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 = ['.*']
```

8 files changed, 87 insertions, 65 deletions

diff --git a/resources/resource_factories/create/create.go b/resources/resource_factories/create/create.go
index 64e2f95a6..f7d0efe64 100644
--- a/resources/resource_factories/create/create.go
+++ b/resources/resource_factories/create/create.go
@@ -154,6 +154,9 @@ func (c *Client) FromString(targetPath, content string) (resource.Resource, erro
 // FromRemote expects one or n-parts of a URL to a resource
 // If you provide multiple parts they will be joined together to the final URL.
 func (c *Client) FromRemote(uri string, options map[string]interface{}) (resource.Resource, error) {
+	if err := c.validateFromRemoteArgs(uri, options); err != nil {
+		return nil, err
+	}
 	rURL, err := url.Parse(uri)
 	if err != nil {
 		return nil, errors.Wrapf(err, "failed to parse URL for resource %s", uri)
@@ -262,6 +265,19 @@ func (c *Client) FromRemote(uri string, options map[string]interface{}) (resourc
 
 }
 
+func (c *Client) validateFromRemoteArgs(uri string, options map[string]interface{}) error {
+	if err := c.rs.ExecHelper.Sec().CheckAllowedHTTPURL(uri); err != nil {
+		return err
+	}
+
+	if method, ok := options["method"].(string); ok {
+		if err := c.rs.ExecHelper.Sec().CheckAllowedHTTPMethod(method); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 func addDefaultHeaders(req *http.Request, accepts ...string) {
 	for _, accept := range accepts {
 		if !hasHeaderValue(req.Header, "Accept", accept) {
diff --git a/resources/resource_spec.go b/resources/resource_spec.go
index 156def363..897c1bbaa 100644
--- a/resources/resource_spec.go
+++ b/resources/resource_spec.go
@@ -26,6 +26,7 @@ import (
 	"github.com/gohugoio/hugo/resources/jsconfig"
 
 	"github.com/gohugoio/hugo/common/herrors"
+	"github.com/gohugoio/hugo/common/hexec"
 
 	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/identity"
@@ -51,6 +52,7 @@ func NewSpec(
 	incr identity.Incrementer,
 	logger loggers.Logger,
 	errorHandler herrors.ErrorSender,
+	execHelper *hexec.Exec,
 	outputFormats output.Formats,
 	mimeTypes media.Types) (*Spec, error) {
 	imgConfig, err := images.DecodeConfig(s.Cfg.GetStringMap("imaging"))
@@ -81,6 +83,7 @@ func NewSpec(
 		Logger:        logger,
 		ErrorSender:   errorHandler,
 		imaging:       imaging,
+		ExecHelper:    execHelper,
 		incr:          incr,
 		MediaTypes:    mimeTypes,
 		OutputFormats: outputFormats,
@@ -120,6 +123,8 @@ type Spec struct {
 	// Holds default filter settings etc.
 	imaging *images.ImageProcessor
 
+	ExecHelper *hexec.Exec
+
 	incr          identity.Incrementer
 	imageCache    *imageCache
 	ResourceCache *ResourceCache
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())
 	}
 
diff --git a/resources/resource_transformers/htesting/testhelpers.go b/resources/resource_transformers/htesting/testhelpers.go
index 21333eccb..674101f03 100644
--- a/resources/resource_transformers/htesting/testhelpers.go
+++ b/resources/resource_transformers/htesting/testhelpers.go
@@ -51,7 +51,7 @@ func NewTestResourceSpec() (*resources.Spec, error) {
 		return nil, err
 	}
 
-	spec, err := resources.NewSpec(s, filecaches, nil, nil, nil, output.DefaultFormats, media.DefaultTypes)
+	spec, err := resources.NewSpec(s, filecaches, nil, nil, nil, nil, output.DefaultFormats, media.DefaultTypes)
 	return spec, err
 }
 
diff --git a/resources/resource_transformers/postcss/postcss.go b/resources/resource_transformers/postcss/postcss.go
index 8104d0336..56cbea156 100644
--- a/resources/resource_transformers/postcss/postcss.go
+++ b/resources/resource_transformers/postcss/postcss.go
@@ -25,8 +25,7 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/cli/safeexec"
-
+	"github.com/gohugoio/hugo/common/collections"
 	"github.com/gohugoio/hugo/common/hexec"
 
 	"github.com/gohugoio/hugo/common/hugo"
@@ -142,22 +141,9 @@ func (t *postcssTransformation) Key() internal.ResourceTransformationKey {
 // npm install -g postcss-cli
 // npm install -g autoprefixer
 func (t *postcssTransformation) Transform(ctx *resources.ResourceTransformationCtx) error {
-	const localPostCSSPath = "node_modules/.bin/"
 	const binaryName = "postcss"
 
-	// Try first in the project's node_modules.
