diff --git a/client/cmd/service.go b/client/cmd/service.go
index 7410d60ea..2a558e6d5 100644
--- a/client/cmd/service.go
+++ b/client/cmd/service.go
@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"net/http"
 	"runtime"
+	"slices"
 	"strings"
 	"sync"
 
@@ -25,6 +26,30 @@ var serviceCmd = &cobra.Command{
 
 const defaultJSONSocket = "unix:///var/run/netbird-http.sock"
 
+// forbiddenServiceEnvVars are the environment variables the service is never
+// registered with, keyed in upper case since these are Windows names. Each one
+// decides where the daemon resolves something it then uses with the privileges
+// of the account it runs under — LocalSystem on Windows, root elsewhere: the
+// executables it runs (PATH, PATHEXT, COMSPEC, SystemRoot, windir) or the
+// directory it writes temporary files in (TEMP, TMP). The daemon needs none of
+// them, and the utilities it shells out to are resolved by absolute path.
+var forbiddenServiceEnvVars = map[string]struct{}{
+	"PATH":       {},
+	"PATHEXT":    {},
+	"SYSTEMROOT": {},
+	"WINDIR":     {},
+	"COMSPEC":    {},
+	"TEMP":       {},
+	"TMP":        {},
+}
+
+// forbiddenServiceEnvPrefixes are the dynamic-loader families, refused whole
+// rather than by name: LD_PRELOAD, DYLD_INSERT_LIBRARIES and their siblings all
+// reach the loader of the process, the set differs per platform and libc, and
+// new members arrive with new OS releases. Listing them one by one is a list
+// that is wrong the moment it is written.
+var forbiddenServiceEnvPrefixes = []string{"LD_", "DYLD_"}
+
 var (
 	serviceName      string
 	serviceEnvVars   []string
@@ -127,8 +152,33 @@ func parseServiceEnvVars(envVars []string) (map[string]string, error) {
 			return nil, fmt.Errorf("empty environment variable key in: %s", env)
 		}
 
+		if isForbiddenServiceEnvVar(key) {
+			return nil, fmt.Errorf("environment variable %s cannot be set on the service: it decides where the service resolves the executables, libraries or temporary files it uses", key)
+		}
+
 		envMap[key] = value
 	}
 
 	return envMap, nil
 }
+
+// isForbiddenServiceEnvVar reports whether name is one the service must not be
+// registered with.
+//
+// The names are matched case-insensitively only on Windows, where they are the
+// same variable however they are spelled. Elsewhere the environment is
+// case-sensitive, so Path and PATH are two different variables and only the
+// exact spelling is the one the loader reads.
+func isForbiddenServiceEnvVar(name string) bool {
+	if runtime.GOOS == "windows" {
+		name = strings.ToUpper(name)
+	}
+
+	if _, forbidden := forbiddenServiceEnvVars[name]; forbidden {
+		return true
+	}
+
+	return slices.ContainsFunc(forbiddenServiceEnvPrefixes, func(prefix string) bool {
+		return strings.HasPrefix(name, prefix)
+	})
+}
diff --git a/client/cmd/service_params.go b/client/cmd/service_params.go
index 750b22ae6..6e2dbec40 100644
--- a/client/cmd/service_params.go
+++ b/client/cmd/service_params.go
@@ -14,6 +14,7 @@ import (
 
 	"github.com/netbirdio/netbird/client/configs"
 	"github.com/netbirdio/netbird/client/internal/daemonaddr"
+	"github.com/netbirdio/netbird/client/internal/elevate"
 	"github.com/netbirdio/netbird/util"
 )
 
@@ -43,10 +44,33 @@ func serviceParamsPath() string {
 
 // loadServiceParams reads saved service parameters from disk.
 // Returns nil with no error if the file does not exist.
+//
+// The file is read by an elevated install and decides the arguments and the
+// environment of the service it then registers, so it is used only when its
+// ownership and permissions are the ones saveServiceParams leaves behind. That
+// restricted ACL is applied when the file is written, which is not necessarily
+// before it is first read, so this is checked rather than assumed. A file that
+// fails the check is treated as absent, and the install proceeds with its
+// defaults.
 func loadServiceParams() (*serviceParams, error) {
 	path := serviceParamsPath()
 
