANs(t *testing.T) {
+	// TODO(#75835): check that certificates we create with CreateCertificate that have malformed SAN values
+	// can be parsed by ParseCertificate. We should eventually restrict this, but for now we have to maintain
+	// this property as people have been relying on it.
+	k, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	badNames := []string{
+		"baredomain",
+		"baredomain.",
+		strings.Repeat("a", 255),
+		strings.Repeat("a", 65) + ".com",
+	}
+	tmpl := &Certificate{
+		EmailAddresses: badNames,
+		DNSNames:       badNames,
+	}
+	b, err := CreateCertificate(rand.Reader, tmpl, tmpl, &k.PublicKey, k)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = ParseCertificate(b)
+	if err != nil {
+		t.Fatalf("Couldn't roundtrip certificate: %v", err)
+	}
+}
+
+func FuzzDomainNameValid(f *testing.F) {
+	f.Fuzz(func(t *testing.T, data string) {
+		domainNameValid(data, false)
+		domainNameValid(data, true)
+	})
+}