-	csiBinPath := filepath.Join(t.rs.WorkingDir, localPostCSSPath, binaryName)
-
-	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
-		}
-	}
+	ex := t.rs.ExecHelper
 
 	var configFile string
 	logger := t.rs.Logger
@@ -179,29 +165,33 @@ func (t *postcssTransformation) Transform(ctx *resources.ResourceTransformationC
 		}
 	}
 
-	var cmdArgs []string
+	var cmdArgs []interface{}
 
 	if configFile != "" {
 		logger.Infoln("postcss: use config file", configFile)
-		cmdArgs = []string{"--config", configFile}
+		cmdArgs = []interface{}{"--config", configFile}
 	}
 
 	if optArgs := t.options.toArgs(); len(optArgs) > 0 {
-		cmdArgs = append(cmdArgs, optArgs...)
-	}
-
-	cmd, err := hexec.SafeCommand(binary, cmdArgs...)
-	if err != nil {
-		return err
+		cmdArgs = append(cmdArgs, collections.StringSliceToInterfaceSlice(optArgs)...)
 	}
 
 	var errBuf bytes.Buffer
 	infoW := loggers.LoggerToWriterWithPrefix(logger.Info(), "postcss")
 
-	cmd.Stdout = ctx.To
-	cmd.Stderr = io.MultiWriter(infoW, &errBuf)
+	stderr := io.MultiWriter(infoW, &errBuf)
+	cmdArgs = append(cmdArgs, hexec.WithStderr(stderr))
+	cmdArgs = append(cmdArgs, hexec.WithStdout(ctx.To))
+	cmdArgs = append(cmdArgs, hexec.WithEnviron(hugo.GetExecEnviron(t.rs.WorkingDir, t.rs.Cfg, t.rs.BaseFs.Assets.Fs)))
 
-	cmd.Env = hugo.GetExecEnviron(t.rs.WorkingDir, t.rs.Cfg, t.rs.BaseFs.Assets.Fs)
+	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
+	}
 
 	stdin, err := cmd.StdinPipe()
 	if err != nil {
@@ -231,6 +221,9 @@ func (t *postcssTransformation) Transform(ctx *resources.ResourceTransformationC
 
 	err = cmd.Run()
 	if err != nil {
+		if hexec.IsNotFound(err) {
+			return herrors.ErrFeatureNotAvailable
+		}
 		return imp.toFileError(errBuf.String())
 	}
 
diff --git a/resources/resource_transformers/tocss/dartsass/client.go b/resources/resource_transformers/tocss/dartsass/client.go
index 1d8250dc5..c2a572d9b 100644
--- a/resources/resource_transformers/tocss/dartsass/client.go
+++ b/resources/resource_transformers/tocss/dartsass/client.go
@@ -33,8 +33,13 @@ const transformationName = "tocss-dart"
 
 func New(fs *filesystems.SourceFilesystem, rs *resources.Spec) (*Client, error) {
 	if !Supports() {
-		return &Client{dartSassNoAvailable: true}, nil
+		return &Client{dartSassNotAvailable: true}, nil
 	}
+
+	if err := rs.ExecHelper.Sec().CheckAllowedExec(dartSassEmbeddedBinaryName); err != nil {
+		return nil, err
+	}
+
 	transpiler, err := godartsass.Start(godartsass.Options{})
 	if err != nil {
 		return nil, err
@@ -43,15 +48,15 @@ func New(fs *filesystems.SourceFilesystem, rs *resources.Spec) (*Client, error)
 }
 
 type Client struct {
-	dartSassNoAvailable bool
-	rs                  *resources.Spec
-	sfs                 *filesystems.SourceFilesystem
