// Copyright 2011 The Go Authors. All rights reserved. // Use of this source code is governed by a BSD-style // license that can be found in the LICENSE file. package x509 import ( "bytes" "errors" "fmt" "net" "net/url" "os" "reflect" "runtime" "strings" "time" "unicode/utf8" ) // ignoreCN disables interpreting Common Name as a hostname. See issue 24151. var ignoreCN = strings.Contains(os.Getenv("GODEBUG"), "x509ignoreCN=1") type InvalidReason int const ( // NotAuthorizedToSign results when a certificate is signed by another // which isn't marked as a CA certificate. NotAuthorizedToSign InvalidReason = iota // Expired results when a certificate has expired, based on the time // given in the VerifyOptions. Expired // CANotAuthorizedForThisName results when an intermediate or root // certificate has a name constraint which doesn't permit a DNS or // other name (including IP address) in the leaf certificate. CANotAuthorizedForThisName // TooManyIntermediates results when a path length constraint is // violated. TooManyIntermediates // IncompatibleUsage results when the certificate's key usage indicates // that it may only be used for a different purpose. IncompatibleUsage // NameMismatch results when the subject name of a parent certificate // does not match the issuer name in the child. NameMismatch // NameConstraintsWithoutSANs results when a leaf certificate doesn't // contain a Subject Alternative Name extension, but a CA certificate // contains name constraints, and the Common Name can be interpreted as // a hostname. // // You can avoid this error by setting the experimental GODEBUG environment // variable to "x509ignoreCN=1", disabling Common Name matching entirely. // This behavior might become the default in the future. NameConstraintsWithoutSANs // UnconstrainedName results when a CA certificate contains permitted // name constraints, but leaf certificate contains a name of an // unsupported or unconstrained type. UnconstrainedName // TooManyConstraints results when the number of comparison operations // needed to check a certificate exceeds the limit set by // VerifyOptions.MaxConstraintComparisions. This limit exists to // prevent pathological certificates can consuming excessive amounts of // CPU time to verify. TooManyConstraints // CANotAuthorizedForExtKeyUsage results when an intermediate or root // certificate does not permit a requested extended key usage. CANotAuthorizedForExtKeyUsage ) // CertificateInvalidError results when an odd error occurs. Users of this // library probably want to handle all these errors uniformly. type CertificateInvalidError struct { Cert *Certificate Reason InvalidReason Detail string } func (e CertificateInvalidError) Error() string { switch e.Reason { case NotAuthorizedToSign: return "x509: certificate is not authorized to sign other certificates" case Expired: return "x509: certificate has expired or is not yet valid: " + e.Detail case CANotAuthorizedForThisName: return "x509: a root or intermediate certificate is not authorized to sign for this name: " + e.Detail case CANotAuthorizedForExtKeyUsage: return "x509: a root or intermediate certificate is not authorized for an extended key usage: " + e.Detail case TooManyIntermediates: return "x509: too many intermediates for path length constraint" case IncompatibleUsage: return "x509: certificate specifies an incompatible key usage" case NameMismatch: return "x509: issuer name does not match subject from issuing certificate" case NameConstraintsWithoutSANs: return "x509: issuer has name constraints but leaf doesn't have a SAN extension" case UnconstrainedName: return "x509: issuer has name constraints but leaf contains unknown or unconstrained name: " + e.Detail } return "x509: unknown error" } // HostnameError results when the set of authorized names doesn't match the // requested name. type HostnameError struct { Certificate *Certificate Host string } func (h HostnameError) Error() string { c := h.Certificate if !c.hasSANExtension() && !validHostname(c.Subject.CommonName) && matchHostnames(toLowerCaseASCII(c.Subject.CommonName), toLowerCaseASCII(h.Host)) { // This would have validated, if it weren't for the validHostname check on Common Name. return "x509: Common Name is not a valid hostname: " + c.Subject.CommonName } var valid string if ip := net.ParseIP(h.Host); ip != nil { // Trying to validate an IP if len(c.IPAddresses) == 0 { return "x509: cannot validate certificate for " + h.Host + " because