Ensure key files have secure permissions.

cmd/core_plugin/agentcrypto/mtls_mds_linux.go

@@ -73,7 +73,7 @@ func (j *CredsJob) writeRootCACert(ctx context.Context, content []byte, outputFi
 	galog.Debugf("Writing root CA cert to %q", outputFile)
 
 	// Write the root CA cert to the output file.
-	if err := j.writeCredentials(ctx, content, outputFile); err != nil {
+	if err := j.writeCredentials(ctx, content, outputFile, 0644); err != nil {
 		return err
 	}
 	galog.Debugf("Successfully wrote root CA cert to %q", outputFile)
@@ -95,16 +95,16 @@ func (j *CredsJob) writeRootCACert(ctx context.Context, content []byte, outputFi
 // key).
 func (j *CredsJob) writeClientCredentials(ctx context.Context, plaintext []byte, outputFile string) error {
 	galog.Debugf("Writing client credentials to %q", outputFile)
-	return j.writeCredentials(ctx, plaintext, outputFile)
+	return j.writeCredentials(ctx, plaintext, outputFile, 0600)
 }
 
 // writeCredentials stores the provided credentials to the output file.
-func (j *CredsJob) writeCredentials(ctx context.Context, certContent []byte, outputFile string) error {
+func (j *CredsJob) writeCredentials(ctx context.Context, certContent []byte, outputFile string, perm os.FileMode) error {
 	// The directory should be executable, but the file does not need to be.
-	if err := os.MkdirAll(filepath.Dir(outputFile), 0655); err != nil {
+	if err := os.MkdirAll(filepath.Dir(outputFile), 0755); err != nil {
 		return err
 	}
-	return file.SaferWriteFile(ctx, certContent, outputFile, file.Options{Perm: 0644})
+	return file.SaferWriteFile(ctx, certContent, outputFile, file.Options{Perm: perm})
 }
 
 // getCAStoreUpdater iterates over known system trust store updaters and returns

cmd/core_plugin/agentcrypto/mtls_mds_linux_test.go

@@ -20,6 +20,7 @@ import (
 	"context"
 	"os"
 	"path/filepath"
+	"syscall"
 	"testing"
 
 	"github.com/GoogleCloudPlatform/google-guest-agent/internal/cfg"
@@ -194,3 +195,45 @@ func TestCertificateDirFromUpdaterError(t *testing.T) {
 		t.Errorf("certificateDirFromUpdater(unknown) succeeded for missing cert dir, want error")
 	}
 }
+
+func TestWriteClientCredentials(t *testing.T) {
+	ctx := context.Background()
+	j := &CredsJob{}
+	tempDir := t.TempDir()
+	outputFile := filepath.Join(tempDir, "test_dir", "client.key")
+
+	oldUmask := syscall.Umask(0)
+	defer syscall.Umask(oldUmask)
+
+	content := []byte("test_credentials_content")
+	if err := j.writeClientCredentials(ctx, content, outputFile); err != nil {
+		t.Fatalf("writeClientCredentials failed unexpectedly: %v", err)
+	}
+
+	// Verify file content
+	got, err := os.ReadFile(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to read written file: %v", err)
+	}
+	if string(got) != string(content) {
+		t.Errorf("Written file content = %q, want %q", string(got), string(content))
+	}
+
+	// Verify file permissions
+	fInfo, err := os.Stat(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to stat file: %v", err)
+	}
+	if fInfo.Mode().Perm() != 0600 {
+		t.Errorf("File permissions = %o, want 0600", fInfo.Mode().Perm())
+	}
+
+	// Verify directory permissions
+	dInfo, err := os.Stat(filepath.Dir(outputFile))
+	if err != nil {
+		t.Fatalf("Failed to stat directory: %v", err)
+	}
+	if dInfo.Mode().Perm() != 0755 {
+		t.Errorf("Directory permissions = %o, want 0755", dInfo.Mode().Perm())
+	}
+}

cmd/core_plugin/agentcrypto/mtls_mds_windows.go

@@ -242,7 +242,7 @@ func (j *CredsJob) writeClientCredentials(ctx context.Context, creds []byte, out
 		galog.Warnf("Could not get previous serial number, will skip cleanup: %v", err)
 	}
 
