Source file src/crypto/x509/verify.go

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package x509
     6  
     7  import (
     8  	"bytes"
     9  	"errors"
    10  	"fmt"
    11  	"net"
    12  	"net/url"
    13  	"reflect"
    14  	"runtime"
    15  	"strings"
    16  	"time"
    17  	"unicode/utf8"
    18  )
    19  
    20  type InvalidReason int
    21  
    22  const (
    23  	// NotAuthorizedToSign results when a certificate is signed by another
    24  	// which isn't marked as a CA certificate.
    25  	NotAuthorizedToSign InvalidReason = iota
    26  	// Expired results when a certificate has expired, based on the time
    27  	// given in the VerifyOptions.
    28  	Expired
    29  	// CANotAuthorizedForThisName results when an intermediate or root
    30  	// certificate has a name constraint which doesn't permit a DNS or
    31  	// other name (including IP address) in the leaf certificate.
    32  	CANotAuthorizedForThisName
    33  	// TooManyIntermediates results when a path length constraint is
    34  	// violated.
    35  	TooManyIntermediates
    36  	// IncompatibleUsage results when the certificate's key usage indicates
    37  	// that it may only be used for a different purpose.
    38  	IncompatibleUsage
    39  	// NameMismatch results when the subject name of a parent certificate
    40  	// does not match the issuer name in the child.
    41  	NameMismatch
    42  	// NameConstraintsWithoutSANs is a legacy error and is no longer returned.
    43  	NameConstraintsWithoutSANs
    44  	// UnconstrainedName results when a CA certificate contains permitted
    45  	// name constraints, but leaf certificate contains a name of an
    46  	// unsupported or unconstrained type.
    47  	UnconstrainedName
    48  	// TooManyConstraints results when the number of comparison operations
    49  	// needed to check a certificate exceeds the limit set by
    50  	// VerifyOptions.MaxConstraintComparisions. This limit exists to
    51  	// prevent pathological certificates can consuming excessive amounts of
    52  	// CPU time to verify.
    53  	TooManyConstraints
    54  	// CANotAuthorizedForExtKeyUsage results when an intermediate or root
    55  	// certificate does not permit a requested extended key usage.
    56  	CANotAuthorizedForExtKeyUsage
    57  )
    58  
    59  // CertificateInvalidError results when an odd error occurs. Users of this
    60  // library probably want to handle all these errors uniformly.
    61  type CertificateInvalidError struct {
    62  	Cert   *Certificate
    63  	Reason InvalidReason
    64  	Detail string
    65  }
    66  
    67  func (e CertificateInvalidError) Error() string {
    68  	switch e.Reason {
    69  	case NotAuthorizedToSign:
    70  		return "x509: certificate is not authorized to sign other certificates"
    71  	case Expired:
    72  		return "x509: certificate has expired or is not yet valid: " + e.Detail
    73  	case CANotAuthorizedForThisName:
    74  		return "x509: a root or intermediate certificate is not authorized to sign for this name: " + e.Detail
    75  	case CANotAuthorizedForExtKeyUsage:
    76  		return "x509: a root or intermediate certificate is not authorized for an extended key usage: " + e.Detail
    77  	case TooManyIntermediates:
    78  		return "x509: too many intermediates for path length constraint"
    79  	case IncompatibleUsage:
    80  		return "x509: certificate specifies an incompatible key usage"
    81  	case NameMismatch:
    82  		return "x509: issuer name does not match subject from issuing certificate"
    83  	case NameConstraintsWithoutSANs:
    84  		return "x509: issuer has name constraints but leaf doesn't have a SAN extension"
    85  	case UnconstrainedName:
    86  		return "x509: issuer has name constraints but leaf contains unknown or unconstrained name: " + e.Detail
    87  	}
    88  	return "x509: unknown error"
    89  }
    90  
    91  // HostnameError results when the set of authorized names doesn't match the
    92  // requested name.
    93  type HostnameError struct {
    94  	Certificate *Certificate
    95  	Host        string
    96  }
    97  
    98  func (h HostnameError) Error() string {
    99  	c := h.Certificate
   100  
   101  	if !c.hasSANExtension() && matchHostnames(c.Subject.CommonName, h.Host) {
   102  		return "x509: certificate relies on legacy Common Name field, use SANs instead"
   103  	}
   104  
   105  	var valid string
   106  	if ip := net.ParseIP(h.Host); ip != nil {
   107  		// Trying to validate an IP
   108  		if len(c.IPAddresses) == 0 {
   109  			return "x509: cannot validate certificate for " + h.Host + " because it doesn't contain any IP SANs"
   110  		}
   111  		for _, san := range c.IPAddresses {
   112  			if len(valid) > 0 {
   113  				valid += ", "
   114  			}
   115  			valid += san.String()
   116  		}
   117  	} else {
   118  		valid = strings.Join(c.DNSNames, ", ")
   119  	}
   120  
   121  	if len(valid) == 0 {
   122  		return "x509: certificate is not valid for any names, but wanted to match " + h.Host
   123  	}
   124  	return "x509: certificate is valid for " + valid + ", not " + h.Host
   125  }
   126  
   127  // UnknownAuthorityError results when the certificate issuer is unknown
   128  type UnknownAuthorityError struct {
   129  	Cert *Certificate
   130  	// hintErr contains an error that may be helpful in determining why an
   131  	// authority wasn't found.
   132  	hintErr error
   133  	// hintCert contains a possible authority certificate that was rejected
   134  	// because of the error in hintErr.