it doesn't contain any IP SANs" } for _, san := range c.IPAddresses { if len(valid) > 0 { valid += ", " } valid += san.String() } } else { if c.commonNameAsHostname() { valid = c.Subject.CommonName } else { valid = strings.Join(c.DNSNames, ", ") } } if len(valid) == 0 { return "x509: certificate is not valid for any names, but wanted to match " + h.Host } return "x509: certificate is valid for " + valid + ", not " + h.Host } // UnknownAuthorityError results when the certificate issuer is unknown type UnknownAuthorityError struct { Cert *Certificate // hintErr contains an error that may be helpful in determining why an // authority wasn't found. hintErr error // hintCert contains a possible authority certificate that was rejected // because of the error in hintErr. hintCert *Certificate } func (e UnknownAuthorityError) Error() string { s := "x509: certificate signed by unknown authority" if e.hintErr != nil { certName := e.hintCert.Subject.CommonName if len(certName) == 0 { if len(e.hintCert.Subject.Organization) > 0 { certName = e.hintCert.Subject.Organization[0] } else { certName = "serial:" + e.hintCert.SerialNumber.String() } } s += fmt.Sprintf(" (possibly because of %q while trying to verify candidate authority certificate %q)", e.hintErr, certName) } return s } // SystemRootsError results when we fail to load the system root certificates. type SystemRootsError struct { Err error } func (se SystemRootsError) Error() string { msg := "x509: failed to load system roots and no roots provided" if se.Err != nil { return msg + "; " + se.Err.Error() } return msg } // errNotParsed is returned when a certificate without ASN.1 contents is // verified. Platform-specific verification needs the ASN.1 contents. var errNotParsed = errors.New("x509: missing ASN.1 contents; use ParseCertificate") // VerifyOptions contains parameters for Certificate.Verify. It's a structure // because other PKIX verification APIs have ended up needing many options. type VerifyOptions struct { DNSName string Intermediates *CertPool Roots *CertPool // if nil, the system roots are used CurrentTime time.Time // if zero, the current time is used // KeyUsage specifies which Extended Key Usage values are acceptable. A leaf // certificate is accepted if it contains any of the listed values. An empty // list means ExtKeyUsageServerAuth. To accept any key usage, include // ExtKeyUsageAny. // // Certificate chains are required to nest these extended key usage values. // (This matches the Windows CryptoAPI behavior, but not the spec.) KeyUsages []ExtKeyUsage // MaxConstraintComparisions is the maximum number of comparisons to // perform when checking a given certificate's name constraints. If // zero, a sensible default is used. This limit prevents pathological // certificates from consuming excessive amounts of CPU time when // validating. MaxConstraintComparisions int } const ( leafCertificate = iota intermediateCertificate rootCertificate ) // rfc2821Mailbox represents a “mailbox” (which is an email address to most // people) by breaking it into the “local” (i.e. before the '@') and “domain” // parts. type rfc2821Mailbox struct { local, domain string } // parseRFC2821Mailbox parses an email address into local and domain parts, // based on the ABNF for a “Mailbox” from RFC 2821. According to RFC 5280, // Section 4.2.1.6 that's correct for an rfc822Name from a certificate: “The // format of an rfc822Name is a "Mailbox" as defined in RFC 2821, Section 4.1.2”. func parseRFC2821Mailbox(in string) (mailbox rfc2821Mailbox, ok bool) { if len(in) == 0 { return mailbox, false } localPartBytes := make([]byte, 0, len(in)/2) if in[0] == '"' { // Quoted-string = DQUOTE *qcontent DQUOTE // non-whitespace-control = %d1-8 / %d11 / %d12 / %d14-31 / %d127 // qcontent = qtext / quoted-pair // qtext = non-whitespace-control / // %d33 / %d35-91 / %d93-126 // quoted-pair = ("\" text) / obs-qp // text = %d1-9 / %d11 / %d12 / %d14-127 / obs-text // // (Names beginning with “obs-” are the obsolete syntax from RFC 2822, // Section 4. Since it has been 16 years, we no longer accept that.) in = in[1:] QuotedString: for { if len(in) == 0 { return mailbox, false } c := in[0] in = in[1:] switch { case c == '"': break QuotedString case c == '\\': // quoted-pair if len(in) == 0 { return mailbox, false } if in[0] == 11 || in[0] == 12 || (1 <= in[0] && in[0] <= 9) || (14 <= in[0] && in[0] <= 127) { localPartBytes = append(localPartBytes, in[0]) in = in[1:] } else { return