package acme import ( "context" "crypto" "crypto/sha256" "crypto/subtle" "crypto/tls" "encoding/asn1" "encoding/base64" "encoding/hex" "encoding/json" "errors" "fmt" "io" "net" "net/url" "reflect" "strings" "time" "go.step.sm/crypto/jose" ) type ChallengeType string const ( // HTTP01 is the http-01 ACME challenge type HTTP01 ChallengeType = "http-01" // DNS01 is the dns-01 ACME challenge type DNS01 ChallengeType = "dns-01" // TLSALPN01 is the tls-alpn-01 ACME challenge type TLSALPN01 ChallengeType = "tls-alpn-01" ) // Challenge represents an ACME response Challenge type. type Challenge struct { ID string `json:"-"` AccountID string `json:"-"` AuthorizationID string `json:"-"` Value string `json:"-"` Type ChallengeType `json:"type"` Status Status `json:"status"` Token string `json:"token"` ValidatedAt string `json:"validated,omitempty"` URL string `json:"url"` Error *Error `json:"error,omitempty"` } // ToLog enables response logging. func (ch *Challenge) ToLog() (interface{}, error) { b, err := json.Marshal(ch) if err != nil { return nil, WrapErrorISE(err, "error marshaling challenge for logging") } return string(b), nil } // Validate attempts to validate the challenge. Stores changes to the Challenge // type using the DB interface. // satisfactorily validated, the 'status' and 'validated' attributes are // updated. func (ch *Challenge) Validate(ctx context.Context, db DB, jwk *jose.JSONWebKey) error { // If already valid or invalid then return without performing validation. if ch.Status != StatusPending { return nil } switch ch.Type { case HTTP01: return http01Validate(ctx, ch, db, jwk) case DNS01: return dns01Validate(ctx, ch, db, jwk) case TLSALPN01: return tlsalpn01Validate(ctx, ch, db, jwk) default: return NewErrorISE("unexpected challenge type '%s'", ch.Type) } } func http01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose.JSONWebKey) error { u := &url.URL{Scheme: "http", Host: http01ChallengeHost(ch.Value), Path: fmt.Sprintf("/.well-known/acme-challenge/%s", ch.Token)} vc := MustClientFromContext(ctx) resp, err := vc.Get(u.String()) if err != nil { return storeError(ctx, db, ch, false, WrapError(ErrorConnectionType, err, "error doing http GET for url %s", u)) } defer resp.Body.Close() if resp.StatusCode >= 400 { return storeError(ctx, db, ch, false, NewError(ErrorConnectionType, "error doing http GET for url %s with status code %d", u, resp.StatusCode)) } body, err := io.ReadAll(resp.Body) if err != nil { return WrapErrorISE(err, "error reading "+ "response body for url %s", u) } keyAuth := strings.TrimSpace(string(body)) expected, err := KeyAuthorization(ch.Token, jwk) if err != nil { return err } if keyAuth != expected { return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType, "keyAuthorization does not match; expected %s, but got %s", expected, keyAuth)) } // Update and store the challenge. ch.Status = StatusValid ch.Error = nil ch.ValidatedAt = clock.Now().Format(time.RFC3339) if err = db.UpdateChallenge(ctx, ch); err != nil { return WrapErrorISE(err, "error updating challenge") } return nil } // http01ChallengeHost checks if a Challenge value is an IPv6 address // and adds square brackets if that's the case, so that it can be used // as a hostname. Returns the original Challenge value as the host to // use in other cases. func http01ChallengeHost(value string) string { if ip := net.ParseIP(value); ip != nil && ip.To4() == nil { value = "[" + value + "]" } return value } func tlsAlert(err error) uint8 { var opErr *net.OpError if errors.As(err, &opErr) { v := reflect.ValueOf(opErr.Err) if v.Kind() == reflect.Uint8 { return uint8(v.Uint()) } } return 0 } func tlsalpn01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose.JSONWebKey) error { config := &tls.Config{ NextProtos: []string{"acme-tls/1"}, // https://tools.ietf.org/html/rfc8737#section-4 // ACME servers that implement "acme-tls/1" MUST only negotiate TLS 1.2 // [RFC5246] or higher when connecting to clients for validation. MinVersion: tls.VersionTLS12, ServerName: serverName(ch), InsecureSkipVerify: true, // we expect a self-signed challenge certificate } hostPort := net.JoinHostPort(ch.Value, "443") vc := MustClientFromContext(ctx) conn, err := vc.TLSDial("tcp", hostPort, config) if err != nil { // With Go 1.17+ tls.Dial fails if there's no overlap between configured // client and server protocols. When this happens the connection is // closed with the error no_application_protocol(120) as required by // RFC7301. See https://golang.org/doc/go1.17#ALPN if tlsAlert(err) == 120 { return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType, "cannot negotiate ALPN acme-tls/1 protocol for tls-alpn-01 challenge")) } return storeError(ctx, db, ch, false, WrapError(ErrorConnectionType, err, "error doing TLS dial for %s", hostPort)) } defer conn.Close() cs := conn.ConnectionState() certs := cs.PeerCertificates if len(certs) == 0 { return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType, "%s challenge for %s resulted in no certificates", ch.Type, ch.Value)) } if cs.NegotiatedProtocol != "acme-tls/1" { return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType, "cannot negotiate ALPN acme-tls/1 protocol for tls-alpn-01 challenge")) } leafCert := certs[0] // if no DNS names present, look for IP address and verify that exactly one exists if len(leafCert.DNSNames) == 0 { if len(leafCert.IPAddresses) != 1 || !leafCert.IPAddresses[0].Equal(net.ParseIP(ch.Value)) { return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType, "incorrect certificate for tls-alpn-01 challenge: leaf certificate must contain a single IP address or DNS name, %v", ch.Value)) } } else { if len(leafCert.DNSNames) != 1 || !strings.EqualFold(leafCert.DNSNames[0], ch.Value) { return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType, "incorrect certificate for tls-alpn-01 challenge: leaf certificate must contain a single IP address or DNS name, %v", ch.Value)) } } idPeAcmeIdentifier := asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 1, 31} idPeAcmeIdentifierV1Obsolete := asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 1, 30, 1} foundIDPeAcmeIdentifierV1Obsolete := false keyAuth, err := KeyAuthorization(ch.Token, jwk) if err != nil { return err } hashedKeyAuth := sha256.Sum256([]byte(keyAuth)) for _, ext := range leafCert.Extensions { if idPeAcmeIdentifier.Equal(ext.Id) { if !ext.Critical { return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType, "incorrect certificate for tls-alpn-01 challenge: acmeValidationV1 extension not critical")) } var extValue []byte rest, err := asn1.Unmarshal(ext.Value, &extValue) if err != nil || len(rest) > 0 || len(hashedKeyAuth) != len(extValue) { return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType, "incorrect certificate for tls-alpn-01 challenge: malformed acmeValidationV1 extension value")) } if subtle.ConstantTimeCompare(hashedKeyAuth[:], extValue) != 1 { return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType, "incorrect certificate for tls-alpn-01 challenge: "+ "expected acmeValidationV1 extension value %s for this challenge but got %s", hex.EncodeToString(hashedKeyAuth[:]), hex.EncodeToString(extValue))) } ch.Status = StatusValid ch.Error = nil ch.ValidatedAt = clock.Now().Format(time.RFC3339) if err = db.UpdateChallenge(ctx, ch); err != nil { return WrapErrorISE(err, "tlsalpn01ValidateChallenge - error updating challenge") } return nil } if idPeAcmeIdentifierV1Obsolete.Equal(ext.Id) { foundIDPeAcmeIdentifierV1Obsolete = true } } if foundIDPeAcmeIdentifierV1Obsolete { return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType, "incorrect certificate for tls-alpn-01 challenge: obsolete id-pe-acmeIdentifier in acmeValidationV1 extension")) } return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType, "incorrect certificate for tls-alpn-01 challenge: missing acmeValidationV1 extension")) } func dns01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose.JSONWebKey) error { // Normalize domain for wildcard DNS names // This is done to avoid making TXT lookups for domains like // _acme-challenge.