Allow identity templates in ssh backend `default_user` field (#16351)

* Allow identity templates in ssh backend `default_user` field

* use correct test expected value

* include api docs for `default_user_template` field

builtin/logical/ssh/backend_test.go

@@ -340,6 +340,147 @@ func TestBackend_AllowedUsersTemplate_WithStaticPrefix(t *testing.T) {
 	)
 }
 
+func TestBackend_DefaultUserTemplate(t *testing.T) {
+	testDefaultUserTemplate(t,
+		"{{ identity.entity.metadata.ssh_username }}",
+		testUserName,
+		map[string]string{
+			"ssh_username": testUserName,
+		},
+	)
+}
+
+func TestBackend_DefaultUserTemplate_WithStaticPrefix(t *testing.T) {
+	testDefaultUserTemplate(t,
+		"user-{{ identity.entity.metadata.ssh_username }}",
+		"user-"+testUserName,
+		map[string]string{
+			"ssh_username": testUserName,
+		},
+	)
+}
+
+func TestBackend_DefaultUserTemplateFalse_AllowedUsersTemplateTrue(t *testing.T) {
+	cluster, userpassToken := getSshCaTestCluster(t, testUserName)
+	defer cluster.Cleanup()
+	client := cluster.Cores[0].Client
+
+	// set metadata "ssh_username" to userpass username
+	tokenLookupResponse, err := client.Logical().Write("/auth/token/lookup", map[string]interface{}{
+		"token": userpassToken,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	entityID := tokenLookupResponse.Data["entity_id"].(string)
+	_, err = client.Logical().Write("/identity/entity/id/"+entityID, map[string]interface{}{
+		"metadata": map[string]string{
+			"ssh_username": testUserName,
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = client.Logical().Write("ssh/roles/my-role", map[string]interface{}{
+		"key_type":                testCaKeyType,
+		"allow_user_certificates": true,
+		"default_user":            "{{identity.entity.metadata.ssh_username}}",
+		// disable user templating but not allowed_user_template and the request should fail
+		"default_user_template":  false,
+		"allowed_users":          "{{identity.entity.metadata.ssh_username}}",
+		"allowed_users_template": true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// sign SSH key as userpass user
+	client.SetToken(userpassToken)
+	_, err = client.Logical().Write("ssh/sign/my-role", map[string]interface{}{
+		"public_key": testCAPublicKey,
+	})
+	if err == nil {
+		t.Errorf("signing request should fail when default_user is not in the allowed_users list, because allowed_users_template is true and default_user_template is not")
+	}
+
+	expectedErrStr := "{{identity.entity.metadata.ssh_username}} is not a valid value for valid_principals"
+	if !strings.Contains(err.Error(), expectedErrStr) {
+		t.Errorf("expected error to include %q but it was: %q", expectedErrStr, err.Error())
+	}
+}
+
+func TestBackend_DefaultUserTemplateFalse_AllowedUsersTemplateFalse(t *testing.T) {
+	cluster, userpassToken := getSshCaTestCluster(t, testUserName)
+	defer cluster.Cleanup()
+	client := cluster.Cores[0].Client
+
+	// set metadata "ssh_username" to userpass username
+	tokenLookupResponse, err := client.Logical().Write("/auth/token/lookup", map[string]interface{}{
+		"token": userpassToken,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	entityID := tokenLookupResponse.Data["entity_id"].(string)
+	_, err = client.Logical().Write("/identity/entity/id/"+entityID, map[string]interface{}{
+		"metadata": map[string]string{
+			"ssh_username": testUserName,
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = client.Logical().Write("ssh/roles/my-role", map[string]interface{}{
+		"key_type":                testCaKeyType,
+		"allow_user_certificates": true,
+		"default_user":            "{{identity.entity.metadata.ssh_username}}",
+		"default_user_template":   false,
+		"allowed_users":           "{{identity.entity.metadata.ssh_username}}",
+		"allowed_users_template":  false,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// sign SSH key as userpass user
+	client.SetToken(userpassToken)
+	signResponse, err := client.Logical().Write("ssh/sign/my-role", map[string]interface{}{
+		"public_key": testCAPublicKey,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// check for the expected valid principals of certificate
+	signedKey := signResponse.Data["signed_key"].(string)
+	key, _ := base64.StdEncoding.DecodeString(strings.Split(signedKey, " ")[1])
+	parsedKey, err := ssh.ParsePublicKey(key)
+	if err != nil {
+		t.Fatal(err)
+	}
+	actualPrincipals := parsedKey.(*ssh.Certificate).ValidPrincipals
+	if len(actualPrincipals) < 1 {
+		t.Fatal(
+			fmt.Sprintf("No ValidPrincipals returned: should have been %v",
+				[]string{"{{identity.entity.metadata.ssh_username}}"}),
+		)
+	}
+	if len(actualPrincipals) > 1 {
+		t.Error(
+			fmt.Sprintf("incorrect number ValidPrincipals, expected only 1: %v should be %v",
+				actualPrincipals, []string{"{{identity.entity.metadata.ssh_username}}"}),
+		)
+	}
+	if actualPrincipals[0] != "{{identity.entity.metadata.ssh_username}}" {
+		t.Fatal(
+			fmt.Sprintf("incorrect ValidPrincipals: %v should be %v",
+				actualPrincipals, []string{"{{identity.entity.metadata.ssh_username}}"}),
+		)
+	}
+}
+
 func newTestingFactory(t *testing.T) func(ctx context.Context, conf *logical.BackendConfig) (logical.Backend, error) {
 	return func(ctx context.Context, conf *logical.BackendConfig) (logical.Backend, error) {
 		defaultLeaseTTLVal := 2 * time.Minute
@@ -1893,6 +2034,65 @@ func getSshCaTestCluster(t *testing.T, userIdentity string) (*vault.TestCluster,
 	return cluster, userpassToken
 }
 
