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Merge pull request #1366 from smallstep/herman/dynamic-scep-webhook

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Dynamic SCEP challenge validation using webhooks
This commit is contained in:
Herman Slatman 2023-05-02 00:53:39 +02:00 committed by GitHub
commit cb1dc8055d
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GPG key ID: 4AEE18F83AFDEB23
10 changed files with 547 additions and 75 deletions
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4
authority/admin/api/webhook.go
View file

@ -57,9 +57,9 @@ func validateWebhook(webhook *linkedca.Webhook) error {
// kind
switch webhook.Kind {
case linkedca.Webhook_ENRICHING, linkedca.Webhook_AUTHORIZING:
case linkedca.Webhook_ENRICHING, linkedca.Webhook_AUTHORIZING, linkedca.Webhook_SCEPCHALLENGE:
default:
return admin.NewError(admin.ErrorBadRequestType, "webhook kind is invalid")
return admin.NewError(admin.ErrorBadRequestType, "webhook kind %q is invalid", webhook.Kind)
}
return nil

20
authority/admin/api/webhook_test.go
View file

@ -180,6 +180,26 @@ func TestWebhookAdminResponder_CreateProvisionerWebhook(t *testing.T) {
statusCode: 400,
}
},
"fail/unsupported-webhook-kind": func(t *testing.T) test {
prov := &linkedca.Provisioner{
Name: "provName",
}
ctx := linkedca.NewContextWithProvisioner(context.Background(), prov)
adminErr := admin.NewError(admin.ErrorBadRequestType, `(line 5:13): invalid value for enum type: "UNSUPPORTED"`)
adminErr.Message = `(line 5:13): invalid value for enum type: "UNSUPPORTED"`
body := []byte(`
{
"name": "metadata",
"url": "https://example.com",
"kind": "UNSUPPORTED",
}`)
return test{
ctx: ctx,
body: body,
err: adminErr,
statusCode: 400,
}
},
"fail/auth.UpdateProvisioner-error": func(t *testing.T) test {
adm := &linkedca.Admin{
Subject: "step",

88
authority/authority.go
View file

@ -545,50 +545,6 @@ func (a *Authority) init() error {
tmplVars.SSH.UserFederatedKeys = append(tmplVars.SSH.UserFederatedKeys, a.sshCAUserFederatedCerts...)
}
// Check if a KMS with decryption capability is required and available
if a.requiresDecrypter() {
if _, ok := a.keyManager.(kmsapi.Decrypter); !ok {
return errors.New("keymanager doesn't provide crypto.Decrypter")
}
}
// TODO: decide if this is a good approach for providing the SCEP functionality
// It currently mirrors the logic for the x509CAService