   135  	hintCert *Certificate
   136  }
   137  
   138  func (e UnknownAuthorityError) Error() string {
   139  	s := "x509: certificate signed by unknown authority"
   140  	if e.hintErr != nil {
   141  		certName := e.hintCert.Subject.CommonName
   142  		if len(certName) == 0 {
   143  			if len(e.hintCert.Subject.Organization) > 0 {
   144  				certName = e.hintCert.Subject.Organization[0]
   145  			} else {
   146  				certName = "serial:" + e.hintCert.SerialNumber.String()
   147  			}
   148  		}
   149  		s += fmt.Sprintf(" (possibly because of %q while trying to verify candidate authority certificate %q)", e.hintErr, certName)
   150  	}
   151  	return s
   152  }
   153  
   154  // SystemRootsError results when we fail to load the system root certificates.
   155  type SystemRootsError struct {
   156  	Err error
   157  }
   158  
   159  func (se SystemRootsError) Error() string {
   160  	msg := "x509: failed to load system roots and no roots provided"
   161  	if se.Err != nil {
   162  		return msg + "; " + se.Err.Error()
   163  	}
   164  	return msg
   165  }
   166  
   167  func (se SystemRootsError) Unwrap() error { return se.Err }
   168  
   169  // errNotParsed is returned when a certificate without ASN.1 contents is
   170  // verified. Platform-specific verification needs the ASN.1 contents.
   171  var errNotParsed = errors.New("x509: missing ASN.1 contents; use ParseCertificate")
   172  
   173  // VerifyOptions contains parameters for Certificate.Verify.
   174  type VerifyOptions struct {
   175  	// DNSName, if set, is checked against the leaf certificate with
   176  	// Certificate.VerifyHostname or the platform verifier.
   177  	DNSName string
   178  
   179  	// Intermediates is an optional pool of certificates that are not trust
   180  	// anchors, but can be used to form a chain from the leaf certificate to a
   181  	// root certificate.
   182  	Intermediates *CertPool
   183  	// Roots is the set of trusted root certificates the leaf certificate needs
   184  	// to chain up to. If nil, the system roots or the platform verifier are used.
   185  	Roots *CertPool
   186  
   187  	// CurrentTime is used to check the validity of all certificates in the
   188  	// chain. If zero, the current time is used.
   189  	CurrentTime time.Time
   190  
   191  	// KeyUsages specifies which Extended Key Usage values are acceptable. A
   192  	// chain is accepted if it allows any of the listed values. An empty list
   193  	// means ExtKeyUsageServerAuth. To accept any key usage, include ExtKeyUsageAny.
   194  	KeyUsages []ExtKeyUsage
   195  
   196  	// MaxConstraintComparisions is the maximum number of comparisons to
   197  	// perform when checking a given certificate's name constraints. If
   198  	// zero, a sensible default is used. This limit prevents pathological
   199  	// certificates from consuming excessive amounts of CPU time when
   200  	// validating. It does not apply to the platform verifier.
   201  	MaxConstraintComparisions int
   202  }
   203  
   204  const (
   205  	leafCertificate = iota
   206  	intermediateCertificate
   207  	rootCertificate
   208  )
   209  
   210  // rfc2821Mailbox represents a “mailbox” (which is an email address to most
   211  // people) by breaking it into the “local” (i.e. before the '@') and “domain”
   212  // parts.
   213  type rfc2821Mailbox struct {
   214  	local, domain string
   215  }
   216  
   217  // parseRFC2821Mailbox parses an email address into local and domain parts,
   218  // based on the ABNF for a “Mailbox” from RFC 2821. According to RFC 5280,
   219  // Section 4.2.1.6 that's correct for an rfc822Name from a certificate: “The
   220  // format of an rfc822Name is a "Mailbox" as defined in RFC 2821, Section 4.1.2”.
   221  func parseRFC2821Mailbox(in string) (mailbox rfc2821Mailbox, ok bool) {
   222  	if len(in) == 0 {
   223  		return mailbox, false
   224  	}
   225  
   226  	localPartBytes := make([]byte, 0, len(in)/2)
   227  
   228  	if in[0] == '"' {
   229  		// Quoted-string = DQUOTE *qcontent DQUOTE
   230  		// non-whitespace-control = %d1-8 / %d11 / %d12 / %d14-31 / %d127
   231  		// qcontent = qtext / quoted-pair
   232  		// qtext = non-whitespace-control /
   233  		//         %d33 / %d35-91 / %d93-126
   234  		// quoted-pair = ("\" text) / obs-qp
   235  		// text = %d1-9 / %d11 / %d12 / %d14-127 / obs-text
   236  		//
   237  		// (Names beginning with “obs-” are the obsolete syntax from RFC 2822,
   238  		// Section 4. Since it has been 16 years, we no longer accept that.)
   239  		in = in[1:]
   240  	QuotedString:
   241  		for {
   242  			if len(in) == 0 {
   243  				return mailbox, false
   244  			}
   245  			c := in[0]
   246  			in = in[1:]
   247  
   248  			switch {
   249  			case c == '"':
   250  				break QuotedString
   251  
   252  			case c == '\\':
   253  				// quoted-pair
   254  				if len(in) == 0 {
   255  					return mailbox, false
   256  				}
   257  				if in[0] == 11 ||
   258  					in[0] == 12 ||
   259  					(1 <= in[0] && in[0] <= 9) ||
   260  					(14 <= in[0] && in[0] <= 127) {
   261  					localPartBytes = append(localPartBytes, in[0])
   262  					in = in[1:]
   263  				} else {
   264  					return mailbox, false
   265  				}
   266  
   267  			case c == 11 ||
   268  				c == 12 ||
   269  				// Space (char 32) is not allowed based on the
   270  				// BNF, but RFC 3696 gives an example that
   271  				// assumes that it is. Several “verified”
   272  				// errata continue to argue about this point.