+func testDefaultUserTemplate(t *testing.T, testDefaultUserTemplate string,
+	expectedValidPrincipal string, testEntityMetadata map[string]string,
+) {
+	cluster, userpassToken := getSshCaTestCluster(t, testUserName)
+	defer cluster.Cleanup()
+	client := cluster.Cores[0].Client
+
+	// set metadata "ssh_username" to userpass username
+	tokenLookupResponse, err := client.Logical().Write("/auth/token/lookup", map[string]interface{}{
+		"token": userpassToken,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	entityID := tokenLookupResponse.Data["entity_id"].(string)
+	_, err = client.Logical().Write("/identity/entity/id/"+entityID, map[string]interface{}{
+		"metadata": testEntityMetadata,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = client.Logical().Write("ssh/roles/my-role", map[string]interface{}{
+		"key_type":                testCaKeyType,
+		"allow_user_certificates": true,
+		"default_user":            testDefaultUserTemplate,
+		"default_user_template":   true,
+		"allowed_users":           testDefaultUserTemplate,
+		"allowed_users_template":  true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// sign SSH key as userpass user
+	client.SetToken(userpassToken)
+	signResponse, err := client.Logical().Write("ssh/sign/my-role", map[string]interface{}{
+		"public_key": testCAPublicKey,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// check for the expected valid principals of certificate
+	signedKey := signResponse.Data["signed_key"].(string)
+	key, _ := base64.StdEncoding.DecodeString(strings.Split(signedKey, " ")[1])
+	parsedKey, err := ssh.ParsePublicKey(key)
+	if err != nil {
+		t.Fatal(err)
+	}
+	actualPrincipals := parsedKey.(*ssh.Certificate).ValidPrincipals
+	if actualPrincipals[0] != expectedValidPrincipal {
+		t.Fatal(
+			fmt.Sprintf("incorrect ValidPrincipals: %v should be %v",
+				actualPrincipals, []string{expectedValidPrincipal}),
+		)
+	}
+}
+
 func testAllowedUsersTemplate(t *testing.T, testAllowedUsersTemplate string,
 	expectedValidPrincipal string, testEntityMetadata map[string]string,
 ) {

builtin/logical/ssh/path_issue_sign.go

@@ -70,7 +70,14 @@ func (b *backend) pathSignIssueCertificateHelper(ctx context.Context, req *logic
 			return logical.ErrorResponse(err.Error()), nil
 		}
 	} else {
-		parsedPrincipals, err = b.calculateValidPrincipals(data, req, role, role.DefaultUser, role.AllowedUsers, strutil.StrListContains)
+		defaultPrincipal := role.DefaultUser
+		if role.DefaultUserTemplate {
+			defaultPrincipal, err = b.renderPrincipal(role.DefaultUser, req)
+			if err != nil {
+				return logical.ErrorResponse(err.Error()), nil
+			}
+		}
+		parsedPrincipals, err = b.calculateValidPrincipals(data, req, role, defaultPrincipal, role.AllowedUsers, strutil.StrListContains)
 		if err != nil {
 			return logical.ErrorResponse(err.Error()), nil
 		}
@@ -136,6 +143,23 @@ func (b *backend) pathSignIssueCertificateHelper(ctx context.Context, req *logic
 	return response, nil
 }
 