if a.requiresSCEPService() && a.scepService == nil {
var options scep.Options
// Read intermediate and create X509 signer and decrypter for default CAS.
options.CertificateChain, err = pemutil.ReadCertificateBundle(a.config.IntermediateCert)
if err != nil {
return err
}
options.CertificateChain = append(options.CertificateChain, a.rootX509Certs...)
options.Signer, err = a.keyManager.CreateSigner(&kmsapi.CreateSignerRequest{
SigningKey: a.config.IntermediateKey,
Password: a.password,
})
if err != nil {
return err
}
if km, ok := a.keyManager.(kmsapi.Decrypter); ok {
options.Decrypter, err = km.CreateDecrypter(&kmsapi.CreateDecrypterRequest{
DecryptionKey: a.config.IntermediateKey,
Password: a.password,
})
if err != nil {
return err
}
}
a.scepService, err = scep.NewService(ctx, options)
if err != nil {
return err
}
// TODO: mimick the x509CAService GetCertificateAuthority here too?
}
if a.config.AuthorityConfig.EnableAdmin {
// Initialize step-ca Admin Database if it's not already initialized using
// WithAdminDB.
@ -684,6 +640,50 @@ func (a *Authority) init() error {
return err
}
// Check if a KMS with decryption capability is required and available
if a.requiresDecrypter() {
if _, ok := a.keyManager.(kmsapi.Decrypter); !ok {
return errors.New("keymanager doesn't provide crypto.Decrypter")
}
}
// TODO: decide if this is a good approach for providing the SCEP functionality
// It currently mirrors the logic for the x509CAService
if a.requiresSCEPService() && a.scepService == nil {
var options scep.Options
// Read intermediate and create X509 signer and decrypter for default CAS.
options.CertificateChain, err = pemutil.ReadCertificateBundle(a.config.IntermediateCert)
if err != nil {
return err
}
options.CertificateChain = append(options.CertificateChain, a.rootX509Certs...)
options.Signer, err = a.keyManager.CreateSigner(&kmsapi.CreateSignerRequest{
SigningKey: a.config.IntermediateKey,
Password: a.password,
})
if err != nil {
return err
}
if km, ok := a.keyManager.(kmsapi.Decrypter); ok {
options.Decrypter, err = km.CreateDecrypter(&kmsapi.CreateDecrypterRequest{
DecryptionKey: a.config.IntermediateKey,
Password: a.password,
})
if err != nil {
return err
}
}
a.scepService, err = scep.NewService(ctx, options)
if err != nil {
return err
}
// TODO: mimick the x509CAService GetCertificateAuthority here too?
}
// Load X509 constraints engine.
//
// This is currently only available in CA mode.