-	workFs              afero.Fs
-	transpiler          *godartsass.Transpiler
+	dartSassNotAvailable bool
+	rs                   *resources.Spec
+	sfs                  *filesystems.SourceFilesystem
+	workFs               afero.Fs
+	transpiler           *godartsass.Transpiler
 }
 
 func (c *Client) ToCSS(res resources.ResourceTransformer, args map[string]interface{}) (resource.Resource, error) {
-	if c.dartSassNoAvailable {
+	if c.dartSassNotAvailable {
 		return res.Transform(resources.NewFeatureNotAvailableTransformer(transformationName, args))
 	}
 	return res.Transform(&transform{c: c, optsm: args})
diff --git a/resources/resource_transformers/tocss/dartsass/transform.go b/resources/resource_transformers/tocss/dartsass/transform.go
index d70d2e6e0..57d9feadb 100644
--- a/resources/resource_transformers/tocss/dartsass/transform.go
+++ b/resources/resource_transformers/tocss/dartsass/transform.go
@@ -21,9 +21,8 @@ import (
 	"path/filepath"
 	"strings"
 
-	"github.com/cli/safeexec"
-
 	"github.com/gohugoio/hugo/common/herrors"
+	"github.com/gohugoio/hugo/common/hexec"
 	"github.com/gohugoio/hugo/htesting"
 	"github.com/gohugoio/hugo/media"
 
@@ -38,16 +37,18 @@ import (
 	"github.com/bep/godartsass"
 )
 
-// See https://github.com/sass/dart-sass-embedded/issues/24
-const stdinPlaceholder = "HUGOSTDIN"
+const (
+	// See https://github.com/sass/dart-sass-embedded/issues/24
+	stdinPlaceholder           = "HUGOSTDIN"
+	dartSassEmbeddedBinaryName = "dart-sass-embedded"
+)
 
 // Supports returns whether dart-sass-embedded is found in $PATH.
 func Supports() bool {
 	if htesting.SupportsAll() {
 		return true
 	}
-	p, err := safeexec.LookPath("dart-sass-embedded")
-	return err == nil && p != ""
+	return hexec.InPath(dartSassEmbeddedBinaryName)
 }
 
 type transform struct {
diff --git a/resources/testhelpers_test.go b/resources/testhelpers_test.go
index 12dc8efe8..14d431644 100644
--- a/resources/testhelpers_test.go
+++ b/resources/testhelpers_test.go
@@ -87,7 +87,7 @@ func newTestResourceSpec(desc specDescriptor) *Spec {
 	filecaches, err := filecache.NewCaches(s)
 	c.Assert(err, qt.IsNil)
 
-	spec, err := NewSpec(s, filecaches, nil, nil, nil, output.DefaultFormats, media.DefaultTypes)
+	spec, err := NewSpec(s, filecaches, nil, nil, nil, nil, output.DefaultFormats, media.DefaultTypes)
 	c.Assert(err, qt.IsNil)
 	return spec
 }
@@ -126,7 +126,7 @@ func newTestResourceOsFs(c *qt.C) (*Spec, string) {
 	filecaches, err := filecache.NewCaches(s)
 	c.Assert(err, qt.IsNil)
 
-	spec, err := NewSpec(s, filecaches, nil, nil, nil, output.DefaultFormats, media.DefaultTypes)
+	spec, err := NewSpec(s, filecaches, nil, nil, nil, nil, output.DefaultFormats, media.DefaultTypes)
 	c.Assert(err, qt.IsNil)
 
 	return spec, workDir