+func (b *backend) renderPrincipal(principal string, req *logical.Request) (string, error) {
+	// Look for templating markers {{ .* }}
+	matched := containsTemplateRegex.MatchString(principal)
+	if matched {
+		if req.EntityID != "" {
+			// Retrieve principal based on template + entityID from request.
+			renderedPrincipal, err := framework.PopulateIdentityTemplate(principal, req.EntityID, b.System())
+			if err != nil {
+				return "", fmt.Errorf("template '%s' could not be rendered -> %s", principal, err)
+			}
+			return renderedPrincipal, nil
+		}
+	}
+	// Static principal
+	return principal, nil
+}
+
 func (b *backend) calculateValidPrincipals(data *framework.FieldData, req *logical.Request, role *sshRole, defaultPrincipal, principalsAllowedByRole string, validatePrincipal func([]string, string) bool) ([]string, error) {
 	validPrincipals := ""
 	validPrincipalsRaw, ok := data.GetOk("valid_principals")
@@ -150,23 +174,12 @@ func (b *backend) calculateValidPrincipals(data *framework.FieldData, req *logic
 	var allowedPrincipals []string
 	for _, principal := range strutil.RemoveDuplicates(strutil.ParseStringSlice(principalsAllowedByRole, ","), false) {
 		if role.AllowedUsersTemplate {
-			// Look for templating markers {{ .* }}
+			rendered, err := b.renderPrincipal(principal, req)
-			matched := containsTemplateRegex.MatchString(principal)
+			if err != nil {
-			if matched {
+				return nil, fmt.Errorf("template '%s' could not be rendered -> %s", principal, err)
-				if req.EntityID != "" {
-					// Retrieve principal based on template + entityID from request.
-					templatePrincipal, err := framework.PopulateIdentityTemplate(principal, req.EntityID, b.System())
-					if err == nil {
-						// Template returned a principal
-						allowedPrincipals = append(allowedPrincipals, templatePrincipal)
-					} else {
-						return nil, fmt.Errorf("template '%s' could not be rendered -> %s", principal, err)
-					}
-				}
-			} else {
-				// Static principal or err template
-				allowedPrincipals = append(allowedPrincipals, principal)
 			}
+			// Template returned a principal
+			allowedPrincipals = append(allowedPrincipals, rendered)
 		} else {
 			// Static principal
 			allowedPrincipals = append(allowedPrincipals, principal)

builtin/logical/ssh/path_roles.go

@@ -40,6 +40,7 @@ type sshRole struct {
 	KeyBits                    int               `mapstructure:"key_bits" json:"key_bits"`
 	AdminUser                  string            `mapstructure:"admin_user" json:"admin_user"`
 	DefaultUser                string            `mapstructure:"default_user" json:"default_user"`
+	DefaultUserTemplate        bool              `mapstructure:"default_user_template" json:"default_user_template"`
 	CIDRList                   string            `mapstructure:"cidr_list" json:"cidr_list"`
 	ExcludeCIDRList            string            `mapstructure:"exclude_cidr_list" json:"exclude_cidr_list"`
 	Port                       int               `mapstructure:"port" json:"port"`
@@ -122,6 +123,15 @@ func pathRoles(b *backend) *framework.Path {
 					Name: "Default Username",
 				},
 			},
+			"default_user_template": {
+				Type: framework.TypeBool,
+				Description: `
+				[Not applicable for Dynamic type] [Not applicable for OTP type] [Optional for CA type]
+				If set, Default user can be specified using identity template policies.
+				Non-templated users are also permitted.
+				`,
+				Default: false,
+			},
 			"cidr_list": {
 				Type: framework.TypeString,
 				Description: `
@@ -558,6 +568,7 @@ func (b *backend) createCARole(allowedUsers, defaultUser, signer string, data *f
 		AllowedUsersTemplate:      data.Get("allowed_users_template").(bool),
 		AllowedDomains:            data.Get("allowed_domains").(string),
 		DefaultUser:               defaultUser,
+		DefaultUserTemplate:       data.Get("default_user_template").(bool),
 		AllowBareDomains:          data.Get("allow_bare_domains").(bool),
 		AllowSubdomains:           data.Get("allow_subdomains").(bool),
 		AllowUserKeyIDs:           data.Get("allow_user_key_ids").(bool),
@@ -740,6 +751,7 @@ func (b *backend) parseRole(role *sshRole) (map[string]interface{}, error) {
 			"allowed_users_template":      role.AllowedUsersTemplate,
 			"allowed_domains":             role.AllowedDomains,
 			"default_user":                role.DefaultUser,
+			"default_user_template":       role.DefaultUserTemplate,
 			"ttl":                         int64(ttl.Seconds()),
 			"max_ttl":                     int64(maxTTL.Seconds()),
 			"allowed_critical_options":    role.AllowedCriticalOptions,

changelog/16351.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+secrets/ssh: Allow the use of Identity templates in the `default_user` field
+```

website/content/api-docs/secret/ssh.mdx

@@ -99,9 +99,15 @@ This endpoint creates or updates a named role.
   create individual roles for each username to ensure absolute isolation between
   usernames. This is required for Dynamic Key type and OTP type.
 
+  When `default_user_template` is set to `true`, this field can contain an identity
+  template with any prefix or suffix, like `ssh-{{identity.entity.id}}-user`.
+
   For the CA type, if you wish this to be a valid principal, it must also be
   in `allowed_users`.
 
+- `default_user_template` `(bool: false)` - If set, `default_users` can be specified
+  using identity template values. A non-templated user is also permitted.
+
 - `cidr_list` `(string: "")` – Specifies a comma separated list of CIDR blocks
   for which the role is applicable for. It is possible that a same set of CIDR
   blocks are part of multiple roles. This is a required parameter, unless the