113
authority/provisioner/scep.go
View file

@ -2,10 +2,16 @@ package provisioner
import (
"context"
"crypto/subtle"
"fmt"
"net/http"
"time"
"github.com/pkg/errors"
"go.step.sm/linkedca"
"github.com/smallstep/certificates/webhook"
)
// SCEP is the SCEP provisioner type, an entity that can authorize the
@ -35,6 +41,7 @@ type SCEP struct {
ctl *Controller
secretChallengePassword string
encryptionAlgorithm int
challengeValidationController *challengeValidationController
}
// GetID returns the provisioner unique identifier.
@ -82,6 +89,67 @@ func (s *SCEP) DefaultTLSCertDuration() time.Duration {
return s.ctl.Claimer.DefaultTLSCertDuration()
}
type challengeValidationController struct {
client *http.Client
webhooks []*Webhook
}
// newChallengeValidationController creates a new challengeValidationController
// that performs challenge validation through webhooks.
func newChallengeValidationController(client *http.Client, webhooks []*Webhook) *challengeValidationController {
scepHooks := []*Webhook{}
for _, wh := range webhooks {
if wh.Kind != linkedca.Webhook_SCEPCHALLENGE.String() {
continue
}
if !isCertTypeOK(wh) {
continue
}
scepHooks = append(scepHooks, wh)
}
return &challengeValidationController{
client: client,
webhooks: scepHooks,
}
}
var (
ErrSCEPChallengeInvalid = errors.New("webhook server did not allow request")
)
// Validate executes zero or more configured webhooks to
// validate the SCEP challenge. If at least one of them indicates
// the challenge value is accepted, validation succeeds. In
// that case, the other webhooks will be skipped. If none of
// the webhooks indicates the value of the challenge was accepted,
// an error is returned.
func (c *challengeValidationController) Validate(ctx context.Context, challenge, transactionID string) error {
for _, wh := range c.webhooks {
req := &webhook.RequestBody{
SCEPChallenge: challenge,
SCEPTransactionID: transactionID,
}
resp, err := wh.DoWithContext(ctx, c.client, req, nil) // TODO(hs): support templated URL? Requires some refactoring
if err != nil {
return fmt.Errorf("failed executing webhook request: %w", err)
}
if resp.Allow {
return nil // return early when response is positive
}
}
return ErrSCEPChallengeInvalid
}
// isCertTypeOK returns whether or not the webhook can be used
// with the SCEP challenge validation webhook controller.
func isCertTypeOK(wh *Webhook) bool {
if wh.CertType == linkedca.Webhook_ALL.String() || wh.CertType == "" {
return true
}
return linkedca.Webhook_X509.String() == wh.CertType
}
// Init initializes and validates the fields of a SCEP type.
func (s *SCEP) Init(config Config) (err error) {
switch {
@ -109,6 +177,11 @@ func (s *SCEP) Init(config Config) (err error) {
return errors.New("only encryption algorithm identifiers from 0 to 4 are valid")
}
s.challengeValidationController = newChallengeValidationController(
config.WebhookClient,
s.GetOptions().GetWebhooks(),
)
// TODO: add other, SCEP specific, options?
s.ctl, err = NewController(s, s.Claims, config, s.Options)
@ -156,3 +229,43 @@ func (s *SCEP) ShouldIncludeRootInChain() bool {
func (s *SCEP) GetContentEncryptionAlgorithm() int {
return s.encryptionAlgorithm
}
// ValidateChallenge validates the provided challenge. It starts by
// selecting the validation method to use, then performs validation
// according to that method.
func (s *SCEP) ValidateChallenge(ctx context.Context, challenge, transactionID string) error {
if s.challengeValidationController == nil {
return fmt.Errorf("provisioner %q wasn't initialized", s.Name)
}
switch s.selectValidationMethod() {
case validationMethodWebhook:
return s.challengeValidationController.Validate(ctx, challenge, transactionID)
default:
if subtle.ConstantTimeCompare([]byte(s.secretChallengePassword), []byte(challenge)) == 0 {
return errors.New("invalid challenge password provided")
}
return nil
}
}
type validationMethod string
const (
validationMethodNone validationMethod = "none"
validationMethodStatic validationMethod = "static"
validationMethodWebhook validationMethod = "webhook"
)
// selectValidationMethod returns the method to validate SCEP
// challenges. If a webhook is configured with kind `SCEPCHALLENGE`,
// the webhook method will be used. If a challenge password is set,
// the static method is used. It will default to the `none` method.
func (s *SCEP) selectValidationMethod() validationMethod {
if len(s.challengeValidationController.webhooks) > 0 {
return validationMethodWebhook
}
if s.secretChallengePassword != "" {
return validationMethodStatic
}
return validationMethodNone
}