   273  				// We choose to accept it.
   274  				c == 32 ||
   275  				c == 33 ||
   276  				c == 127 ||
   277  				(1 <= c && c <= 8) ||
   278  				(14 <= c && c <= 31) ||
   279  				(35 <= c && c <= 91) ||
   280  				(93 <= c && c <= 126):
   281  				// qtext
   282  				localPartBytes = append(localPartBytes, c)
   283  
   284  			default:
   285  				return mailbox, false
   286  			}
   287  		}
   288  	} else {
   289  		// Atom ("." Atom)*
   290  	NextChar:
   291  		for len(in) > 0 {
   292  			// atext from RFC 2822, Section 3.2.4
   293  			c := in[0]
   294  
   295  			switch {
   296  			case c == '\\':
   297  				// Examples given in RFC 3696 suggest that
   298  				// escaped characters can appear outside of a
   299  				// quoted string. Several “verified” errata
   300  				// continue to argue the point. We choose to
   301  				// accept it.
   302  				in = in[1:]
   303  				if len(in) == 0 {
   304  					return mailbox, false
   305  				}
   306  				fallthrough
   307  
   308  			case ('0' <= c && c <= '9') ||
   309  				('a' <= c && c <= 'z') ||
   310  				('A' <= c && c <= 'Z') ||
   311  				c == '!' || c == '#' || c == '$' || c == '%' ||
   312  				c == '&' || c == '\'' || c == '*' || c == '+' ||
   313  				c == '-' || c == '/' || c == '=' || c == '?' ||
   314  				c == '^' || c == '_' || c == '`' || c == '{' ||
   315  				c == '|' || c == '}' || c == '~' || c == '.':
   316  				localPartBytes = append(localPartBytes, in[0])
   317  				in = in[1:]
   318  
   319  			default:
   320  				break NextChar
   321  			}
   322  		}
   323  
   324  		if len(localPartBytes) == 0 {
   325  			return mailbox, false
   326  		}
   327  
   328  		// From RFC 3696, Section 3:
   329  		// “period (".") may also appear, but may not be used to start
   330  		// or end the local part, nor may two or more consecutive
   331  		// periods appear.”
   332  		twoDots := []byte{'.', '.'}
   333  		if localPartBytes[0] == '.' ||
   334  			localPartBytes[len(localPartBytes)-1] == '.' ||
   335  			bytes.Contains(localPartBytes, twoDots) {
   336  			return mailbox, false
   337  		}
   338  	}
   339  
   340  	if len(in) == 0 || in[0] != '@' {
   341  		return mailbox, false
   342  	}
   343  	in = in[1:]
   344  
   345  	// The RFC species a format for domains, but that's known to be
   346  	// violated in practice so we accept that anything after an '@' is the
   347  	// domain part.
   348  	if _, ok := domainToReverseLabels(in); !ok {
   349  		return mailbox, false
   350  	}
   351  
   352  	mailbox.local = string(localPartBytes)
   353  	mailbox.domain = in
   354  	return mailbox, true
   355  }
   356  
   357  // domainToReverseLabels converts a textual domain name like foo.example.com to
   358  // the list of labels in reverse order, e.g. ["com", "example", "foo"].
   359  func domainToReverseLabels(domain string) (reverseLabels []string, ok bool) {
   360  	for len(domain) > 0 {
   361  		if i := strings.LastIndexByte(domain, '.'); i == -1 {
   362  			reverseLabels = append(reverseLabels, domain)
   363  			domain = ""
   364  		} else {
   365  			reverseLabels = append(reverseLabels, domain[i+1:])
   366  			domain = domain[:i]
   367  		}
   368  	}
   369  
   370  	if len(reverseLabels) > 0 && len(reverseLabels[0]) == 0 {
   371  		// An empty label at the end indicates an absolute value.
   372  		return nil, false
   373  	}
   374  
   375  	for _, label := range reverseLabels {
   376  		if len(label) == 0 {
   377  			// Empty labels are otherwise invalid.
   378  			return nil, false
   379  		}
   380  
   381  		for _, c := range label {
   382  			if c < 33 || c > 126 {
   383  				// Invalid character.
   384  				return nil, false
   385  			}
   386  		}
   387  	}
   388  
   389  	return reverseLabels, true
   390  }
   391  
   392  func matchEmailConstraint(mailbox rfc2821Mailbox, constraint string) (bool, error) {
   393  	// If the constraint contains an @, then it specifies an exact mailbox
   394  	// name.
   395  	if strings.Contains(constraint, "@") {
   396  		constraintMailbox, ok := parseRFC2821Mailbox(constraint)
   397  		if !ok {
   398  			return false, fmt.Errorf("x509: internal error: cannot parse constraint %q", constraint)
   399  		}
   400  		return mailbox.local == constraintMailbox.local && strings.EqualFold(mailbox.domain, constraintMailbox.domain), nil
   401  	}
   402  
   403  	// Otherwise the constraint is like a DNS constraint of the domain part
   404  	// of the mailbox.
