cli: Only allow HTTPS URLs with `inject` (#7940)

The CLI may access manifests over insecure channels, potentially
allowing MITM-attacks to run arbitrary code.

This change modifies `inject` to only support `https` URLs to mitigate
this risk.

This change addresses a security review finding (`TOB-LKDTM-4`).

Signed-off-by: Oliver Gould <ver@buoyant.io>

cli/cmd/inject.go

@@ -70,7 +70,7 @@ sub-folders, or coming from stdin.`,
   kubectl get deploy -o yaml | linkerd inject - | kubectl apply -f -
 
   # Injecting a file from a remote URL
-  linkerd inject http://url.to/yml | kubectl apply -f -
+  linkerd inject https://url.to/yml | kubectl apply -f -
 
   # Inject all the resources inside a folder and its sub-folders.
   linkerd inject <folder> | kubectl apply -f -`,

cli/cmd/inject_test.go

@@ -577,7 +577,7 @@ func TestInjectFilePath(t *testing.T) {
 	})
 }
 
-func TestValidURL(t *testing.T) {
+func TestToURL(t *testing.T) {
 	// if the string follows a URL pattern, true has to be returned
 	// if not false is returned
 
@@ -593,9 +593,9 @@ func TestValidURL(t *testing.T) {
 	}
 
 	for url, expectedValue := range tests {
-		value := isValidURL(url)
-		if value != expectedValue {
-			t.Errorf("Result mismatch for %s. expected %v, but got %v", url, expectedValue, value)
+		_, ok := toURL(url)
+		if ok != expectedValue {
+			t.Errorf("Result mismatch for %s. expected %v, but got %v", url, expectedValue, ok)
 		}
 	}
 

cli/cmd/inject_util.go

@@ -10,6 +10,7 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/linkerd/linkerd2/pkg/inject"
 	corev1 "k8s.io/api/core/v1"
@@ -142,17 +143,19 @@ func processList(bytes []byte, rt resourceTransformer) ([]byte, []inject.Report,
 // Read all the resource files found in path into a slice of readers.
 // path can be either a file, directory or stdin.
 func read(path string) ([]io.Reader, error) {
-	var (
-		in  []io.Reader
-		err error
-	)
 	if path == "-" {
-		in = append(in, os.Stdin)
-	} else if isValidURL(path) {
-		resp, err := http.Get(path)
+		return []io.Reader{os.Stdin}, nil
+	}
+
+	if url, ok := toURL(path); ok {
+		if strings.ToLower(url.Scheme) != "https" {
+			return nil, fmt.Errorf("only HTTPS URLs are allowed")
+		}
+		resp, err := http.Get(url.String())
 		if err != nil {
 			return nil, err
 		}
+		defer resp.Body.Close()
 
 		if resp.StatusCode != http.StatusOK {
 			return nil, fmt.Errorf("unable to read URL %q, server reported %s, status code=%d", path, resp.Status, resp.StatusCode)
@@ -164,26 +167,21 @@ func read(path string) ([]io.Reader, error) {
 		if err != nil {
 			return nil, err
 		}
-		resp.Body.Close()
-		in = append(in, buf)
-	} else {
-		in, err = walk(path)
-		if err != nil {
-			return nil, err
-		}
+
+		return []io.Reader{buf}, nil
 	}
 
-	return in, nil
+	return walk(path)
 }
 
 // checks if the given string is a valid URL
-func isValidURL(path string) bool {
+func toURL(path string) (*url.URL, bool) {
 	u, err := url.ParseRequestURI(path)
-	if err != nil {
-		return false
+	if err == nil && u.Host != "" && u.Scheme != "" {
+		return u, true
 	}
 
-	return u.Host != "" && u.Scheme != ""
+	return nil, false
 }
 
 // walk walks the file tree rooted at path. path may be a file or a directory.