Do not return ephemeral values to unsupported plugins

Because ephemeral values are likely to contain secrets,
return ErrSensitive for plugins that do not support it
to prevent unintended disclosure.

plugin/server.go

@@ -10,6 +10,7 @@ import (
 	hcl "github.com/hashicorp/hcl/v2"
 	"github.com/terraform-linters/tflint-plugin-sdk/hclext"
 	"github.com/terraform-linters/tflint-plugin-sdk/plugin/plugin2host"
+	"github.com/terraform-linters/tflint-plugin-sdk/terraform/lang/marks"
 	sdk "github.com/terraform-linters/tflint-plugin-sdk/tflint"
 	"github.com/terraform-linters/tflint/terraform"
 	"github.com/terraform-linters/tflint/tflint"
@@ -145,7 +146,10 @@ func (s *GRPCServer) EvaluateExpr(expr hcl.Expression, opts sdk.EvaluateExprOpti
 
 	// SDK v0.16+ introduces client-side handling of unknown/NULL/sensitive values.
 	if s.clientSDKVersion != nil && s.clientSDKVersion.GreaterThanOrEqual(version.Must(version.NewVersion("0.16.0"))) {
-		return val, nil
+		// Before SDK v0.22, ephemeral marks are ignored, so retrun ErrSensitive to prevent secrets ​​from being leaked.
+		if !marks.Contains(val, marks.Ephemeral) || s.clientSDKVersion.GreaterThanOrEqual(version.Must(version.NewVersion("0.22.0"))) {
+			return val, nil
+		}
 	}
 
 	if val.ContainsMarked() {

plugin/server_test.go

@@ -457,6 +457,11 @@ variable "sensitive" {
 	default   = "foo"
 }
 
+variable "ephemeral" {
+	ephemeral = true
+	default   = "foo"
+}
+
 variable "no_default" {}
 
 variable "null" {
@@ -472,6 +477,7 @@ variable "foo" {
 	server := NewGRPCServer(runner, rootRunner, runner.Files(), SDKVersion)
 
 	sdkv15 := version.Must(version.NewVersion("0.15.0"))
+	sdkv21 := version.Must(version.NewVersion("0.21.0"))
 
 	// test util functions
 	hclExpr := func(expr string) hcl.Expression {
@@ -542,6 +548,15 @@ variable "foo" {
 				return err == nil || !errors.Is(err, sdk.ErrSensitive)
 			},
 		},
+		{
+			Name: "sensitive value (SDK v0.21)",
+			Args: func() (hcl.Expression, sdk.EvaluateExprOption) {
+				return hclExpr(`var.sensitive`), sdk.EvaluateExprOption{WantType: &cty.String, ModuleCtx: sdk.SelfModuleCtxType}
+			},
+			Want:       cty.StringVal("foo").Mark(marks.Sensitive),
+			SDKVersion: sdkv21,
+			ErrCheck:   neverHappend,
+		},
 		{
 			Name: "no default",
 			Args: func() (hcl.Expression, sdk.EvaluateExprOption) {
@@ -622,6 +637,37 @@ variable "foo" {
 				return err == nil || !errors.Is(err, sdk.ErrNullValue)
 			},
 		},
+		{
+			Name: "ephemeral value",
+			Args: func() (hcl.Expression, sdk.EvaluateExprOption) {
+				return hclExpr(`var.ephemeral`), sdk.EvaluateExprOption{WantType: &cty.String, ModuleCtx: sdk.SelfModuleCtxType}
+			},
+			Want:     cty.StringVal("foo").Mark(marks.Ephemeral),
+			ErrCheck: neverHappend,
+		},
+		{
+			Name: "ephemeral value (SDK v0.21)",
+			Args: func() (hcl.Expression, sdk.EvaluateExprOption) {
+				return hclExpr(`var.ephemeral`), sdk.EvaluateExprOption{WantType: &cty.String, ModuleCtx: sdk.SelfModuleCtxType}
+			},
+			Want:       cty.NullVal(cty.NilType),
+			SDKVersion: sdkv21,
+			ErrCheck: func(err error) bool {
+				return err == nil || !errors.Is(err, sdk.ErrSensitive)
+			},
+		},
+		{
+			Name: "ephemeral value in object (SDK v0.21)",
+			Args: func() (hcl.Expression, sdk.EvaluateExprOption) {
+				ty := cty.Object(map[string]cty.Type{"value": cty.String})
+				return hclExpr(`{ value = var.ephemeral }`), sdk.EvaluateExprOption{WantType: &ty, ModuleCtx: sdk.SelfModuleCtxType}
+			},
+			Want:       cty.NullVal(cty.NilType),
+			SDKVersion: sdkv21,
+			ErrCheck: func(err error) bool {
+				return err == nil || !errors.Is(err, sdk.ErrSensitive)
+			},
+		},
 	}
 
 	for _, test := range tests {