   405  	return matchDomainConstraint(mailbox.domain, constraint)
   406  }
   407  
   408  func matchURIConstraint(uri *url.URL, constraint string) (bool, error) {
   409  	// From RFC 5280, Section 4.2.1.10:
   410  	// “a uniformResourceIdentifier that does not include an authority
   411  	// component with a host name specified as a fully qualified domain
   412  	// name (e.g., if the URI either does not include an authority
   413  	// component or includes an authority component in which the host name
   414  	// is specified as an IP address), then the application MUST reject the
   415  	// certificate.”
   416  
   417  	host := uri.Host
   418  	if len(host) == 0 {
   419  		return false, fmt.Errorf("URI with empty host (%q) cannot be matched against constraints", uri.String())
   420  	}
   421  
   422  	if strings.Contains(host, ":") && !strings.HasSuffix(host, "]") {
   423  		var err error
   424  		host, _, err = net.SplitHostPort(uri.Host)
   425  		if err != nil {
   426  			return false, err
   427  		}
   428  	}
   429  
   430  	if strings.HasPrefix(host, "[") && strings.HasSuffix(host, "]") ||
   431  		net.ParseIP(host) != nil {
   432  		return false, fmt.Errorf("URI with IP (%q) cannot be matched against constraints", uri.String())
   433  	}
   434  
   435  	return matchDomainConstraint(host, constraint)
   436  }
   437  
   438  func matchIPConstraint(ip net.IP, constraint *net.IPNet) (bool, error) {
   439  	if len(ip) != len(constraint.IP) {
   440  		return false, nil
   441  	}
   442  
   443  	for i := range ip {
   444  		if mask := constraint.Mask[i]; ip[i]&mask != constraint.IP[i]&mask {
   445  			return false, nil
   446  		}
   447  	}
   448  
   449  	return true, nil
   450  }
   451  
   452  func matchDomainConstraint(domain, constraint string) (bool, error) {
   453  	// The meaning of zero length constraints is not specified, but this
   454  	// code follows NSS and accepts them as matching everything.
   455  	if len(constraint) == 0 {
   456  		return true, nil
   457  	}
   458  
   459  	domainLabels, ok := domainToReverseLabels(domain)
   460  	if !ok {
   461  		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", domain)
   462  	}
   463  
   464  	// RFC 5280 says that a leading period in a domain name means that at
   465  	// least one label must be prepended, but only for URI and email
   466  	// constraints, not DNS constraints. The code also supports that
   467  	// behaviour for DNS constraints.
   468  
   469  	mustHaveSubdomains := false
   470  	if constraint[0] == '.' {
   471  		mustHaveSubdomains = true
   472  		constraint = constraint[1:]
   473  	}
   474  
   475  	constraintLabels, ok := domainToReverseLabels(constraint)
   476  	if !ok {
   477  		return false, fmt.Errorf("x509: internal error: cannot parse domain %q", constraint)
   478  	}
   479  
   480  	if len(domainLabels) < len(constraintLabels) ||
   481  		(mustHaveSubdomains && len(domainLabels) == len(constraintLabels)) {
   482  		return false, nil
   483  	}
   484  
   485  	for i, constraintLabel := range constraintLabels {
   486  		if !strings.EqualFold(constraintLabel, domainLabels[i]) {
   487  			return false, nil
   488  		}
   489  	}
   490  
   491  	return true, nil
   492  }
   493  
   494  // checkNameConstraints checks that c permits a child certificate to claim the
   495  // given name, of type nameType. The argument parsedName contains the parsed
   496  // form of name, suitable for passing to the match function. The total number
   497  // of comparisons is tracked in the given count and should not exceed the given
   498  // limit.
   499  func (c *Certificate) checkNameConstraints(count *int,
   500  	maxConstraintComparisons int,
   501  	nameType string,
   502  	name string,
   503  	parsedName interface{},
   504  	match func(parsedName, constraint interface{}) (match bool, err error),
   505  	permitted, excluded interface{}) error {
   506  
   507  	excludedValue := reflect.ValueOf(excluded)
   508  
   509  	*count += excludedValue.Len()
   510  	if *count > maxConstraintComparisons {
   511  		return CertificateInvalidError{c, TooManyConstraints, ""}
   512  	}
   513  
   514  	for i := 0; i < excludedValue.Len(); i++ {
   515  		constraint := excludedValue.Index(i).Interface()
   516  		match, err := match(parsedName, constraint)
   517  		if err != nil {
   518  			return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
   519  		}
   520  
   521  		if match {
   522  			return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is excluded by constraint %q", nameType, name, constraint)}
   523  		}
   524  	}
   525  
   526  	permittedValue := reflect.ValueOf(permitted)
   527  
   528  	*count += permittedValue.Len()
   529  	if *count > maxConstraintComparisons {
   530  		return CertificateInvalidError{c, TooManyConstraints, ""}
   531  	}
   532  
   533  	ok := true
   534  	for i := 0; i < permittedValue.Len(); i++ {
   535  		constraint := permittedValue.Index(i).Interface()
   536  
   537  		var err error
   538  		if ok, err = match(parsedName, constraint); err != nil {
   539  			return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()}
   540  		}
   541  
   542  		if ok {
   543  			break
   544  		}
   545  	}
   546  
   547  	if !ok {
   548  		return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is not permitted by any constraint", nameType, name)}
   549  	}
   550  
   551  	return nil
   552  }
   553  
   554  // isValid performs validity checks on c given that it is a candidate to append
   555  // to the chain in currentChain.