-	if err := file.SaferWriteFile(ctx, creds, outputFile, file.Options{Perm: 0644}); err != nil {
+	if err := file.SaferWriteFile(ctx, creds, outputFile, file.Options{Perm: 0600}); err != nil {
 		return fmt.Errorf("failed to write client key: %w", err)
 	}
 	galog.Debugf("Successfully wrote client credentials to %q", outputFile)
@@ -256,7 +256,7 @@ func (j *CredsJob) writeClientCredentials(ctx context.Context, creds []byte, out
 
 	galog.V(1).Debugf("Writing PFX file to %q", pfxFile)
 	p := filepath.Join(filepath.Dir(outputFile), pfxFile)
-	if err := file.SaferWriteFile(ctx, pfx, p, file.Options{Perm: 0644}); err != nil {
+	if err := file.SaferWriteFile(ctx, pfx, p, file.Options{Perm: 0600}); err != nil {
 		return fmt.Errorf("failed to write PFX file: %w", err)
 	}
 	galog.V(1).Debugf("Successfully wrote PFX file to %q", p)

cmd/core_plugin/workloadcertrefresh/refresher.go

@@ -192,7 +192,7 @@ func writeWorkloadIdentities(destDir string, wisMd []byte) (string, error) {
 		return "", fmt.Errorf("error writing certificates.pem: %w", err)
 	}
 
-	if err := os.WriteFile(filepath.Join(destDir, "private_key.pem"), []byte(wis.WorkloadCredentials[spiffeID].PrivateKeyPem), 0644); err != nil {
+	if err := os.WriteFile(filepath.Join(destDir, "private_key.pem"), []byte(wis.WorkloadCredentials[spiffeID].PrivateKeyPem), 0600); err != nil {
 		return "", fmt.Errorf("error writing private_key.pem: %w", err)
 	}
 	return spiffeID, nil
@@ -449,7 +449,7 @@ func (j *RefresherJob) refreshCredsWithGRPC(ctx context.Context, contentDir stri
 	}
 
 	galog.Debugf("Writing workload certificates private key to %s", contentDir)
-	if err := os.WriteFile(filepath.Join(contentDir, "private_key.pem"), certs.GetPrivateKeyPem(), 0644); err != nil {
+	if err := os.WriteFile(filepath.Join(contentDir, "private_key.pem"), certs.GetPrivateKeyPem(), 0600); err != nil {
 		return fmt.Errorf("error writing private_key.pem: %w", err)
 	}
 	galog.Debugf("Writing workload certificates certificate chain to %s", contentDir)

cmd/core_plugin/workloadcertrefresh/refresher_test.go

@@ -21,6 +21,7 @@ import (
 	"net"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strconv"
 	"testing"
 	"time"
@@ -175,6 +176,16 @@ func TestWriteWorkloadIdentities(t *testing.T) {
 	if string(gotPvtPem) != pvtPem {
 		t.Errorf("writeWorkloadIdentities(%s,%s) wrote %q, expected to write %q", dir, resp, string(gotPvtPem), pvtPem)
 	}
+
+	if runtime.GOOS != "windows" {
+		info, err := os.Stat(filepath.Join(dir, "private_key.pem"))
+		if err != nil {
+			t.Errorf("failed to stat private_key.pem: %v", err)
+		}
+		if info.Mode().Perm() != 0600 {
+			t.Errorf("private_key.pem has permissions %o, want 0600", info.Mode().Perm())
+		}
+	}
 }
 
 func TestFindDomainError(t *testing.T) {
@@ -354,6 +365,20 @@ func TestRefreshCreds(t *testing.T) {
 				if string(got) != test.content {
 					t.Errorf("refreshCreds(ctx, %+v) wrote %q, want content %q", out, string(got), test.content)
 				}
+
+				if runtime.GOOS != "windows" {
+					info, err := os.Stat(test.path)
+					if err != nil {
+						t.Errorf("failed to stat file %q: %v", test.path, err)
+					}
+					expectedPerm := os.FileMode(0644)
+					if filepath.Base(test.path) == "private_key.pem" {
+						expectedPerm = 0600
+					}
+					if info.Mode().Perm() != expectedPerm {
+						t.Errorf("file %q has permissions %o, want %o", test.path, info.Mode().Perm(), expectedPerm)
+					}
+				}
 			})
 		}