diff --git a/cmd/root.go b/cmd/root.go
index e61c8a6..58ad97f 100644
--- a/cmd/root.go
+++ b/cmd/root.go
@@ -60,6 +60,14 @@ projects of your OpenProject instance.`,
 			os.Exit(1)
 		}
 
+		if insecure, mode := configuration.InsecureConfigPermissions(); insecure {
+			path := configuration.ConfigFilePath()
+			printer.Warning(fmt.Sprintf(
+				"config file %s is accessible by other users (mode %#o); it stores your API token. Run 'chmod 600 \"%s\"' to restrict access.",
+				path, mode, path,
+			))
+		}
+
 		if host == "" && explicit {
 			printer.Error(openerrors.Custom(fmt.Sprintf(
 				"Profile %q not found. Run 'op login --profile %s' to create it.",
diff --git a/components/configuration/filemode_test.go b/components/configuration/filemode_test.go
index 89c02f6..7ebfa47 100644
--- a/components/configuration/filemode_test.go
+++ b/components/configuration/filemode_test.go
@@ -6,6 +6,7 @@ package configuration_test
 import (
 	"os"
 	"path/filepath"
+	"runtime"
 	"testing"
 
 	"github.com/opf/openproject-cli/components/configuration"
@@ -27,3 +28,49 @@ func TestWriteConfigForProfile_FileModeIs0600(t *testing.T) {
 		t.Errorf("config file mode = %04o, want 0600 (token must not be world-readable)", mode)
 	}
 }
+
+// A config written by the CLI (mode 0600) must not be reported as insecure.
+func TestInsecureConfigPermissions_SecureFile(t *testing.T) {
+	setupTempConfig(t)
+
+	if err := configuration.WriteConfigForProfile("default", "https://example.com", "secret-token"); err != nil {
+		t.Fatal(err)
+	}
+
+	if insecure, mode := configuration.InsecureConfigPermissions(); insecure {
+		t.Errorf("0600 config reported insecure (mode %#o)", mode)
+	}
+}
+
+// A config readable by group or other users must be reported as insecure so the
+// CLI can warn that the API token may leak.
+func TestInsecureConfigPermissions_WorldReadableFile(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("Unix permission bits are not meaningful on Windows")
+	}
+	setupTempConfig(t)
+
+	if err := configuration.WriteConfigForProfile("default", "https://example.com", "secret-token"); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Chmod(configuration.ConfigFilePath(), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	insecure, mode := configuration.InsecureConfigPermissions()
+	if !insecure {
+		t.Errorf("0644 config not reported insecure (mode %#o)", mode)
+	}
+	if mode.Perm() != 0644 {
+		t.Errorf("reported mode = %#o, want 0644", mode.Perm())
+	}
+}
+
+// A missing config file is not insecure.
+func TestInsecureConfigPermissions_MissingFile(t *testing.T) {
+	setupTempConfig(t)
+
+	if insecure, _ := configuration.InsecureConfigPermissions(); insecure {
+		t.Error("missing config file reported insecure")
+	}
+}
diff --git a/components/configuration/profiles.go b/components/configuration/profiles.go
index 9bff0f1..fac9ef4 100644
--- a/components/configuration/profiles.go
+++ b/components/configuration/profiles.go
@@ -5,6 +5,7 @@ import (
 	"net/url"
 	"os"
 	"regexp"
+	"runtime"
 	"sort"
 	"strings"
 
@@ -84,6 +85,31 @@ func DeleteProfile(profile string) error {
 	return deleteProfile(profile)
 }
 
