fix: restrict cmd template function for remote dependency configs

Prevent arbitrary command execution from untrusted remote configuration
dependencies by disabling the cmd template function when processing
remote configs.

Author: markoyoungson

pkg/skaffold/parser/config.go

@@ -347,6 +347,12 @@ func processEachDependency(ctx context.Context, d latest.ConfigDependency, cfgOp
 	cfgOpts.file = path
 	cfgOpts.selection = d.Names
 	cfgOpts.isRemote = isRemoteCfg
+	// Disable the `cmd` template function for remote/untrusted config dependencies
+	// to prevent arbitrary command execution via malicious skaffold.yaml files.
+	if isRemoteCfg {
+		util.CmdAllowed = false
+		defer func() { util.CmdAllowed = true }()
+	}
 	depConfigs, _, err := getConfigs(ctx, cfgOpts, opts, r)
 	if err != nil {
 		return nil, err

pkg/skaffold/util/env_template.go

@@ -37,6 +37,10 @@ var (
 	funcsMap  = template.FuncMap{
 		"cmd": runCmdFunc,
 	}
+	// CmdAllowed controls whether the `cmd` template function is permitted.
+	// Set to false when processing remote/untrusted config dependencies to
+	// prevent arbitrary command execution.
+	CmdAllowed = true
 )
 
 // ExpandEnvTemplate parses and executes template s with an optional environment map
@@ -140,6 +144,9 @@ func MapToFlag(m map[string]*string, flag string) ([]string, error) {
 }
 
 func runCmdFunc(name string, args ...string) (string, error) {
+	if !CmdAllowed {
+		return "", fmt.Errorf("the 'cmd' template function is not allowed in remote dependency configs for security reasons")
+	}
 	cmd := exec.Command(name, args...)
 	out, err := RunCmdOut(context.TODO(), cmd)
 	return strings.TrimSpace(string(out)), err

pkg/skaffold/util/env_template_test.go

@@ -18,6 +18,7 @@ package util
 
 import (
 	"fmt"
+	"strings"
 	"testing"
 
 	"github.com/GoogleContainerTools/skaffold/v2/testutil"
@@ -180,6 +181,25 @@ func TestMapToFlag(t *testing.T) {
 	}
 }
 
+func TestCmdBlockedWhenNotAllowed(t *testing.T) {
+	testutil.Run(t, "cmd blocked in remote config context", func(t *testutil.T) {
+		t.Override(&CmdAllowed, false)
+		_, err := ExpandEnvTemplate(`{{cmd "echo" "hello"}}`, nil)
+		t.CheckError(true, err)
+		if err != nil && !strings.Contains(err.Error(), "not allowed in remote dependency") {
+			t.Errorf("expected 'not allowed in remote dependency' error, got: %v", err)
+		}
+	})
+
+	testutil.Run(t, "cmd allowed in local config context", func(t *testutil.T) {
+		t.Override(&CmdAllowed, true)
+		t.Override(&DefaultExecCommand, testutil.CmdRunOut("echo hello", "hello"))
+		out, err := ExpandEnvTemplate(`{{cmd "echo" "hello"}}`, nil)
+		t.CheckNoError(err)
+		t.CheckDeepEqual("hello", out)
+	})
+}
+
 func TestRunCmdFunc(t *testing.T) {
 	tests := []struct {
 		description     string