fix(terraform): allow nullable value for default values of vars

pkg/scanners/terraform/parser/evaluator.go

@@ -347,7 +347,7 @@ func (e *evaluator) evaluateVariable(b *terraform.Block) (cty.Value, error) {
 		return cty.NilVal, fmt.Errorf("cannot resolve variable with no attributes")
 	}
 	if def, exists := attributes["default"]; exists {
-		return def.Value(), nil
+		return def.NullableValue(), nil
 	}
 	return cty.NilVal, fmt.Errorf("no value found")
 }

pkg/scanners/terraform/parser/parser_test.go

@@ -633,3 +633,36 @@ module "registry" {
 	require.NoError(t, err)
 	require.Len(t, modules, 2)
 }
+
+func Test_NullDefaultValueForVar(t *testing.T) {
+	fs := testutil.CreateFS(t, map[string]string{
+		"test.tf": `
+variable "bucket_name" {
+  type    = string
+  default = null
+}
+
+resource "aws_s3_bucket" "default" {
+  bucket = var.bucket_name != null ? var.bucket_name : "default"
+}
+`,
+	})
+
+	parser := New(fs, "", OptionStopOnHCLError(true))
+	if err := parser.ParseFS(context.TODO(), "."); err != nil {
+		t.Fatal(err)
+	}
+	modules, _, err := parser.EvaluateAll(context.TODO())
+	require.NoError(t, err)
+	require.Len(t, modules, 1)
+
+	rootModule := modules[0]
+
+	blocks := rootModule.GetResourcesByType("aws_s3_bucket")
+	require.Len(t, blocks, 1)
+	block := blocks[0]
+
+	attr := block.GetAttribute("bucket")
+	require.NotNil(t, attr)
+	assert.Equal(t, "default", attr.Value().AsString())
+}

pkg/terraform/attribute.go

@@ -253,6 +253,23 @@ func (a *Attribute) Value() (ctyVal cty.Value) {
 	return ctyVal
 }
 
+// Allows a null value for a variable https://developer.hashicorp.com/terraform/language/expressions/types#null
+func (a *Attribute) NullableValue() (ctyVal cty.Value) {
+	if a == nil {
+		return cty.NilVal
+	}
+	defer func() {
+		if err := recover(); err != nil {
+			ctyVal = cty.NilVal
+		}
+	}()
+	ctyVal, _ = a.hclAttribute.Expr.Value(a.ctx.Inner())
+	if !ctyVal.IsKnown() {
+		return cty.NilVal
+	}
+	return ctyVal
+}
+
 func (a *Attribute) Name() string {
 	if a == nil {
 		return ""