mailbox, false } case c == 11 || c == 12 || // Space (char 32) is not allowed based on the // BNF, but RFC 3696 gives an example that // assumes that it is. Several “verified” // errata continue to argue about this point. // We choose to accept it. c == 32 || c == 33 || c == 127 || (1 <= c && c <= 8) || (14 <= c && c <= 31) || (35 <= c && c <= 91) || (93 <= c && c <= 126): // qtext localPartBytes = append(localPartBytes, c) default: return mailbox, false } } } else { // Atom ("." Atom)* NextChar: for len(in) > 0 { // atext from RFC 2822, Section 3.2.4 c := in[0] switch { case c == '\\': // Examples given in RFC 3696 suggest that // escaped characters can appear outside of a // quoted string. Several “verified” errata // continue to argue the point. We choose to // accept it. in = in[1:] if len(in) == 0 { return mailbox, false } fallthrough case ('0' <= c && c <= '9') || ('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z') || c == '!' || c == '#' || c == '$' || c == '%' || c == '&' || c == '\'' || c == '*' || c == '+' || c == '-' || c == '/' || c == '=' || c == '?' || c == '^' || c == '_' || c == '`' || c == '{' || c == '|' || c == '}' || c == '~' || c == '.': localPartBytes = append(localPartBytes, in[0]) in = in[1:] default: break NextChar } } if len(localPartBytes) == 0 { return mailbox, false } // From RFC 3696, Section 3: // “period (".") may also appear, but may not be used to start // or end the local part, nor may two or more consecutive // periods appear.” twoDots := []byte{'.', '.'} if localPartBytes[0] == '.' || localPartBytes[len(localPartBytes)-1] == '.' || bytes.Contains(localPartBytes, twoDots) { return mailbox, false } } if len(in) == 0 || in[0] != '@' { return mailbox, false } in = in[1:] // The RFC species a format for domains, but that's known to be // violated in practice so we accept that anything after an '@' is the // domain part. if _, ok := domainToReverseLabels(in); !ok { return mailbox, false } mailbox.local = string(localPartBytes) mailbox.domain = in return mailbox, true } // domainToReverseLabels converts a textual domain name like foo.example.com to // the list of labels in reverse order, e.g. ["com", "example", "foo"]. func domainToReverseLabels(domain string) (reverseLabels []string, ok bool) { for len(domain) > 0 { if i := strings.LastIndexByte(domain, '.'); i == -1 { reverseLabels = append(reverseLabels, domain) domain = "" } else { reverseLabels = append(reverseLabels, domain[i+1:]) domain = domain[:i] } } if len(reverseLabels) > 0 && len(reverseLabels[0]) == 0 { // An empty label at the end indicates an absolute value. return nil, false } for _, label := range reverseLabels { if len(label) == 0 { // Empty labels are otherwise invalid. return nil, false } for _, c := range label { if c < 33 || c > 126 { // Invalid character. return nil, false } } } return reverseLabels, true } func matchEmailConstraint(mailbox rfc2821Mailbox, constraint string) (bool, error) { // If the constraint contains an @, then it specifies an exact mailbox // name. if strings.Contains(constraint, "@") { constraintMailbox, ok := parseRFC2821Mailbox(constraint) if !ok { return false, fmt.Errorf("x509: internal error: cannot parse constraint %q", constraint) } return mailbox.local == constraintMailbox.local && strings.EqualFold(mailbox.domain, constraintMailbox.domain), nil } // Otherwise the constraint is like a DNS constraint of the domain part // of the mailbox. return matchDomainConstraint(mailbox.domain, constraint) } func matchURIConstraint(uri *url.URL, constraint string) (bool, error) { // From RFC 5280, Section 4.2.1.10: // “a uniformResourceIdentifier that does not include an authority // component with a host name specified as a fully qualified domain // name (e.g., if the URI either does not include an authority // component or includes an authority component in which the host name // is specified as an IP address), then the application MUST reject the // certificate.” host := uri.Host if len(host) == 0 { return false, fmt.Errorf("URI with empty host (%q) cannot be matched against constraints", uri.String()) } if strings.Contains(host, ":") && !strings.HasSuffix(host, "]") { var err error host, _, err = net.SplitHostPort(uri.Host) if err != nil { return false, err } } if strings.HasPrefix(host, "[") && strings.HasSuffix(host, "]") || net.ParseIP(host) != nil { return false, fmt.Errorf("URI with IP (%q) cannot be matched against constraints", uri.String()) } return