   556  func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *VerifyOptions) error {
   557  	if len(c.UnhandledCriticalExtensions) > 0 {
   558  		return UnhandledCriticalExtension{}
   559  	}
   560  
   561  	if len(currentChain) > 0 {
   562  		child := currentChain[len(currentChain)-1]
   563  		if !bytes.Equal(child.RawIssuer, c.RawSubject) {
   564  			return CertificateInvalidError{c, NameMismatch, ""}
   565  		}
   566  	}
   567  
   568  	now := opts.CurrentTime
   569  	if now.IsZero() {
   570  		now = time.Now()
   571  	}
   572  	if now.Before(c.NotBefore) {
   573  		return CertificateInvalidError{
   574  			Cert:   c,
   575  			Reason: Expired,
   576  			Detail: fmt.Sprintf("current time %s is before %s", now.Format(time.RFC3339), c.NotBefore.Format(time.RFC3339)),
   577  		}
   578  	} else if now.After(c.NotAfter) {
   579  		return CertificateInvalidError{
   580  			Cert:   c,
   581  			Reason: Expired,
   582  			Detail: fmt.Sprintf("current time %s is after %s", now.Format(time.RFC3339), c.NotAfter.Format(time.RFC3339)),
   583  		}
   584  	}
   585  
   586  	maxConstraintComparisons := opts.MaxConstraintComparisions
   587  	if maxConstraintComparisons == 0 {
   588  		maxConstraintComparisons = 250000
   589  	}
   590  	comparisonCount := 0
   591  
   592  	var leaf *Certificate
   593  	if certType == intermediateCertificate || certType == rootCertificate {
   594  		if len(currentChain) == 0 {
   595  			return errors.New("x509: internal error: empty chain when appending CA cert")
   596  		}
   597  		leaf = currentChain[0]
   598  	}
   599  
   600  	if (certType == intermediateCertificate || certType == rootCertificate) &&
   601  		c.hasNameConstraints() && leaf.hasSANExtension() {
   602  		err := forEachSAN(leaf.getSANExtension(), func(tag int, data []byte) error {
   603  			switch tag {
   604  			case nameTypeEmail:
   605  				name := string(data)
   606  				mailbox, ok := parseRFC2821Mailbox(name)
   607  				if !ok {
   608  					return fmt.Errorf("x509: cannot parse rfc822Name %q", mailbox)
   609  				}
   610  
   611  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "email address", name, mailbox,
   612  					func(parsedName, constraint interface{}) (bool, error) {
   613  						return matchEmailConstraint(parsedName.(rfc2821Mailbox), constraint.(string))
   614  					}, c.PermittedEmailAddresses, c.ExcludedEmailAddresses); err != nil {
   615  					return err
   616  				}
   617  
   618  			case nameTypeDNS:
   619  				name := string(data)
   620  				if _, ok := domainToReverseLabels(name); !ok {
   621  					return fmt.Errorf("x509: cannot parse dnsName %q", name)
   622  				}
   623  
   624  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "DNS name", name, name,
   625  					func(parsedName, constraint interface{}) (bool, error) {
   626  						return matchDomainConstraint(parsedName.(string), constraint.(string))
   627  					}, c.PermittedDNSDomains, c.ExcludedDNSDomains); err != nil {
   628  					return err
   629  				}
   630  
   631  			case nameTypeURI:
   632  				name := string(data)
   633  				uri, err := url.Parse(name)
   634  				if err != nil {
   635  					return fmt.Errorf("x509: internal error: URI SAN %q failed to parse", name)
   636  				}
   637  
   638  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "URI", name, uri,
   639  					func(parsedName, constraint interface{}) (bool, error) {
   640  						return matchURIConstraint(parsedName.(*url.URL), constraint.(string))
   641  					}, c.PermittedURIDomains, c.ExcludedURIDomains); err != nil {
   642  					return err
   643  				}
   644  
   645  			case nameTypeIP:
   646  				ip := net.IP(data)
   647  				if l := len(ip); l != net.IPv4len && l != net.IPv6len {
   648  					return fmt.Errorf("x509: internal error: IP SAN %x failed to parse", data)
   649  				}
   650  
   651  				if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "IP address", ip.String(), ip,
   652  					func(parsedName, constraint interface{}) (bool, error) {
   653  						return matchIPConstraint(parsedName.(net.IP), constraint.(*net.IPNet))
   654  					}, c.PermittedIPRanges, c.ExcludedIPRanges); err != nil {
   655  					return err
   656  				}
   657  
   658  			default:
   659  				// Unknown SAN types are ignored.
   660  			}
   661  
   662  			return nil
   663  		})
   664  
   665  		if err != nil {
   666  			return err
   667  		}
   668  	}
   669  
   670  	// KeyUsage status flags are ignored. From Engineering Security, Peter
   671  	// Gutmann: A European government CA marked its signing certificates as
   672  	// being valid for encryption only, but no-one noticed. Another
   673  	// European CA marked its signature keys as not being valid for
   674  	// signatures. A different CA marked its own trusted root certificate
   675  	// as being invalid for certificate signing. Another national CA
   676  	// distributed a certificate to be used to encrypt data for the
   677  	// country’s tax authority that was marked as only being usable for
   678  	// digital signatures but not for encryption. Yet another CA reversed
   679  	// the order of the bit flags in the keyUsage due to confusion over
   680  	// encoding endianness, essentially setting a random keyUsage in
   681  	// certificates that it issued. Another CA created a self-invalidating
   682  	// certificate by adding a certificate policy statement stipulating
   683  	// that the certificate had to be used strictly as specified in the
   684  	// keyUsage, and a keyUsage containing a flag indicating that the RSA
   685  	// encryption key could only be used for Diffie-Hellman key agreement.
