Add some basic security policies with sensible defaults

This ommmit contains some security hardening measures for the Hugo build runtime.

There are some rarely used features in Hugo that would be good to have disabled by default. One example would be the "external helpers".

For `asciidoctor` and some others we use Go's `os/exec` package to start a new process.

These are a predefined set of binary names, all loaded from `PATH` and with a predefined set of arguments. Still, if you don't use `asciidoctor` in your project, you might as well have it turned off.

You can configure your own in the new `security` configuration section, but the defaults are configured to create a minimal amount of site breakage. And if that do happen, you will get clear instructions in the loa about what to do.

The default configuration is listed below. Note that almost all of these options are regular expression _whitelists_ (a string or a slice); the value `none` will block all.

```toml
[security]
  enableInlineShortcodes = false
  [security.exec]
    allow = ['^dart-sass-embedded$', '^go$', '^npx$', '^postcss$']
    osEnv = ['(?i)^(PATH|PATHEXT|APPDATA|TMP|TEMP|TERM)$']

  [security.funcs]
    getenv = ['^HUGO_']

  [security.http]
    methods = ['(?i)GET|POST']
    urls = ['.*']
```

1 files changed, 24 insertions, 22 deletions

diff --git a/resources/resource_transformers/babel/babel.go b/resources/resource_transformers/babel/babel.go
index e291b210b..c20a131f6 100644
--- a/resources/resource_transformers/babel/babel.go
+++ b/resources/resource_transformers/babel/babel.go
@@ -23,7 +23,6 @@ import (
 	"regexp"
 	"strconv"
 
-	"github.com/cli/safeexec"
 	"github.com/gohugoio/hugo/common/hexec"
 	"github.com/gohugoio/hugo/common/loggers"
 
@@ -59,8 +58,8 @@ func DecodeOptions(m map[string]interface{}) (opts Options, err error) {
 	return
 }
 
-func (opts Options) toArgs() []string {
-	var args []string
+func (opts Options) toArgs() []interface{} {
+	var args []interface{}
 
 	// external is not a known constant on the babel command line
 	// .sourceMaps must be a boolean, "inline", "both", or undefined
@@ -115,21 +114,12 @@ func (t *babelTransformation) Key() internal.ResourceTransformationKey {
 // npm install -g @babel/preset-env
 // Instead of installing globally, you can also install everything as a dev-dependency (--save-dev instead of -g)
 func (t *babelTransformation) Transform(ctx *resources.ResourceTransformationCtx) error {
-	const localBabelPath = "node_modules/.bin/"
 	const binaryName = "babel"
 
-	// Try first in the project's node_modules.
-	csiBinPath := filepath.Join(t.rs.WorkingDir, localBabelPath, binaryName)
+	ex := t.rs.ExecHelper
 
-	binary := csiBinPath
-
-	if _, err := safeexec.LookPath(binary); err != nil {
-		// Try PATH
-		binary = binaryName
-		if _, err := safeexec.LookPath(binary); err != nil {
-			// This may be on a CI server etc. Will fall back to pre-built assets.
-			return herrors.ErrFeatureNotAvailable
-		}
+	if err := ex.Sec().CheckAllowedExec(binaryName); err != nil {
+		return err
 	}
 
 	var configFile string
@@ -157,11 +147,11 @@ func (t *babelTransformation) Transform(ctx *resources.ResourceTransformationCtx
 
 	ctx.ReplaceOutPathExtension(".js")
 
-	var cmdArgs []string
+	var cmdArgs []interface{}
 
 	if configFile != "" {
 		logger.Infoln("babel: use config file", configFile)
-		cmdArgs = []string{"--config-file", configFile}
+		cmdArgs = []interface{}{"--config-file", configFile}
 	}
 
 	if optArgs := t.options.toArgs(); len(optArgs) > 0 {
@@ -178,18 +168,27 @@ func (t *babelTransformation) Transform(ctx *resources.ResourceTransformationCtx
 	}
 
 	cmdArgs = append(cmdArgs, "--out-file="+compileOutput.Name())
+	stderr := io.MultiWriter(infoW, &errBuf)
+	cmdArgs = append(cmdArgs, hexec.WithStderr(stderr))
+	cmdArgs = append(cmdArgs, hexec.WithStdout(stderr))
+	cmdArgs = append(cmdArgs, hexec.WithEnviron(hugo.GetExecEnviron(t.rs.WorkingDir, t.rs.Cfg, t.rs.BaseFs.Assets.Fs)))
+
 	defer os.Remove(compileOutput.Name())
 
-	cmd, err := hexec.SafeCommand(binary, cmdArgs...)
+	// ARGA [--no-install babel --config-file /private/var/folders/_g/j3j21hts4fn7__h04w2x8gb40000gn/T/hugo-test-babel812882892/babel.config.js --source-maps --filename=js/main2.js --out-file=/var/folders/_g/j3j21hts4fn7__h04w2x8gb40000gn/T/compileOut-2237820197.js]
+	//      [--no-install babel --config-file /private/var/folders/_g/j3j21hts4fn7__h04w2x8gb40000gn/T/hugo-test-babel332846848/babel.config.js --filename=js/main.js --out-file=/var/folders/_g/j3j21hts4fn7__h04w2x8gb40000gn/T/compileOut-1451390834.js 0x10304ee60 0x10304ed60 0x10304f060]
+	cmd, err := ex.Npx(binaryName, cmdArgs...)
+
 	if err != nil {
+		if hexec.IsNotFound(err) {
+			// This may be on a CI server etc. Will fall back to pre-built assets.
+			return herrors.ErrFeatureNotAvailable
+		}
 		return err
 	}
 
-	cmd.Stderr = io.MultiWriter(infoW, &errBuf)
-	cmd.Stdout = cmd.Stderr
-	cmd.Env = hugo.GetExecEnviron(t.rs.WorkingDir, t.rs.Cfg, t.rs.BaseFs.Assets.Fs)
-
 	stdin, err := cmd.StdinPipe()
+
 	if err != nil {
 		return err
 	}
@@ -201,6 +200,9 @@ func (t *babelTransformation) Transform(ctx *resources.ResourceTransformationCtx
 
 	err = cmd.Run()
 	if err != nil {
+		if hexec.IsNotFound(err) {
+			return herrors.ErrFeatureNotAvailable
+		}
 		return errors.Wrap(err, errBuf.String())
 	}