matchDomainConstraint(host, constraint) } func matchIPConstraint(ip net.IP, constraint *net.IPNet) (bool, error) { if len(ip) != len(constraint.IP) { return false, nil } for i := range ip { if mask := constraint.Mask[i]; ip[i]&mask != constraint.IP[i]&mask { return false, nil } } return true, nil } func matchDomainConstraint(domain, constraint string) (bool, error) { // The meaning of zero length constraints is not specified, but this // code follows NSS and accepts them as matching everything. if len(constraint) == 0 { return true, nil } domainLabels, ok := domainToReverseLabels(domain) if !ok { return false, fmt.Errorf("x509: internal error: cannot parse domain %q", domain) } // RFC 5280 says that a leading period in a domain name means that at // least one label must be prepended, but only for URI and email // constraints, not DNS constraints. The code also supports that // behaviour for DNS constraints. mustHaveSubdomains := false if constraint[0] == '.' { mustHaveSubdomains = true constraint = constraint[1:] } constraintLabels, ok := domainToReverseLabels(constraint) if !ok { return false, fmt.Errorf("x509: internal error: cannot parse domain %q", constraint) } if len(domainLabels) < len(constraintLabels) || (mustHaveSubdomains && len(domainLabels) == len(constraintLabels)) { return false, nil } for i, constraintLabel := range constraintLabels { if !strings.EqualFold(constraintLabel, domainLabels[i]) { return false, nil } } return true, nil } // checkNameConstraints checks that c permits a child certificate to claim the // given name, of type nameType. The argument parsedName contains the parsed // form of name, suitable for passing to the match function. The total number // of comparisons is tracked in the given count and should not exceed the given // limit. func (c *Certificate) checkNameConstraints(count *int, maxConstraintComparisons int, nameType string, name string, parsedName interface{}, match func(parsedName, constraint interface{}) (match bool, err error), permitted, excluded interface{}) error { excludedValue := reflect.ValueOf(excluded) *count += excludedValue.Len() if *count > maxConstraintComparisons { return CertificateInvalidError{c, TooManyConstraints, ""} } for i := 0; i < excludedValue.Len(); i++ { constraint := excludedValue.Index(i).Interface() match, err := match(parsedName, constraint) if err != nil { return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()} } if match { return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is excluded by constraint %q", nameType, name, constraint)} } } permittedValue := reflect.ValueOf(permitted) *count += permittedValue.Len() if *count > maxConstraintComparisons { return CertificateInvalidError{c, TooManyConstraints, ""} } ok := true for i := 0; i < permittedValue.Len(); i++ { constraint := permittedValue.Index(i).Interface() var err error if ok, err = match(parsedName, constraint); err != nil { return CertificateInvalidError{c, CANotAuthorizedForThisName, err.Error()} } if ok { break } } if !ok { return CertificateInvalidError{c, CANotAuthorizedForThisName, fmt.Sprintf("%s %q is not permitted by any constraint", nameType, name)} } return nil } // isValid performs validity checks on c given that it is a candidate to append // to the chain in currentChain. func (c *Certificate) isValid(certType int, currentChain []*Certificate, opts *VerifyOptions) error { if len(c.UnhandledCriticalExtensions) > 0 { return UnhandledCriticalExtension{} } if len(currentChain) > 0 { child := currentChain[len(currentChain)-1] if !bytes.Equal(child.RawIssuer, c.RawSubject) { return CertificateInvalidError{c, NameMismatch, ""} } } now := opts.CurrentTime if now.IsZero() { now = time.Now() } if now.Before(c.NotBefore) { return CertificateInvalidError{ Cert: c, Reason: Expired, Detail: fmt.Sprintf("current time %s is before %s", now.Format(time.RFC3339), c.NotBefore.Format(time.RFC3339)), } } else if now.After(c.NotAfter) { return CertificateInvalidError{ Cert: c, Reason: Expired, Detail: fmt.Sprintf("current time %s is after %s", now.Format(time.RFC3339), c.NotAfter.Format(time.RFC3339)), } } maxConstraintComparisons := opts.MaxConstraintComparisions if maxConstraintComparisons == 0 { maxConstraintComparisons = 250000 } comparisonCount := 0 var leaf *Certificate if certType == intermediateCertificate || certType == rootCertificate { if len(currentChain) == 0 { return errors.New("x509: internal error: empty chain when appending CA cert") } leaf = currentChain[0] } checkNameConstraints := (certType == intermediateCertificate || certType == rootCertificate) && c.hasNameConstraints() if checkNameConstraints && leaf.commonNameAsHostname() { // This is the deprecated, legacy case of depending on the commonName as // a hostname. We don't enforce name constraints against the CN, but // VerifyHostname will look for hostnames in there if there are no SANs. // In order to ensure VerifyHostname will not accept an unchecked name, // return an error here. return CertificateInvalidError{c, NameConstraintsWithoutSANs, ""} } else if checkNameConstraints && leaf.hasSANExtension() { err := forEachSAN(leaf.getSANExtension(), func(tag int, data []byte) error { switch tag { case nameTypeEmail: name := string(data) mailbox, ok := parseRFC2821Mailbox(name) if !ok { return fmt.Errorf("x509: cannot parse rfc822Name %q", mailbox) } if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "email address", name, mailbox, func(parsedName, constraint interface{}) (bool, error) { return matchEmailConstraint(parsedName.(rfc2821Mailbox), constraint.(string)) }, c.PermittedEmailAddresses, c.ExcludedEmailAddresses); err != nil { return err } 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)) }, c.PermittedDNSDomains, c.ExcludedDNSDomains); err != nil { return err } case nameTypeURI: name := string(data) uri, err := url.Parse(name) if err != nil { return fmt.Errorf("x509: internal error: URI SAN %q failed to parse", name) } if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "URI", name, uri, func(parsedName, constraint interface{}) (bool, error) { return matchURIConstraint(parsedName.(*url.URL), constraint.(string)) }, c.PermittedURIDomains, c.ExcludedURIDomains); err != nil { return err } case nameTypeIP: ip := net.IP(data) if l := len(ip); l != net.IPv4len && l != net.IPv6len { return fmt.Errorf("x509: internal error: IP SAN %x failed to parse", data) } if err := c.checkNameConstraints(&comparisonCount, maxConstraintComparisons, "IP address", ip.String(), ip, func(parsedName, constraint interface{}) (bool, error) { return matchIPConstraint(parsedName.(net.IP), constraint.(*net.IPNet)) }, c.PermittedIPRanges, c.ExcludedIPRanges); err != nil { return err } default: // Unknown SAN types are ignored. } return nil }) if err != nil { return err } } // KeyUsage status flags are ignored. From Engineering Security, Peter // Gutmann: A European government CA marked its signing certificates as // being valid for encryption only, but no-one noticed. Another // European CA marked its signature keys as not being valid for // signatures. A different CA marked its own trusted root certificate // as being invalid for certificate signing. Another national CA // distributed a certificate to be used to encrypt data for the // country’s tax authority that was marked as only being usable for // digital signatures but not for encryption. Yet another CA reversed // the order of the bit flags in the keyUsage due to confusion over // encoding endianness, essentially setting a random keyUsage in // certificates that it issued. Another CA created a self-invalidating // certificate by adding a certificate policy statement stipulating // that the certificate had to be used strictly as specified in the // keyUsage, and a keyUsage containing a flag indicating that the RSA // encryption key could only be used for Diffie-Hellman key agreement. if certType == intermediateCertificate && (!c.BasicConstraintsValid || !c.IsCA) { return CertificateInvalidError{c, NotAuthorizedToSign, ""} } if c.BasicConstraintsValid && c.MaxPathLen >= 0 { numIntermediates := len(currentChain) - 1 if numIntermediates > c.MaxPathLen { return CertificateInvalidError{c, TooManyIntermediates, ""} } } return nil } // Verify attempts to verify c by building one or more chains from c to a // certificate in opts.Roots, using certificates in opts.Intermediates if // needed. If successful, it returns one or more chains where the first // element of the chain is c and the last element is from opts.Roots. // // If opts.Roots is nil and system roots are unavailable the returned error // will be of type SystemRootsError. // // Name constraints in the intermediates will be applied to all names claimed // in the chain, not just opts.DNSName. Thus it is invalid for a leaf to claim // example.com if an intermediate doesn't permit it, even if example.com is not // the name being validated. Note that DirectoryName constraints are not // supported. // // Extended Key Usage values are enforced down a chain, so an intermediate or // root that enumerates EKUs prevents a leaf from asserting an EKU not in that // list. // // WARNING: this function doesn't do any revocation checking. func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err error) { // Platform-specific verification needs the ASN.1 contents so // this makes the behavior consistent across platforms. if len(c.Raw) == 0 { return nil, errNotParsed } if opts.Intermediates != nil { for _, intermediate := range opts.Intermediates.certs { if len(intermediate.Raw) == 0 { return nil, errNotParsed } } } // Use Windows's own verification and chain building. if opts.Roots == nil && runtime.GOOS == "windows" { return c.systemVerify(&opts) } if opts.Roots == nil { opts.Roots = systemRootsPool() if opts.Roots == nil { return nil, SystemRootsError{systemRootsErr} } } err = c.isValid(leafCertificate, nil, &opts) if err != nil { return } if len(opts.DNSName) > 0 { err = c.VerifyHostname(opts.DNSName) if err != nil { return } } var candidateChains [][]*Certificate if opts.Roots.contains(c) { candidateChains = append(candidateChains, []*Certificate{c}) } else { if candidateChains, err = c.buildChains(nil, []*Certificate{c}, nil, &opts); err != nil { return nil, err } } keyUsages := opts.KeyUsages if len(keyUsages) == 0 { keyUsages = []ExtKeyUsage{ExtKeyUsageServerAuth} } // If any key usage is acceptable then we're done. for _, usage := range keyUsages { if usage == ExtKeyUsageAny { return candidateChains, nil } } for _, candidate := range candidateChains { if checkChainForKeyUsage(candidate, keyUsages) { chains = append(chains, candidate) } } if len(chains) == 0 { return nil, CertificateInvalidError{c, IncompatibleUsage, ""} } return chains, nil } func appendToFreshChain(chain []*Certificate, cert *Certificate) []*Certificate { n := make([]*Certificate, len(chain)+1) copy(n, chain) n[len(chain)] = cert return n } // maxChainSignatureChecks is the maximum number of CheckSignatureFrom calls // that an invocation of buildChains will (tranistively) make. Most chains are // less than 15 certificates long, so this leaves space for multiple chains and // for failed checks due to different intermediates having the same Subject. const maxChainSignatureChecks = 100 func (c *Certificate) buildChains(cache map[*Certificate][][]*Certificate, currentChain []*Certificate, sigChecks *int, opts *VerifyOptions) (chains [][]*Certificate, err error) { var ( hintErr error hintCert *Certificate ) considerCandidate := func(certType int, candidate *Certificate) { for _, cert := range currentChain { if cert.Equal(candidate) { return } } if sigChecks == nil { sigChecks = new(int) } *sigChecks++ if *sigChecks > maxChainSignatureChecks { err = errors.New("x509: signature check attempts limit reached while verifying certificate chain") return } if err := c.CheckSignatureFrom(candidate); err != nil { if hintErr == nil { hintErr = err hintCert = candidate } return } err = candidate.isValid(certType, currentChain, opts) if err != nil { return } switch certType { case rootCertificate: chains = append(chains, appendToFreshChain(currentChain, candidate)) case intermediateCertificate: if cache == nil { cache = make(map[*Certificate][][]*Certificate) } childChains, ok := cache[candidate] if !ok { childChains, err = candidate.buildChains(cache, appendToFreshChain(currentChain, candidate), sigChecks, opts) cache[candidate] = childChains } chains = append(chains, childChains...) } } for _, rootNum := range opts.Roots.findPotentialParents(c) { considerCandidate(rootCertificate, opts.Roots.certs[rootNum]) } for _, intermediateNum := range opts.Intermediates.findPotentialParents(c) { considerCandidate(intermediateCertificate, opts.Intermediates.certs[intermediateNum]) } if len(chains) > 0 { err = nil } if len(chains) == 0 && err == nil { err = UnknownAuthorityError{c, hintErr, hintCert} } return } // validHostname reports whether host is a valid hostname that can be matched or // matched against according to RFC 6125 2.2, with some leniency to accommodate // legacy values. func validHostname(host string) bool { host = strings.TrimSuffix(host, ".") if len(host) == 0 { return false } for i, part := range strings.Split(host, ".") { if part == "" { // Empty label. return false } if i == 0 && part == "*" { // Only allow full left-most wildcards, as those are the only ones // we match, and matching literal '*' characters is probably never // the expected behavior. continue } for j, c := range part { if 'a' <= c && c <= 'z' { continue } if '0' <= c && c <= '9' { continue } if 'A' <= c && c <= 'Z' { continue } if c == '-' && j != 0 { continue } if c == '_' || c == ':' { // Not valid characters in hostnames, but commonly // found in deployments outside the WebPKI. continue } return false } } return true } // commonNameAsHostname reports whether the Common Name field should be // considered the hostname that the certificate is valid for. This is a legacy // behavior, disabled if the Subject Alt Name extension is present. // // It applies the strict validHostname check to the Common Name field, so that // certificates without SANs can still be validated against CAs with name // constraints if there is no risk the CN would be matched as a hostname. // See NameConstraintsWithoutSANs and issue 24151. func (c *Certificate) commonNameAsHostname() bool { return !ignoreCN && !c.hasSANExtension() && validHostname(c.Subject.CommonName) } func matchHostnames(pattern, host string) bool { host = strings.TrimSuffix(host, ".") pattern = strings.TrimSuffix(pattern, ".") if len(pattern) == 0 || len(host) == 0 { return false } patternParts := strings.Split(pattern, ".") hostParts := strings.Split(host, ".") if len(patternParts) != len(hostParts) { return false } for i, patternPart := range patternParts { if i == 0 && patternPart == "*" { continue } if patternPart != hostParts[i] { return false } } return true } // toLowerCaseASCII returns a lower-case version of in. See RFC 6125 6.4.1. We use // an explicitly ASCII function to avoid any sharp corners resulting from // performing Unicode operations on DNS labels. func toLowerCaseASCII(in string) string { // If the string is already lower-case then there's nothing to do. isAlreadyLowerCase := true for _, c := range in { if c == utf8.RuneError { // If we get a UTF-8 error then there might be // upper-case ASCII bytes in the invalid sequence. isAlreadyLowerCase = false break } if 'A' <= c && c <= 'Z' { isAlreadyLowerCase = false break } } if isAlreadyLowerCase { return in } out := []byte(in) for i, c := range out { if 'A' <= c && c <= 'Z' { out[i] += 'a' - 'A' } } return string(out) } // VerifyHostname returns nil if c is a valid certificate for the named host. // Otherwise it returns an error describing the mismatch. func (c *Certificate) VerifyHostname(h string) error { // IP addresses may be written in [ ]. candidateIP := h if len(h) >= 3 && h[0] == '[' && h[len(h)-1] == ']' { candidateIP = h[1 : len(h)-1] } if ip := net.ParseIP(candidateIP); ip != nil { // We only match IP addresses against IP SANs. // See RFC 6125, Appendix B.2. for _, candidate := range c.IPAddresses { if ip.Equal(candidate) { return nil } } return HostnameError{c, candidateIP} } lowered := toLowerCaseASCII(h) if c.commonNameAsHostname() { if matchHostnames(toLowerCaseASCII(c.Subject.CommonName), lowered) { return nil } } else { for _, match := range c.DNSNames { if matchHostnames(toLowerCaseASCII(match), lowered) { return nil } } } return HostnameError{c, h} } func checkChainForKeyUsage(chain []*Certificate, keyUsages []ExtKeyUsage) bool { usages := make([]ExtKeyUsage, len(keyUsages)) copy(usages, keyUsages) if len(chain) == 0 { return false } usagesRemaining := len(usages) // We walk down the list and cross out any usages that aren't supported // by each certificate. If we cross out all the usages, then the chain // is unacceptable. NextCert: for i := len(chain) - 1; i >= 0; i-- { cert := chain[i] if len(cert.ExtKeyUsage) == 0 && len(cert.UnknownExtKeyUsage) == 0 { // The certificate doesn't have any extended key usage specified. continue } for _, usage := range cert.ExtKeyUsage { if usage == ExtKeyUsageAny { // The certificate is explicitly good for any usage. continue NextCert } } const invalidUsage ExtKeyUsage = -1 NextRequestedUsage: for i, requestedUsage := range usages { if requestedUsage == invalidUsage { continue } for _, usage := range cert.ExtKeyUsage { if requestedUsage == usage { continue NextRequestedUsage } else if requestedUsage == ExtKeyUsageServerAuth && (usage == ExtKeyUsageNetscapeServerGatedCrypto || usage == ExtKeyUsageMicrosoftServerGatedCrypto) { // In order to support COMODO // certificate chains, we have to // accept Netscape or Microsoft SGC // usages as equal to ServerAuth. continue NextRequestedUsage } } usages[i] = invalidUsage usagesRemaining-- if usagesRemaining == 0 { return false } } } return true }