*.example.com // Instead perform txt lookup for _acme-challenge.example.com domain := strings.TrimPrefix(ch.Value, "*.") vc := MustClientFromContext(ctx) txtRecords, err := vc.LookupTxt("_acme-challenge." + domain) if err != nil { return storeError(ctx, db, ch, false, WrapError(ErrorDNSType, err, "error looking up TXT records for domain %s", domain)) } expectedKeyAuth, err := KeyAuthorization(ch.Token, jwk) if err != nil { return err } h := sha256.Sum256([]byte(expectedKeyAuth)) expected := base64.RawURLEncoding.EncodeToString(h[:]) var found bool for _, r := range txtRecords { if r == expected { found = true break } } if !found { return storeError(ctx, db, ch, false, NewError(ErrorRejectedIdentifierType, "keyAuthorization does not match; expected %s, but got %s", expectedKeyAuth, txtRecords)) } // Update and store the challenge. ch.Status = StatusValid ch.Error = nil ch.ValidatedAt = clock.Now().Format(time.RFC3339) if err = db.UpdateChallenge(ctx, ch); err != nil { return WrapErrorISE(err, "error updating challenge") } return nil } // serverName determines the SNI HostName to set based on an acme.Challenge // for TLS-ALPN-01 challenges RFC8738 states that, if HostName is an IP, it // should be the ARPA address https://datatracker.ietf.org/doc/html/rfc8738#section-6. // It also references TLS Extensions [RFC6066]. func serverName(ch *Challenge) string { var serverName string ip := net.ParseIP(ch.Value) if ip != nil { serverName = reverseAddr(ip) } else { serverName = ch.Value } return serverName } // reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP // address addr suitable for rDNS (PTR) record lookup or an error if it fails // to parse the IP address. // Implementation taken and adapted from https://golang.org/src/net/dnsclient.go?s=780:834#L20 func reverseAddr(ip net.IP) (arpa string) { if ip.To4() != nil { return uitoa(uint(ip[15])) + "." + uitoa(uint(ip[14])) + "." + uitoa(uint(ip[13])) + "." + uitoa(uint(ip[12])) + ".in-addr.arpa." } // Must be IPv6 buf := make([]byte, 0, len(ip)*4+len("ip6.arpa.")) // Add it, in reverse, to the buffer for i := len(ip) - 1; i >= 0; i-- { v := ip[i] buf = append(buf, hexit[v&0xF], '.', hexit[v>>4], '.') } // Append "ip6.arpa." and return (buf already has the final .) buf = append(buf, "ip6.arpa."...) return string(buf) } // Convert unsigned integer to decimal string. // Implementation taken from https://golang.org/src/net/parse.go func uitoa(val uint) string { if val == 0 { // avoid string allocation return "0" } var buf [20]byte // big enough for 64bit value base 10 i := len(buf) - 1 for val >= 10 { q := val / 10 buf[i] = byte('0' + val - q*10) i-- val = q } // val < 10 buf[i] = byte('0' + val) return string(buf[i:]) } const hexit = "0123456789abcdef" // KeyAuthorization creates the ACME key authorization value from a token // and a jwk. func KeyAuthorization(token string, jwk *jose.JSONWebKey) (string, error) { thumbprint, err := jwk.Thumbprint(crypto.SHA256) if err != nil { return "", WrapErrorISE(err, "error generating JWK thumbprint") } encPrint := base64.RawURLEncoding.EncodeToString(thumbprint) return fmt.Sprintf("%s.%s", token, encPrint), nil } // storeError the given error to an ACME error and saves using the DB interface. func storeError(ctx context.Context, db DB, ch *Challenge, markInvalid bool, err *Error) error { ch.Error = err if markInvalid { ch.Status = StatusInvalid } if err := db.UpdateChallenge(ctx, ch); err != nil { return WrapErrorISE(err, "failure saving error to acme challenge") } return nil }