   686  
   687  	if certType == intermediateCertificate && (!c.BasicConstraintsValid || !c.IsCA) {
   688  		return CertificateInvalidError{c, NotAuthorizedToSign, ""}
   689  	}
   690  
   691  	if c.BasicConstraintsValid && c.MaxPathLen >= 0 {
   692  		numIntermediates := len(currentChain) - 1
   693  		if numIntermediates > c.MaxPathLen {
   694  			return CertificateInvalidError{c, TooManyIntermediates, ""}
   695  		}
   696  	}
   697  
   698  	return nil
   699  }
   700  
   701  // Verify attempts to verify c by building one or more chains from c to a
   702  // certificate in opts.Roots, using certificates in opts.Intermediates if
   703  // needed. If successful, it returns one or more chains where the first
   704  // element of the chain is c and the last element is from opts.Roots.
   705  //
   706  // If opts.Roots is nil, the platform verifier might be used, and
   707  // verification details might differ from what is described below. If system
   708  // roots are unavailable the returned error will be of type SystemRootsError.
   709  //
   710  // Name constraints in the intermediates will be applied to all names claimed
   711  // in the chain, not just opts.DNSName. Thus it is invalid for a leaf to claim
   712  // example.com if an intermediate doesn't permit it, even if example.com is not
   713  // the name being validated. Note that DirectoryName constraints are not
   714  // supported.
   715  //
   716  // Name constraint validation follows the rules from RFC 5280, with the
   717  // addition that DNS name constraints may use the leading period format
   718  // defined for emails and URIs. When a constraint has a leading period
   719  // it indicates that at least one additional label must be prepended to
   720  // the constrained name to be considered valid.
   721  //
   722  // Extended Key Usage values are enforced nested down a chain, so an intermediate
   723  // or root that enumerates EKUs prevents a leaf from asserting an EKU not in that
   724  // list. (While this is not specified, it is common practice in order to limit
   725  // the types of certificates a CA can issue.)
   726  //
   727  // WARNING: this function doesn't do any revocation checking.
   728  func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err error) {
   729  	// Platform-specific verification needs the ASN.1 contents so
   730  	// this makes the behavior consistent across platforms.
   731  	if len(c.Raw) == 0 {
   732  		return nil, errNotParsed
   733  	}
   734  	for i := 0; i < opts.Intermediates.len(); i++ {
   735  		c, err := opts.Intermediates.cert(i)
   736  		if err != nil {
   737  			return nil, fmt.Errorf("crypto/x509: error fetching intermediate: %w", err)
   738  		}
   739  		if len(c.Raw) == 0 {
   740  			return nil, errNotParsed
   741  		}
   742  	}
   743  
   744  	// Use Windows's own verification and chain building.
   745  	if opts.Roots == nil && runtime.GOOS == "windows" {
   746  		return c.systemVerify(&opts)
   747  	}
   748  
   749  	if opts.Roots == nil {
   750  		opts.Roots = systemRootsPool()
   751  		if opts.Roots == nil {
   752  			return nil, SystemRootsError{systemRootsErr}
   753  		}
   754  	}
   755  
   756  	err = c.isValid(leafCertificate, nil, &opts)
   757  	if err != nil {
   758  		return
   759  	}
   760  
   761  	if len(opts.DNSName) > 0 {
   762  		err = c.VerifyHostname(opts.DNSName)
   763  		if err != nil {
   764  			return
   765  		}
   766  	}
   767  
   768  	var candidateChains [][]*Certificate
   769  	if opts.Roots.contains(c) {
   770  		candidateChains = append(candidateChains, []*Certificate{c})
   771  	} else {
   772  		if candidateChains, err = c.buildChains(nil, []*Certificate{c}, nil, &opts); err != nil {
   773  			return nil, err
   774  		}
   775  	}
   776  
   777  	keyUsages := opts.KeyUsages
   778  	if len(keyUsages) == 0 {
   779  		keyUsages = []ExtKeyUsage{ExtKeyUsageServerAuth}
   780  	}
   781  
   782  	// If any key usage is acceptable then we're done.
   783  	for _, usage := range keyUsages {
   784  		if usage == ExtKeyUsageAny {
   785  			return candidateChains, nil
   786  		}
   787  	}
   788  
   789  	for _, candidate := range candidateChains {
   790  		if checkChainForKeyUsage(candidate, keyUsages) {
   791  			chains = append(chains, candidate)
   792  		}
   793  	}
   794  
   795  	if len(chains) == 0 {
   796  		return nil, CertificateInvalidError{c, IncompatibleUsage, ""}
   797  	}
   798  
   799  	return chains, nil
   800  }
   801  
   802  func appendToFreshChain(chain []*Certificate, cert *Certificate) []*Certificate {
   803  	n := make([]*Certificate, len(chain)+1)
   804  	copy(n, chain)
   805  	n[len(chain)] = cert
   806  	return n
   807  }
   808  
   809  // maxChainSignatureChecks is the maximum number of CheckSignatureFrom calls
   810  // that an invocation of buildChains will (transitively) make. Most chains are
   811  // less than 15 certificates long, so this leaves space for multiple chains and
   812  // for failed checks due to different intermediates having the same Subject.