-	data, err := os.ReadFile(path)
+	// Resolve links first so the checks apply to the file that is actually read.
+	// Since the check covers every directory above it as well, nobody who fails
+	// it can swap the file between here and the read below.
+	resolved, err := filepath.EvalSymlinks(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil //nolint:nilnil
+		}
+		return nil, fmt.Errorf("resolve service params %s: %w", path, err)
+	}
+
+	if err := elevate.CheckOnlyOwnerWritable(resolved); err != nil {
+		return nil, fmt.Errorf("refusing to read service params from %s: %w", resolved, err)
+	}
+
+	data, err := os.ReadFile(resolved)
 	if err != nil {
 		if os.IsNotExist(err) {
 			return nil, nil //nolint:nilnil
@@ -182,10 +206,16 @@ func applyServiceParams(cmd *cobra.Command, params *serviceParams) {
 // If --service-env was explicitly set to empty, all saved env vars are cleared.
 // If --service-env was not set, saved env vars are used entirely.
 func applyServiceEnvParams(cmd *cobra.Command, params *serviceParams) {
+	// A forbidden name explicitly passed on the command line is an error the
+	// operator is told about, but one restored from a file written by an older
+	// version is dropped: an install that refuses to run would leave the host
+	// without a daemon over a variable nobody is asking for any more.
+	saved := dropForbiddenServiceEnvVars(cmd, params.ServiceEnvVars)
+
 	if !cmd.Flags().Changed("service-env") {
-		if len(params.ServiceEnvVars) > 0 {
+		if len(saved) > 0 {
 			// No explicit env vars: rebuild serviceEnvVars from saved params.
-			serviceEnvVars = envMapToSlice(params.ServiceEnvVars)
+			serviceEnvVars = envMapToSlice(saved)
 		}
 		return
 	}
@@ -204,13 +234,13 @@ func applyServiceEnvParams(cmd *cobra.Command, params *serviceParams) {
 		return
 	}
 
-	if len(params.ServiceEnvVars) == 0 {
+	if len(saved) == 0 {
 		return
 	}
 
 	// Merge saved values underneath explicit ones.
-	merged := make(map[string]string, len(params.ServiceEnvVars)+len(explicit))
-	maps.Copy(merged, params.ServiceEnvVars)
+	merged := make(map[string]string, len(saved)+len(explicit))
+	maps.Copy(merged, saved)
 	maps.Copy(merged, explicit) // explicit wins on conflict
 	serviceEnvVars = envMapToSlice(merged)
 }
@@ -233,6 +263,20 @@ var resetParamsCmd = &cobra.Command{
 	},
 }
 
+// dropForbiddenServiceEnvVars returns the saved entries that may still be
+// registered on the service, reporting every one it leaves behind.
+func dropForbiddenServiceEnvVars(cmd *cobra.Command, saved map[string]string) map[string]string {
+	kept := make(map[string]string, len(saved))
+	for key, value := range saved {
+		if isForbiddenServiceEnvVar(key) {
+			cmd.PrintErrf("Warning: ignoring saved service environment variable %s: it decides where the service resolves the executables, libraries or temporary files it uses\n", key)
+			continue
+		}
+		kept[key] = value
+	}
+	return kept
+}
+
 // envMapToSlice converts a map of env vars to a KEY=VALUE slice.
 func envMapToSlice(m map[string]string) []string {
 	s := make([]string, 0, len(m))
diff --git a/client/cmd/service_params_test.go b/client/cmd/service_params_test.go
index 94f98a0ce..1f83374cb 100644
--- a/client/cmd/service_params_test.go
+++ b/client/cmd/service_params_test.go
@@ -9,6 +9,7 @@ import (
 	"go/token"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 
@@ -353,6 +354,59 @@ func TestApplyServiceEnvParams_NotChanged(t *testing.T) {
 	assert.Equal(t, map[string]string{"FROM_SAVED": "val"}, result)
 }
 