342
authority/provisioner/scep_test.go Normal file
View file

@ -0,0 +1,342 @@
package provisioner
import (
"context"
"encoding/json"
"errors"
"net/http"
"net/http/httptest"
"testing"
"github.com/stretchr/testify/assert"
"github.com/stretchr/testify/require"
"go.step.sm/linkedca"
)
func Test_challengeValidationController_Validate(t *testing.T) {
type request struct {
Challenge string `json:"scepChallenge"`
TransactionID string `json:"scepTransactionID"`
}
type response struct {
Allow bool `json:"allow"`
}
nokServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
req := &request{}
err := json.NewDecoder(r.Body).Decode(req)
require.NoError(t, err)
assert.Equal(t, "not-allowed", req.Challenge)
assert.Equal(t, "transaction-1", req.TransactionID)
b, err := json.Marshal(response{Allow: false})
require.NoError(t, err)
w.WriteHeader(200)
w.Write(b)
}))
okServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
req := &request{}
err := json.NewDecoder(r.Body).Decode(req)
require.NoError(t, err)
assert.Equal(t, "challenge", req.Challenge)
assert.Equal(t, "transaction-1", req.TransactionID)
b, err := json.Marshal(response{Allow: true})
require.NoError(t, err)
w.WriteHeader(200)
w.Write(b)
}))
type fields struct {
client *http.Client
webhooks []*Webhook
}
type args struct {
challenge string
transactionID string
}
tests := []struct {
name string
fields fields
args args
server *httptest.Server
expErr error
}{
{
name: "fail/no-webhook",
fields: fields{http.DefaultClient, nil},
args: args{"no-webhook", "transaction-1"},
expErr: errors.New("webhook server did not allow request"),
},
{
name: "fail/wrong-cert-type",
fields: fields{http.DefaultClient, []*Webhook{
{
Kind: linkedca.Webhook_SCEPCHALLENGE.String(),
CertType: linkedca.Webhook_SSH.String(),
},
}},
args: args{"wrong-cert-type", "transaction-1"},
expErr: errors.New("webhook server did not allow request"),
},
{
name: "fail/wrong-secret-value",
fields: fields{http.DefaultClient, []*Webhook{
{
ID: "webhook-id-1",
Name: "webhook-name-1",
Secret: "{{}}",
Kind: linkedca.Webhook_SCEPCHALLENGE.String(),
CertType: linkedca.Webhook_X509.String(),
URL: okServer.URL,
},
}},
args: args{
challenge: "wrong-secret-value",
transactionID: "transaction-1",
},
expErr: errors.New("failed executing webhook request: illegal base64 data at input byte 0"),
},
{
name: "fail/not-allowed",
fields: fields{http.DefaultClient, []*Webhook{
{
ID: "webhook-id-1",
Name: "webhook-name-1",
Secret: "MTIzNAo=",
Kind: linkedca.Webhook_SCEPCHALLENGE.String(),
CertType: linkedca.Webhook_X509.String(),
URL: nokServer.URL,
},
}},
args: args{
challenge: "not-allowed",
transactionID: "transaction-1",
},
server: nokServer,
expErr: errors.New("webhook server did not allow request"),
},
{
name: "ok",
fields: fields{http.DefaultClient, []*Webhook{
{
ID: "webhook-id-1",
Name: "webhook-name-1",
Secret: "MTIzNAo=",
Kind: linkedca.Webhook_SCEPCHALLENGE.String(),
CertType: linkedca.Webhook_X509.String(),
URL: okServer.URL,
},
}},
args: args{
challenge: "challenge",
transactionID: "transaction-1",
},
server: okServer,
},
}
for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
c := newChallengeValidationController(tt.fields.client, tt.fields.webhooks)
if tt.server != nil {
defer tt.server.Close()
}
ctx := context.Background()
err := c.Validate(ctx, tt.args.challenge, tt.args.transactionID)
if tt.expErr != nil {
assert.EqualError(t, err, tt.expErr.Error())
return
}
assert.NoError(t, err)
})
}
}
func TestController_isCertTypeOK(t *testing.T) {
assert.True(t, isCertTypeOK(&Webhook{CertType: linkedca.Webhook_X509.String()}))
assert.True(t, isCertTypeOK(&Webhook{CertType: linkedca.Webhook_ALL.String()}))
assert.True(t, isCertTypeOK(&Webhook{CertType: ""}))
assert.False(t, isCertTypeOK(&Webhook{CertType: linkedca.Webhook_SSH.String()}))
}
