preventing replay attack on MFA passcodes

vault/external_tests/identity/login_mfa_totp_test.go

@@ -2,7 +2,9 @@ package identity
 
 import (
 	"fmt"
+	"strings"
 	"testing"
+	"time"
 
 	"github.com/hashicorp/vault/api"
 	"github.com/hashicorp/vault/builtin/credential/userpass"
@@ -115,7 +117,7 @@ func TestLoginMfaGenerateTOTPRoleTest(t *testing.T) {
 		// create a config
 		resp1, err := client.Logical().Write("identity/mfa/method/totp", map[string]interface{}{
 			"issuer":    "yCorp",
-			"period":    10000,
+			"period":    5,
 			"algorithm": "SHA1",
 			"digits":    6,
 			"skew":      1,
@@ -206,6 +208,10 @@ func TestLoginMfaGenerateTOTPRoleTest(t *testing.T) {
 			t.Fatalf("MFA failed: %v", err)
 		}
 
+		if len(secret.Warnings) == 0 || !strings.Contains(strings.Join(secret.Warnings, ""), "A login request was issued that is subject to MFA validation") {
+			t.Fatalf("first phase of login did not have a warning")
+		}
+
 		if secret.Auth == nil || secret.Auth.MFARequirement == nil {
 			t.Fatalf("two phase login returned nil MFARequirement")
 		}
@@ -229,6 +235,15 @@ func TestLoginMfaGenerateTOTPRoleTest(t *testing.T) {
 		}
 
 		// validation
+		// waiting for 5 seconds so that a fresh code could be generated
+		time.Sleep(5 * time.Second)
+		// getting a fresh totp passcode for the validation step
+		totpResp, err := client.Logical().Read("totp/code/loginMFA")
+		if err != nil {
+			t.Fatalf("failed to create totp passcode: %v", err)
+		}
+		totpPasscode = totpResp.Data["code"].(string)
+
 		secret, err = user2Client.Logical().Write("sys/mfa/validate", map[string]interface{}{
 			"mfa_request_id": secret.Auth.MFARequirement.MFARequestID,
 			"mfa_payload": map[string][]string{
@@ -251,6 +266,19 @@ func TestLoginMfaGenerateTOTPRoleTest(t *testing.T) {
 			t.Fatalf("two phase login returned nil MFARequirement")
 		}
 
+		_, err = user2Client.Logical().Write("sys/mfa/validate", map[string]interface{}{
+			"mfa_request_id": secret.Auth.MFARequirement.MFARequestID,
+			"mfa_payload": map[string][]string{
+				methodID: {totpPasscode},
+			},
+		})
+		if err == nil {
+			t.Fatalf("MFA succeeded with an already used passcode")
+		}
+		if !strings.Contains(err.Error(), "code already used") {
+			t.Fatalf("expected error message to mention code already used")
+		}
+
 		// Destroy the secret so that the token can self generate
 		_, err = userClient.Logical().Write(fmt.Sprintf("identity/mfa/method/totp/admin-destroy"), map[string]interface{}{
 			"entity_id": entityID,

vault/login_mfa.go

@@ -35,6 +35,7 @@ import (
 	"github.com/hashicorp/vault/sdk/logical"
 	"github.com/hashicorp/vault/vault/quotas"
 	"github.com/mitchellh/mapstructure"
+	"github.com/patrickmn/go-cache"
 	otplib "github.com/pquerna/otp"
 	totplib "github.com/pquerna/otp/totp"
 )
@@ -99,6 +100,7 @@ type MFABackend struct {
 	mfaLogger   hclog.Logger
 	namespacer  Namespacer
 	methodTable string
+	usedCodes   *cache.Cache
 }
 
 type LoginMFABackend struct {
@@ -138,6 +140,7 @@ func NewMFABackend(core *Core, logger hclog.Logger, prefix string, schemaFuncs [
 		mfaLogger:   logger.Named("mfa"),
 		namespacer:  core,
 		methodTable: prefix,
+		usedCodes:   cache.New(0, 30*time.Second),
 	}
 }
 