+func TestParseServiceEnvVars_RejectsForbiddenNames(t *testing.T) {
+	for _, env := range []string{"PATH=C:\\somewhere", "LD_PRELOAD=/tmp/lib.so", "DYLD_FALLBACK_LIBRARY_PATH=/tmp"} {
+		_, err := parseServiceEnvVars([]string{"KEEP=me", env})
+		require.Errorf(t, err, "%s selects what the service resolves and must be refused", env)
+	}
+}
+
+func TestIsForbiddenServiceEnvVar(t *testing.T) {
+	// The loader families are matched by prefix, so a name nobody has heard of
+	// yet is refused too.
+	for _, name := range []string{
+		"PATH", "PATHEXT", "COMSPEC", "SYSTEMROOT", "WINDIR", "TEMP", "TMP",
+		"LD_PRELOAD", "LD_AUDIT", "DYLD_INSERT_LIBRARIES", "DYLD_FALLBACK_FRAMEWORK_PATH",
+	} {
+		assert.Truef(t, isForbiddenServiceEnvVar(name), "%s must be refused", name)
+	}
+
+	// The prefix must not swallow names that merely start with the same letters.
+	for _, name := range []string{"NB_LOG_LEVEL", "NB_WG_DEBUG", "HTTPS_PROXY", "LDAP_URL", "DYLDX"} {
+		assert.Falsef(t, isForbiddenServiceEnvVar(name), "%s has no reason to be refused", name)
+	}
+
+	// On Windows a variable is the same one however it is spelled; elsewhere
+	// Path and PATH are two variables and only the exact one is read.
+	if runtime.GOOS == "windows" {
+		assert.True(t, isForbiddenServiceEnvVar("Path"))
+		assert.True(t, isForbiddenServiceEnvVar("ld_preload"))
+	} else {
+		assert.False(t, isForbiddenServiceEnvVar("Path"))
+		assert.False(t, isForbiddenServiceEnvVar("ld_preload"))
+	}
+}
+
+func TestApplyServiceEnvParams_DropsForbiddenSavedNames(t *testing.T) {
+	origServiceEnvVars := serviceEnvVars
+	t.Cleanup(func() { serviceEnvVars = origServiceEnvVars })
+
+	serviceEnvVars = nil
+
+	cmd := &cobra.Command{}
+	cmd.Flags().StringSlice("service-env", nil, "")
+
+	saved := &serviceParams{
+		ServiceEnvVars: map[string]string{"PATH": "C:\\attacker", "NB_LOG_FORMAT": "json"},
+	}
+
+	applyServiceEnvParams(cmd, saved)
+
+	result, err := parseServiceEnvVars(serviceEnvVars)
+	require.NoError(t, err, "a saved PATH must be dropped rather than fail the install")
+	assert.Equal(t, map[string]string{"NB_LOG_FORMAT": "json"}, result)
+}
+
 func TestApplyServiceEnvParams_ExplicitEmptyClears(t *testing.T) {
 	origServiceEnvVars := serviceEnvVars
 	t.Cleanup(func() { serviceEnvVars = origServiceEnvVars })
diff --git a/client/cmd/service_params_trust_test.go b/client/cmd/service_params_trust_test.go
new file mode 100644
index 000000000..1cf564445
--- /dev/null
+++ b/client/cmd/service_params_trust_test.go
@@ -0,0 +1,57 @@
+//go:build !windows && !ios && !android
+
+package cmd
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/netbirdio/netbird/client/configs"
+)
+
+// The Windows equivalent of this is the ACL check in
+// elevate.CheckOnlyOwnerWritable, covered by that package's own tests; here the
+// point is that loadServiceParams asks the question at all.
+func TestLoadServiceParams_RefusesWorldWritableFile(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	original := configs.StateDir
+	t.Cleanup(func() { configs.StateDir = original })
+	configs.StateDir = tmpDir
+
+	path := filepath.Join(tmpDir, serviceParamsFile)
+	require.NoError(t, os.WriteFile(path, []byte(`{"log_level":"debug"}`), 0o666))
+	// WriteFile is subject to the umask, so set the bits that matter explicitly.
+	require.NoError(t, os.Chmod(path, 0o666))
+
+	params, err := loadServiceParams()
+	require.Error(t, err, "a service.json anyone can rewrite must not be trusted")
+	assert.Nil(t, params)
+
+	require.NoError(t, os.Chmod(path, 0o600))
+	params, err = loadServiceParams()
+	require.NoError(t, err)
+	require.NotNil(t, params)
+	assert.Equal(t, "debug", params.LogLevel)
+}
+
+func TestLoadServiceParams_RefusesWorldWritableDirectory(t *testing.T) {
+	tmpDir := t.TempDir()
+	stateDir := filepath.Join(tmpDir, "state")
+	require.NoError(t, os.Mkdir(stateDir, 0o777))
+	require.NoError(t, os.Chmod(stateDir, 0o777))
+
+	original := configs.StateDir
+	t.Cleanup(func() { configs.StateDir = original })
+	configs.StateDir = stateDir
+
+	require.NoError(t, os.WriteFile(filepath.Join(stateDir, serviceParamsFile), []byte(`{}`), 0o600))
+
+	params, err := loadServiceParams()
+	require.Error(t, err, "a service.json in a directory anyone can replace entries in must not be trusted")
+	assert.Nil(t, params)
+}
diff --git a/client/firewall/uspfilter/interface_allower_windows.go b/client/firewall/uspfilter/interface_allower_windows.go
index 7f525e28c..4cd0fe969 100644
--- a/client/firewall/uspfilter/interface_allower_windows.go
+++ b/client/firewall/uspfilter/interface_allower_windows.go
@@ -9,6 +9,7 @@ import (
 	log "github.com/sirupsen/logrus"
 