   813  const maxChainSignatureChecks = 100
   814  
   815  func (c *Certificate) buildChains(cache map[*Certificate][][]*Certificate, currentChain []*Certificate, sigChecks *int, opts *VerifyOptions) (chains [][]*Certificate, err error) {
   816  	var (
   817  		hintErr  error
   818  		hintCert *Certificate
   819  	)
   820  
   821  	considerCandidate := func(certType int, candidate *Certificate) {
   822  		for _, cert := range currentChain {
   823  			if cert.Equal(candidate) {
   824  				return
   825  			}
   826  		}
   827  
   828  		if sigChecks == nil {
   829  			sigChecks = new(int)
   830  		}
   831  		*sigChecks++
   832  		if *sigChecks > maxChainSignatureChecks {
   833  			err = errors.New("x509: signature check attempts limit reached while verifying certificate chain")
   834  			return
   835  		}
   836  
   837  		if err := c.CheckSignatureFrom(candidate); err != nil {
   838  			if hintErr == nil {
   839  				hintErr = err
   840  				hintCert = candidate
   841  			}
   842  			return
   843  		}
   844  
   845  		err = candidate.isValid(certType, currentChain, opts)
   846  		if err != nil {
   847  			return
   848  		}
   849  
   850  		switch certType {
   851  		case rootCertificate:
   852  			chains = append(chains, appendToFreshChain(currentChain, candidate))
   853  		case intermediateCertificate:
   854  			if cache == nil {
   855  				cache = make(map[*Certificate][][]*Certificate)
   856  			}
   857  			childChains, ok := cache[candidate]
   858  			if !ok {
   859  				childChains, err = candidate.buildChains(cache, appendToFreshChain(currentChain, candidate), sigChecks, opts)
   860  				cache[candidate] = childChains
   861  			}
   862  			chains = append(chains, childChains...)
   863  		}
   864  	}
   865  
   866  	for _, root := range opts.Roots.findPotentialParents(c) {
   867  		considerCandidate(rootCertificate, root)
   868  	}
   869  	for _, intermediate := range opts.Intermediates.findPotentialParents(c) {
   870  		considerCandidate(intermediateCertificate, intermediate)
   871  	}
   872  
   873  	if len(chains) > 0 {
   874  		err = nil
   875  	}
   876  	if len(chains) == 0 && err == nil {
   877  		err = UnknownAuthorityError{c, hintErr, hintCert}
   878  	}
   879  
   880  	return
   881  }
   882  
   883  func validHostnamePattern(host string) bool { return validHostname(host, true) }
   884  func validHostnameInput(host string) bool   { return validHostname(host, false) }
   885  
   886  // validHostname reports whether host is a valid hostname that can be matched or
   887  // matched against according to RFC 6125 2.2, with some leniency to accommodate
   888  // legacy values.
   889  func validHostname(host string, isPattern bool) bool {
   890  	if !isPattern {
   891  		host = strings.TrimSuffix(host, ".")
   892  	}
   893  	if len(host) == 0 {
   894  		return false
   895  	}
   896  
   897  	for i, part := range strings.Split(host, ".") {
   898  		if part == "" {
   899  			// Empty label.
   900  			return false
   901  		}
   902  		if isPattern && i == 0 && part == "*" {
   903  			// Only allow full left-most wildcards, as those are the only ones
   904  			// we match, and matching literal '*' characters is probably never
   905  			// the expected behavior.
   906  			continue
   907  		}
   908  		for j, c := range part {
   909  			if 'a' <= c && c <= 'z' {
   910  				continue
   911  			}
   912  			if '0' <= c && c <= '9' {
   913  				continue
   914  			}
   915  			if 'A' <= c && c <= 'Z' {
   916  				continue
   917  			}
   918  			if c == '-' && j != 0 {
   919  				continue
   920  			}
   921  			if c == '_' {
   922  				// Not a valid character in hostnames, but commonly
   923  				// found in deployments outside the WebPKI.
   924  				continue
   925  			}
   926  			return false
   927  		}
   928  	}
   929  
   930  	return true
   931  }
   932  
   933  func matchExactly(hostA, hostB string) bool {
   934  	if hostA == "" || hostA == "." || hostB == "" || hostB == "." {
   935  		return false
   936  	}
   937  	return toLowerCaseASCII(hostA) == toLowerCaseASCII(hostB)
   938  }
   939  
   940  func matchHostnames(pattern, host string) bool {
   941  	pattern = toLowerCaseASCII(pattern)
   942  	host = toLowerCaseASCII(strings.TrimSuffix(host, "."))
   943  
   944  	if len(pattern) == 0 || len(host) == 0 {
   945  		return false
   946  	}
   947  
   948  	patternParts := strings.Split(pattern, ".")
   949  	hostParts := strings.Split(host, ".")
   950  
   951  	if len(patternParts) != len(hostParts) {
   952  		return false
   953  	}
   954  
   955  	for i, patternPart := range patternParts {
   956  		if i == 0 && patternPart == "*" {
   957  			continue
   958  		}
   959  		if patternPart != hostParts[i] {
   960  			return false
   961  		}
   962  	}
   963  
   964  	return true
   965  }
   966  
   967  // toLowerCaseASCII returns a lower-case version of in. See RFC 6125 6.4.1. We use
   968  // an explicitly ASCII function to avoid any sharp corners resulting from
   969  // performing Unicode operations on DNS labels.
   970  func toLowerCaseASCII(in string) string {
   971  	// If the string is already lower-case then there's nothing to do.