func Test_selectValidationMethod(t *testing.T) {
tests := []struct {
name string
p *SCEP
want validationMethod
}{
{"webhooks", &SCEP{
Name: "SCEP",
Type: "SCEP",
Options: &Options{
Webhooks: []*Webhook{
{
Kind: linkedca.Webhook_SCEPCHALLENGE.String(),
},
},
},
}, "webhook"},
{"challenge", &SCEP{
Name: "SCEP",
Type: "SCEP",
ChallengePassword: "pass",
}, "static"},
{"challenge-with-different-webhook", &SCEP{
Name: "SCEP",
Type: "SCEP",
Options: &Options{
Webhooks: []*Webhook{
{
Kind: linkedca.Webhook_AUTHORIZING.String(),
},
},
},
ChallengePassword: "pass",
}, "static"},
{"none", &SCEP{
Name: "SCEP",
Type: "SCEP",
}, "none"},
{"none-with-different-webhook", &SCEP{
Name: "SCEP",
Type: "SCEP",
Options: &Options{
Webhooks: []*Webhook{
{
Kind: linkedca.Webhook_AUTHORIZING.String(),
},
},
},
}, "none"},
}
for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
err := tt.p.Init(Config{Claims: globalProvisionerClaims})
require.NoError(t, err)
got := tt.p.selectValidationMethod()
assert.Equal(t, tt.want, got)
})
}
}
func TestSCEP_ValidateChallenge(t *testing.T) {
type request struct {
Challenge string `json:"scepChallenge"`
TransactionID string `json:"scepTransactionID"`
}
type response struct {
Allow bool `json:"allow"`
}
okServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
req := &request{}
err := json.NewDecoder(r.Body).Decode(req)
require.NoError(t, err)
assert.Equal(t, "webhook-challenge", req.Challenge)
assert.Equal(t, "webhook-transaction-1", req.TransactionID)
b, err := json.Marshal(response{Allow: true})
require.NoError(t, err)
w.WriteHeader(200)
w.Write(b)
}))
type args struct {
challenge string
transactionID string
}
tests := []struct {
name string
p *SCEP
server *httptest.Server
args args
expErr error
}{
{"ok/webhooks", &SCEP{
Name: "SCEP",
Type: "SCEP",
Options: &Options{
Webhooks: []*Webhook{
{
ID: "webhook-id-1",
Name: "webhook-name-1",
Secret: "MTIzNAo=",
Kind: linkedca.Webhook_SCEPCHALLENGE.String(),
CertType: linkedca.Webhook_X509.String(),
URL: okServer.URL,
},
},
},
}, okServer, args{"webhook-challenge", "webhook-transaction-1"},
nil,
},
{"fail/webhooks-secret-configuration", &SCEP{
Name: "SCEP",
Type: "SCEP",
Options: &Options{
Webhooks: []*Webhook{
{
ID: "webhook-id-1",
Name: "webhook-name-1",
Secret: "{{}}",
Kind: linkedca.Webhook_SCEPCHALLENGE.String(),
CertType: linkedca.Webhook_X509.String(),
URL: okServer.URL,
},
},
},
}, nil, args{"webhook-challenge", "webhook-transaction-1"},
errors.New("failed executing webhook request: illegal base64 data at input byte 0"),
},
{"ok/static-challenge", &SCEP{
Name: "SCEP",
Type: "SCEP",
Options: &Options{},
ChallengePassword: "secret-static-challenge",
}, nil, args{"secret-static-challenge", "static-transaction-1"},
nil,
},
{"fail/wrong-static-challenge", &SCEP{
Name: "SCEP",
Type: "SCEP",
Options: &Options{},
ChallengePassword: "secret-static-challenge",
}, nil, args{"the-wrong-challenge-secret", "static-transaction-1"},
errors.New("invalid challenge password provided"),
},
{"ok/no-challenge", &SCEP{
Name: "SCEP",
Type: "SCEP",
Options: &Options{},
ChallengePassword: "",
}, nil, args{"", "static-transaction-1"},
nil,
},
{"fail/no-challenge-but-provided", &SCEP{
Name: "SCEP",
Type: "SCEP",
Options: &Options{},
ChallengePassword: "",
}, nil, args{"a-challenge-value", "static-transaction-1"},
errors.New("invalid challenge password provided"),
},
}
for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
if tt.server != nil {
defer tt.server.Close()
}
err := tt.p.Init(Config{Claims: globalProvisionerClaims, WebhookClient: http.DefaultClient})
require.NoError(t, err)
ctx := context.Background()
err = tt.p.ValidateChallenge(ctx, tt.args.challenge, tt.args.transactionID)
if tt.expErr != nil {
assert.EqualError(t, err, tt.expErr.Error())
return
}
assert.NoError(t, err)
})
}
}