 	nberrors "github.com/netbirdio/netbird/client/errors"
+	"github.com/netbirdio/netbird/client/internal/wincmd"
 )
 
 type action string
@@ -91,7 +92,7 @@ func manageFirewallRule(ruleName string, action action, extraArgs ...string) err
 	if action == addRule {
 		args = append(args, extraArgs...)
 	}
-	netshCmd := GetSystem32Command("netsh")
+	netshCmd := wincmd.System32("netsh")
 	cmd := exec.Command(netshCmd, args...)
 	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
 	return cmd.Run()
@@ -100,7 +101,7 @@ func manageFirewallRule(ruleName string, action action, extraArgs ...string) err
 func isWindowsFirewallReachable() bool {
 	args := []string{"advfirewall", "show", "allprofiles", "state"}
 
-	netshCmd := GetSystem32Command("netsh")
+	netshCmd := wincmd.System32("netsh")
 
 	cmd := exec.Command(netshCmd, args...)
 	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
@@ -117,23 +118,10 @@ func isWindowsFirewallReachable() bool {
 func isFirewallRuleActive(ruleName string) bool {
 	args := []string{"advfirewall", "firewall", "show", "rule", "name=" + ruleName}
 
-	netshCmd := GetSystem32Command("netsh")
+	netshCmd := wincmd.System32("netsh")
 
 	cmd := exec.Command(netshCmd, args...)
 	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
 	_, err := cmd.Output()
 	return err == nil
 }
-
-// GetSystem32Command checks if a command can be found in the system path and returns it. In case it can't find it
-// in the path it will return the full path of a command assuming C:\windows\system32 as the base path.
-func GetSystem32Command(command string) string {
-	_, err := exec.LookPath(command)
-	if err == nil {
-		return command
-	}
-
-	log.Tracef("Command %s not found in PATH, using C:\\windows\\system32\\%s.exe path", command, command)
-
-	return "C:\\windows\\system32\\" + command + ".exe"
-}
diff --git a/client/iface/iface_destroy_windows.go b/client/iface/iface_destroy_windows.go
index 0bfa4e211..54c0014c4 100644
--- a/client/iface/iface_destroy_windows.go
+++ b/client/iface/iface_destroy_windows.go
@@ -6,27 +6,14 @@ import (
 	"fmt"
 	"os/exec"
 