+// ConfigFilePath returns the path of the config file. It honours
+// $XDG_CONFIG_HOME/$HOME, so it is absolute only when those are. Exposed so
+// callers can name the file in user-facing messages.
+func ConfigFilePath() string {
+	return configFile()
+}
+
+// InsecureConfigPermissions reports whether the config file exists with
+// permissions that let group or other users access it. The file stores API
+// tokens (and is written with mode 0600), so callers should warn the user when
+// this returns true. The second value is the file's permission bits. Unix
+// permission bits are not meaningful on Windows, so it always reports false
+// there; a missing or unreadable file is likewise not reported as insecure.
+func InsecureConfigPermissions() (insecure bool, mode os.FileMode) {
+	if runtime.GOOS == "windows" {
+		return false, 0
+	}
+	info, err := os.Stat(configFile())
+	if err != nil {
+		return false, 0
+	}
+	mode = info.Mode().Perm()
+	return mode&0077 != 0, mode
+}
+
 // AllProfiles returns every profile stored in the config file.
 func AllProfiles() ([]*Profile, error) {
 	if err := ensureConfigDir(); err != nil {
diff --git a/components/printer/common.go b/components/printer/common.go
index 7b3b33f..e3ef4d3 100644
--- a/components/printer/common.go
+++ b/components/printer/common.go
@@ -2,6 +2,7 @@ package printer
 
 import (
 	"encoding/json"
+	"fmt"
 
 	"github.com/opf/openproject-cli/components/errors"
 )
@@ -20,6 +21,12 @@ func Input(prompt string) {
 	activePrinter.Printf(prompt)
 }
 
+// Warning writes a non-fatal diagnostic to standard error so it never corrupts
+// machine-readable output (e.g. JSON) written to standard out.
+func Warning(msg string) {
+	activePrinter.Eprintln(fmt.Sprintf("%s %s", Yellow("[WARNING]"), msg))
+}
+
 func Done() {
 	activePrinter.Println(Green("DONE"))
 }
diff --git a/components/printer/console_printer.go b/components/printer/console_printer.go
index a60002d..bf7c8e7 100644
--- a/components/printer/console_printer.go
+++ b/components/printer/console_printer.go
@@ -1,6 +1,9 @@
 package printer
 
-import "fmt"
+import (
+	"fmt"
+	"os"
+)
 
 type ConsolePrinter struct{}
 
@@ -11,3 +14,7 @@ func (printer *ConsolePrinter) Printf(format string, a ...any) (n int, err error
 func (printer *ConsolePrinter) Println(a ...any) (n int, err error) {
 	return fmt.Println(a...)
 }
+
+func (printer *ConsolePrinter) Eprintln(a ...any) (n int, err error) {
+	return fmt.Fprintln(os.Stderr, a...)
+}
diff --git a/components/printer/printer.go b/components/printer/printer.go
index 00f0c24..776acae 100644
--- a/components/printer/printer.go
+++ b/components/printer/printer.go
@@ -3,6 +3,9 @@ package printer
 type Printer interface {
 	Printf(format string, a ...any) (n int, err error)
 	Println(a ...any) (n int, err error)
+	// Eprintln writes a line to standard error, keeping diagnostics out of the
+	// machine-readable output written to standard out.
+	Eprintln(a ...any) (n int, err error)
 }
 
 var activePrinter Printer
diff --git a/components/printer/testing_printer.go b/components/printer/testing_printer.go
index faea265..82f2c31 100644
--- a/components/printer/testing_printer.go
+++ b/components/printer/testing_printer.go
@@ -18,6 +18,12 @@ func (printer *TestingPrinter) Println(a ...any) (n int, err error) {
 	return len(printer.Result), nil
 }
 
+func (printer *TestingPrinter) Eprintln(a ...any) (n int, err error) {
+	printer.Result += fmt.Sprintln(a...)
+
+	return len(printer.Result), nil
+}
+
 func (printer *TestingPrinter) Reset() {
 	printer.Result = ""
 }
diff --git a/components/printer/warning_test.go b/components/printer/warning_test.go
new file mode 100644
index 0000000..327b51d
--- /dev/null
+++ b/components/printer/warning_test.go
@@ -0,0 +1,21 @@
+package printer_test
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/opf/openproject-cli/components/printer"
+)
+
+func TestWarning(t *testing.T) {
+	testingPrinter.Reset()
+
+	printer.Warning("watch out")
+
+	if !strings.Contains(testingPrinter.Result, "[WARNING]") {
+		t.Errorf("warning missing [WARNING] tag: %q", testingPrinter.Result)
+	}
+	if !strings.Contains(testingPrinter.Result, "watch out") {
+		t.Errorf("warning missing message: %q", testingPrinter.Result)
+	}
+}