@@ -553,7 +556,7 @@ func (b *LoginMFABackend) handleMFALoginValidate(ctx context.Context, req *logic
 	for _, eConfig := range matchedMfaEnforcementList {
 		err = b.Core.validateLoginMFA(ctx, eConfig, entity, req.Connection.RemoteAddr, mfaCreds)
 		if err != nil {
-			return logical.ErrorResponse("failed to satisfy enforcement %s", eConfig.Name), logical.ErrPermissionDenied
+			return logical.ErrorResponse(fmt.Sprintf("failed to satisfy enforcement %s. error: %s", eConfig.Name, err.Error())), logical.ErrPermissionDenied
 		}
 	}
 
@@ -1566,9 +1569,16 @@ func (c *Core) validateDuo(ctx context.Context, creds []string, mConfig *mfa.Con
 		return fmt.Errorf("invalid response from Duo preauth: %q", preauth.Response.Result)
 	}
 
+	var usedName string
 	options := []func(*url.Values){}
 	factor := "push"
 	if passcode != "" {
+		usedName = fmt.Sprintf("%s_%s", mConfig.ID, passcode)
+		_, ok := c.loginMFABackend.usedCodes.Get(usedName)
+		if ok {
+			return fmt.Errorf("code already used; wait until the next time period")
+		}
+
 		factor = "passcode"
 		options = append(options, authapi.AuthPasscode(passcode))
 	} else {
@@ -1610,6 +1620,10 @@ func (c *Core) validateDuo(ctx context.Context, creds []string, mConfig *mfa.Con
 		case "deny":
 			return fmt.Errorf("duo authentication failed: %q", statusResult.Response.Status_Msg)
 		case "allow":
+			err = c.loginMFABackend.usedCodes.Add(usedName, nil, 30*time.Second)
+			if err != nil {
+				return fmt.Errorf("error adding code to used cache: %w", err)
+			}
 			return nil
 		}
 
@@ -1962,6 +1976,13 @@ func (c *Core) validateTOTP(ctx context.Context, creds []string, entityMethodSec
 		return fmt.Errorf("more than one TOTP passcode supplied")
 	}
 
+	usedName := fmt.Sprintf("%s_%s", configID, creds[0])
+
+	_, ok := c.loginMFABackend.usedCodes.Get(usedName)
+	if ok {
+		return fmt.Errorf("code already used; wait until the next time period")
+	}
+
 	totpSecret := entityMethodSecret.GetTOTPSecret()
 	if totpSecret == nil {
 		return fmt.Errorf("entity does not contain the TOTP secret")
@@ -1992,6 +2013,17 @@ func (c *Core) validateTOTP(ctx context.Context, creds []string, entityMethodSec
 		return fmt.Errorf("failed to validate TOTP passcode")
 	}
 
+	// Adding the used code to the cache
+	// Take the key skew, add two for behind and in front, and multiply that by
+	// the period to cover the full possibility of the validity of the key
+	err = c.loginMFABackend.usedCodes.Add(usedName, nil, time.Duration(
+		int64(time.Second)*
+			int64(totpSecret.Period)*
+			int64(2+totpSecret.Skew)))
+	if err != nil {
+		return fmt.Errorf("error adding code to used cache: %w", err)
+	}
+
 	return nil
 }
 

vault/request_handling.go

@@ -1483,6 +1483,7 @@ func (c *Core) handleLoginRequest(ctx context.Context, req *logical.Request) (re
 				resp.Auth = &logical.Auth{
 					MFARequirement: mfaRequirement,
 				}
+				resp.AddWarning("A login request was issued that is subject to MFA validation. Please make sure to validate the login by sending another request to mfa/validate endpoint.")
 				// going to return early before generating the token
 				// the user receives the mfaRequirement, and need to use the
 				// login MFA validate endpoint to get the token