9
authority/provisioner/webhook.go
View file

@ -107,6 +107,13 @@ type Webhook struct {
}
func (w *Webhook) Do(client *http.Client, reqBody *webhook.RequestBody, data any) (*webhook.ResponseBody, error) {
ctx, cancel := context.WithTimeout(context.Background(), time.Second*10)
defer cancel()
return w.DoWithContext(ctx, client, reqBody, data)
}
func (w *Webhook) DoWithContext(ctx context.Context, client *http.Client, reqBody *webhook.RequestBody, data any) (*webhook.ResponseBody, error) {
tmpl, err := template.New("url").Funcs(templates.StepFuncMap()).Parse(w.URL)
if err != nil {
return nil, err
@ -129,8 +136,6 @@ func (w *Webhook) Do(client *http.Client, reqBody *webhook.RequestBody, data any
reqBody.Token = tmpl[sshutil.TokenKey]
}
*/
ctx, cancel := context.WithTimeout(context.Background(), time.Second*10)
defer cancel()
reqBody.Timestamp = time.Now()

12
scep/api/api.go
View file

@ -305,6 +305,8 @@ func PKIOperation(ctx context.Context, req request) (Response, error) {
// NOTE: at this point we have sufficient information for returning nicely signed CertReps
csr := msg.CSRReqMessage.CSR
transactionID := string(msg.TransactionID)
challengePassword := msg.CSRReqMessage.ChallengePassword
// NOTE: we're blocking the RenewalReq if the challenge does not match, because otherwise we don't have any authentication.
// The macOS SCEP client performs renewals using PKCSreq. The CertNanny SCEP client will use PKCSreq with challenge too, it seems,
@ -312,13 +314,11 @@ func PKIOperation(ctx context.Context, req request) (Response, error) {
// a certificate exists; then it will use RenewalReq. Adding the challenge check here may be a small breaking change for clients.
// We'll have to see how it works out.
if msg.MessageType == microscep.PKCSReq || msg.MessageType == microscep.RenewalReq {
challengeMatches, err := auth.MatchChallengePassword(ctx, msg.CSRReqMessage.ChallengePassword)
if err != nil {
return createFailureResponse(ctx, csr, msg, microscep.BadRequest, errors.New("error when checking password"))
if err := auth.ValidateChallenge(ctx, challengePassword, transactionID); err != nil {
if errors.Is(err, provisioner.ErrSCEPChallengeInvalid) {
return createFailureResponse(ctx, csr, msg, microscep.BadRequest, err)
}
if !challengeMatches {
// TODO: can this be returned safely to the client? In the end, if the password was correct, that gains a bit of info too.
return createFailureResponse(ctx, csr, msg, microscep.BadRequest, errors.New("wrong password provided"))
return createFailureResponse(ctx, csr, msg, microscep.BadRequest, errors.New("failed validating challenge password"))
}
}

27
scep/authority.go
View file

@ -2,7 +2,6 @@ package scep
import (
"context"
"crypto/subtle"
"crypto/x509"
"errors"
"fmt"
@ -456,24 +455,6 @@ func (a *Authority) CreateFailureResponse(ctx context.Context, csr *x509.Certifi
return crepMsg, nil
}
// MatchChallengePassword verifies a SCEP challenge password
func (a *Authority) MatchChallengePassword(ctx context.Context, password string) (bool, error) {
p, err := provisionerFromContext(ctx)
if err != nil {
return false, err
}
if subtle.ConstantTimeCompare([]byte(p.GetChallengePassword()), []byte(password)) == 1 {
return true, nil
}
// TODO: support dynamic challenges, i.e. a list of challenges instead of one?
// That's probably a bit harder to configure, though; likely requires some data store
// that can be interacted with more easily, via some internal API, for example.
return false, nil
}
// GetCACaps returns the CA capabilities
func (a *Authority) GetCACaps(ctx context.Context) []string {
p, err := provisionerFromContext(ctx)
@ -494,3 +475,11 @@ func (a *Authority) GetCACaps(ctx context.Context) []string {
return caps
}
func (a *Authority) ValidateChallenge(ctx context.Context, challenge, transactionID string) error {
p, err := provisionerFromContext(ctx)
if err != nil {
return err
}
return p.ValidateChallenge(ctx, challenge, transactionID)
}

2
scep/provisioner.go
View file

@ -14,8 +14,8 @@ type Provisioner interface {
GetName() string
DefaultTLSCertDuration() time.Duration
GetOptions() *provisioner.Options
GetChallengePassword() string
GetCapabilities() []string
ShouldIncludeRootInChain() bool
GetContentEncryptionAlgorithm() int
ValidateChallenge(ctx context.Context, challenge, transactionID string) error
}

3
webhook/types.go
View file

@ -68,4 +68,7 @@ type RequestBody struct {
X509Certificate *X509Certificate `json:"x509Certificate,omitempty"`
SSHCertificateRequest *SSHCertificateRequest `json:"sshCertificateRequest,omitempty"`
SSHCertificate *SSHCertificate `json:"sshCertificate,omitempty"`
// Only set for SCEP challenge validation requests
SCEPChallenge string `json:"scepChallenge,omitempty"`
SCEPTransactionID string `json:"scepTransactionID,omitempty"`
}