-	log "github.com/sirupsen/logrus"
+	"github.com/netbirdio/netbird/client/internal/wincmd"
 )
 
 func (w *WGIface) Destroy() error {
-	netshCmd := GetSystem32Command("netsh")
+	netshCmd := wincmd.System32("netsh")
 	out, err := exec.Command(netshCmd, "interface", "set", "interface", w.Name(), "admin=disable").CombinedOutput()
 	if err != nil {
 		return fmt.Errorf("failed to remove interface %s: %w - %s", w.Name(), err, out)
 	}
 	return nil
 }
-
-// GetSystem32Command checks if a command can be found in the system path and returns it. In case it can't find it
-// in the path it will return the full path of a command assuming C:\windows\system32 as the base path.
-func GetSystem32Command(command string) string {
-	_, err := exec.LookPath(command)
-	if err == nil {
-		return command
-	}
-
-	log.Tracef("Command %s not found in PATH, using C:\\windows\\system32\\%s.exe path", command, command)
-
-	return "C:\\windows\\system32\\" + command + ".exe"
-}
diff --git a/client/internal/elevate/trusted.go b/client/internal/elevate/trusted.go
index c11054c45..98e05fde5 100644
--- a/client/internal/elevate/trusted.go
+++ b/client/internal/elevate/trusted.go
@@ -6,6 +6,17 @@ import (
 	"path/filepath"
 )
 
+// CheckOnlyOwnerWritable reports an error unless path, and every directory
+// leading to it, is owned by an account that can already act with the privileges
+// the caller holds, and is writable by nobody else.
+//
+// Exported for callers outside elevation that read a file while privileged and
+// then act on what it says: the same question this package asks of an
+// executable, asked of a configuration file.
+func CheckOnlyOwnerWritable(path string) error {
+	return checkOnlyOwnerWritable(path)
+}
+
 // trustedSelf returns the path of this executable, provided it is one we are
 // willing to have run as root.
 //
diff --git a/client/internal/wincmd/system32_windows.go b/client/internal/wincmd/system32_windows.go
new file mode 100644
index 000000000..36aa258b5
--- /dev/null
+++ b/client/internal/wincmd/system32_windows.go
@@ -0,0 +1,30 @@
+// Package wincmd locates the Windows utilities the client shells out to.
+package wincmd
+
+import (
+	"path/filepath"
+
+	log "github.com/sirupsen/logrus"
+	"golang.org/x/sys/windows"
+)
+
+// defaultSystem32Dir is where the system directory is on every supported
+// install, used only when the API that reports it fails.
+const defaultSystem32Dir = `C:\Windows\System32`
+
+// System32 returns the full path of a Windows utility under the system
+// directory.
+//
+// PATH is deliberately not consulted. The daemon runs as LocalSystem with an
+// environment of its own, so whoever can place an entry in that PATH chooses
+// which binary runs with those privileges. The system directory is read from
+// the API rather than from %SystemRoot% for the same reason.
+func System32(command string) string {
+	sysDir, err := windows.GetSystemDirectory()
+	if err != nil {
+		log.Warnf("Failed to locate the Windows system directory, falling back to %s: %v", defaultSystem32Dir, err)
+		sysDir = defaultSystem32Dir
+	}
+
+	return filepath.Join(sysDir, command+".exe")
+}
diff --git a/client/internal/wincmd/system32_windows_test.go b/client/internal/wincmd/system32_windows_test.go
new file mode 100644
index 000000000..0d31d7ee7
--- /dev/null
+++ b/client/internal/wincmd/system32_windows_test.go
@@ -0,0 +1,31 @@
+package wincmd
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSystem32IgnoresPATH(t *testing.T) {
+	// A directory holding something that would win a PATH lookup, in front of
+	// everything else: the daemon runs as LocalSystem, so a PATH entry must not
+	// be able to decide what it executes.
+	planted := t.TempDir()
+	require.NoError(t, os.WriteFile(filepath.Join(planted, "netsh.exe"), []byte("not really netsh"), 0o600))
+	t.Setenv("PATH", planted+string(os.PathListSeparator)+os.Getenv("PATH"))
+
+	got := System32("netsh")
+
+	assert.True(t, filepath.IsAbs(got), "the path must be absolute, got %q", got)
+	assert.NotContains(t, got, planted, "a PATH entry must not be consulted")
+	assert.True(t, strings.EqualFold(filepath.Base(got), "netsh.exe"), "unexpected file name in %q", got)
+
+	// The system directory is what Windows reports it to be, not %SystemRoot%,
+	// which the same caller could have set alongside PATH.
+	t.Setenv("SystemRoot", planted)
+	assert.Equal(t, got, System32("netsh"), "%SystemRoot% must not move the lookup")
+}