   972  	isAlreadyLowerCase := true
   973  	for _, c := range in {
   974  		if c == utf8.RuneError {
   975  			// If we get a UTF-8 error then there might be
   976  			// upper-case ASCII bytes in the invalid sequence.
   977  			isAlreadyLowerCase = false
   978  			break
   979  		}
   980  		if 'A' <= c && c <= 'Z' {
   981  			isAlreadyLowerCase = false
   982  			break
   983  		}
   984  	}
   985  
   986  	if isAlreadyLowerCase {
   987  		return in
   988  	}
   989  
   990  	out := []byte(in)
   991  	for i, c := range out {
   992  		if 'A' <= c && c <= 'Z' {
   993  			out[i] += 'a' - 'A'
   994  		}
   995  	}
   996  	return string(out)
   997  }
   998  
   999  // VerifyHostname returns nil if c is a valid certificate for the named host.
  1000  // Otherwise it returns an error describing the mismatch.
  1001  //
  1002  // IP addresses can be optionally enclosed in square brackets and are checked
  1003  // against the IPAddresses field. Other names are checked case insensitively
  1004  // against the DNSNames field. If the names are valid hostnames, the certificate
  1005  // fields can have a wildcard as the left-most label.
  1006  //
  1007  // Note that the legacy Common Name field is ignored.
  1008  func (c *Certificate) VerifyHostname(h string) error {
  1009  	// IP addresses may be written in [ ].
  1010  	candidateIP := h
  1011  	if len(h) >= 3 && h[0] == '[' && h[len(h)-1] == ']' {
  1012  		candidateIP = h[1 : len(h)-1]
  1013  	}
  1014  	if ip := net.ParseIP(candidateIP); ip != nil {
  1015  		// We only match IP addresses against IP SANs.
  1016  		// See RFC 6125, Appendix B.2.
  1017  		for _, candidate := range c.IPAddresses {
  1018  			if ip.Equal(candidate) {
  1019  				return nil
  1020  			}
  1021  		}
  1022  		return HostnameError{c, candidateIP}
  1023  	}
  1024  
  1025  	candidateName := toLowerCaseASCII(h) // Save allocations inside the loop.
  1026  	validCandidateName := validHostnameInput(candidateName)
  1027  
  1028  	for _, match := range c.DNSNames {
  1029  		// Ideally, we'd only match valid hostnames according to RFC 6125 like
  1030  		// browsers (more or less) do, but in practice Go is used in a wider
  1031  		// array of contexts and can't even assume DNS resolution. Instead,
  1032  		// always allow perfect matches, and only apply wildcard and trailing
  1033  		// dot processing to valid hostnames.
  1034  		if validCandidateName && validHostnamePattern(match) {
  1035  			if matchHostnames(match, candidateName) {
  1036  				return nil
  1037  			}
  1038  		} else {
  1039  			if matchExactly(match, candidateName) {
  1040  				return nil
  1041  			}
  1042  		}
  1043  	}
  1044  
  1045  	return HostnameError{c, h}
  1046  }
  1047  
  1048  func checkChainForKeyUsage(chain []*Certificate, keyUsages []ExtKeyUsage) bool {
  1049  	usages := make([]ExtKeyUsage, len(keyUsages))
  1050  	copy(usages, keyUsages)
  1051  
  1052  	if len(chain) == 0 {
  1053  		return false
  1054  	}
  1055  
  1056  	usagesRemaining := len(usages)
  1057  
  1058  	// We walk down the list and cross out any usages that aren't supported
  1059  	// by each certificate. If we cross out all the usages, then the chain
  1060  	// is unacceptable.
  1061  
  1062  NextCert:
  1063  	for i := len(chain) - 1; i >= 0; i-- {
  1064  		cert := chain[i]
  1065  		if len(cert.ExtKeyUsage) == 0 && len(cert.UnknownExtKeyUsage) == 0 {
  1066  			// The certificate doesn't have any extended key usage specified.
  1067  			continue
  1068  		}
  1069  
  1070  		for _, usage := range cert.ExtKeyUsage {
  1071  			if usage == ExtKeyUsageAny {
  1072  				// The certificate is explicitly good for any usage.
  1073  				continue NextCert
  1074  			}
  1075  		}
  1076  
  1077  		const invalidUsage ExtKeyUsage = -1
  1078  
  1079  	NextRequestedUsage:
  1080  		for i, requestedUsage := range usages {
  1081  			if requestedUsage == invalidUsage {
  1082  				continue
  1083  			}
  1084  
  1085  			for _, usage := range cert.ExtKeyUsage {
  1086  				if requestedUsage == usage {
  1087  					continue NextRequestedUsage
  1088  				} else if requestedUsage == ExtKeyUsageServerAuth &&
  1089  					(usage == ExtKeyUsageNetscapeServerGatedCrypto ||
  1090  						usage == ExtKeyUsageMicrosoftServerGatedCrypto) {
  1091  					// In order to support COMODO
  1092  					// certificate chains, we have to
  1093  					// accept Netscape or Microsoft SGC
  1094  					// usages as equal to ServerAuth.
  1095  					continue NextRequestedUsage
  1096  				}
  1097  			}
  1098  
  1099  			usages[i] = invalidUsage
  1100  			usagesRemaining--
  1101  			if usagesRemaining == 0 {
  1102  				return false
  1103  			}
  1104  		}
  1105  	}
  1106  
  1107  	return true
  1108  }
  1109  

View as plain text