From a7b88cf1764d6056bebee1519660e104340c4ee1 Mon Sep 17 00:00:00 2001
From: David Levy <dlevy@microsoft.com>
Date: Thu, 5 Feb 2026 20:08:08 -0600
Subject: [PATCH] fix: validate config file extension before use

Provide clear error message when user specifies a config file
with unsupported extension (e.g., .txt, .json). Only .yaml,
.yml, and no extension are allowed.
---
 internal/config/config.go     | 14 ++++++++---
 internal/config/viper.go      | 24 +++++++++++++++++-
 internal/config/viper_test.go | 47 +++++++++++++++++++++++++++++++++--
 3 files changed, 78 insertions(+), 7 deletions(-)

diff --git a/internal/config/config.go b/internal/config/config.go
index 1b24695c..0bc9490b 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -4,13 +4,14 @@
 package config
 
 import (
+	"os"
+	"path/filepath"
+	"testing"
+
 	. "github.com/microsoft/go-sqlcmd/cmd/modern/sqlconfig"
 	"github.com/microsoft/go-sqlcmd/internal/io/file"
 	"github.com/microsoft/go-sqlcmd/internal/io/folder"
 	"github.com/microsoft/go-sqlcmd/internal/pal"
-	"os"
-	"path/filepath"
-	"testing"
 )
 
 var config Sqlconfig
@@ -26,8 +27,13 @@ func SetFileName(name string) {
 
 	filename = name
 
+	// Validate extension before creating the file
+	err := validateConfigFileExtension(filename)
+	checkErr(err)
+
 	file.CreateEmptyIfNotExists(filename)
-	configureViper(filename)
+	err = configureViper(filename)
+	checkErr(err)
 }
 
 func SetFileNameForTest(t *testing.T) {
diff --git a/internal/config/viper.go b/internal/config/viper.go
index 9409fd8d..33a9b373 100644
--- a/internal/config/viper.go
+++ b/internal/config/viper.go
@@ -5,6 +5,10 @@ package config
 
 import (
 	"bytes"
+	"path/filepath"
+	"strings"
+
+	"github.com/microsoft/go-sqlcmd/internal/localizer"
 	"github.com/microsoft/go-sqlcmd/internal/pal"
 	"github.com/spf13/viper"
 	"gopkg.in/yaml.v2"
@@ -56,16 +60,34 @@ func GetConfigFileUsed() string {
 	return viper.ConfigFileUsed()
 }
 
+// validateConfigFileExtension checks if the config file has a supported extension.
+// Allows .yaml, .yml, and no extension (for default sqlconfig file).
+func validateConfigFileExtension(configFile string) error {
+	ext := strings.ToLower(filepath.Ext(configFile))
+	if ext == "" || ext == ".yaml" || ext == ".yml" {
+		return nil
+	}
+	return localizer.Errorf(
+		"Configuration files must use YAML format (.yaml or .yml extension). "+
+			"File '%s' has unsupported extension '%s'.",
+		configFile, ext)
+}
+
 // configureViper initializes the Viper library with the given configuration file.
 // This function sets the configuration file type to "yaml" and sets the environment variable prefix to "SQLCMD".
 // It also sets the configuration file to use to the one provided as an argument to the function.
 // This function is intended to be called at the start of the application to configure Viper before any other code uses it.
-func configureViper(configFile string) {
+func configureViper(configFile string) error {
 	if configFile == "" {
 		panic("Must provide configFile")
 	}
 
+	if err := validateConfigFileExtension(configFile); err != nil {
+		return err
+	}
+
 	viper.SetConfigType("yaml")
 	viper.SetEnvPrefix("SQLCMD")
 	viper.SetConfigFile(configFile)
+	return nil
 }
diff --git a/internal/config/viper_test.go b/internal/config/viper_test.go
index 3a606f6d..a4ddc755 100644
--- a/internal/config/viper_test.go
+++ b/internal/config/viper_test.go
@@ -4,16 +4,59 @@
 package config
 
 import (
-	"github.com/stretchr/testify/assert"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func Test_configureViper(t *testing.T) {
 	assert.Panics(t, func() {
-		configureViper("")
+		_ = configureViper("")
 	})
 }
 
+func Test_configureViperValidExtensions(t *testing.T) {
+	tests := []string{"config.yaml", "config.yml", "sqlconfig", "/path/to/config.YAML"}
+	for _, name := range tests {
+		t.Run(name, func(t *testing.T) {
+			err := configureViper(name)
+			assert.NoError(t, err)
+		})
+	}
+}
+
+func Test_configureViperInvalidExtension(t *testing.T) {
+	err := configureViper("config.txt")
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "YAML format")
+	assert.Contains(t, err.Error(), ".txt")
+}
+
+func Test_validateConfigFileExtension(t *testing.T) {
+	tests := []struct {
+		name    string
+		file    string
+		wantErr bool
+	}{
+		{"yaml extension", "config.yaml", false},
+		{"yml extension", "config.yml", false},
+		{"no extension", "sqlconfig", false},
+		{"uppercase YAML", "config.YAML", false},
+		{"txt extension", "config.txt", true},
+		{"json extension", "config.json", true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := validateConfigFileExtension(tt.file)
+			if tt.wantErr {
+				assert.Error(t, err)
+			} else {
+				assert.NoError(t, err)
+			}
+		})
+	}
+}
+
 func Test_Load(t *testing.T) {
 	SetFileNameForTest(t)
 	Clean()