diff --git a/admin/class-boldgrid-backup-admin-folder-exclusion.php b/admin/class-boldgrid-backup-admin-folder-exclusion.php
index 6bd0a7ef..43ac544e 100644
--- a/admin/class-boldgrid-backup-admin-folder-exclusion.php
+++ b/admin/class-boldgrid-backup-admin-folder-exclusion.php
@@ -26,7 +26,7 @@ class Boldgrid_Backup_Admin_Folder_Exclusion {
 	 * @since 1.6.0
 	 * @var   string
 	 */
-	public $default_exclude = '.git,node_modules,wp-content/cache';
+	public $default_exclude = '.git,node_modules,wp-content/cache,core.*';
 
 	/**
 	 * The default include value.
diff --git a/tests/admin/test-class-boldgrid-backup-admin-folder-exclusion.php b/tests/admin/test-class-boldgrid-backup-admin-folder-exclusion.php
new file mode 100644
index 00000000..4716151b
--- /dev/null
+++ b/tests/admin/test-class-boldgrid-backup-admin-folder-exclusion.php
@@ -0,0 +1,136 @@
+<?php
+/**
+ * File: test-class-boldgrid-backup-admin-folder-exclusion.php
+ *
+ * @link  https://www.boldgrid.com
+ * @since SINCEVERSION
+ *
+ * @package    Boldgrid_Backup
+ * @subpackage Boldgrid_Backup/tests/admin
+ * @copyright  BoldGrid
+ * @version    $Id$
+ * @author     BoldGrid <support@boldgrid.com>
+ */
+
+/**
+ * Class: Test_Boldgrid_Backup_Admin_Folder_Exclusion
+ *
+ * @since SINCEVERSION
+ */
+class Test_Boldgrid_Backup_Admin_Folder_Exclusion extends WP_UnitTestCase {
+	/**
+	 * Get a fresh folder exclusion object.
+	 *
+	 * @since SINCEVERSION
+	 *
+	 * @return Boldgrid_Backup_Admin_Folder_Exclusion
+	 */
+	private function get_folder_exclusion() {
+		$core = new Boldgrid_Backup_Admin_Core();
+
+		// Ensure we are not in a forced "full backup" state, which bypasses stored/custom values.
+		$core->is_backup_full                 = false;
+		$core->is_archiving_update_protection = false;
+		$core->pre_auto_update                = false;
+		$core->is_backup_now                  = false;
+
+		return new Boldgrid_Backup_Admin_Folder_Exclusion( $core );
+	}
+
+	/**
+	 * Test that Linux core dumps are excluded by default.
+	 *
+	 * Issue #538: exclude Linux core dump files (core.*) from backups by default.
+	 *
+	 * @since SINCEVERSION
+	 */
+	public function test_core_dumps_in_default_exclude() {
+		$folder_exclusion = $this->get_folder_exclusion();
+
+		$this->assertStringContainsString(
+			'core.*',
+			$folder_exclusion->default_exclude,
+			'The default exclude list should contain the core.* pattern.'
+		);
+	}
+
+	/**
+	 * Test the core.* pattern matches Linux core dump filenames.
+	 *
+	 * @since SINCEVERSION
+	 */
+	public function test_core_dump_pattern_matches() {
+		$folder_exclusion = $this->get_folder_exclusion();
+
+		// Core dumps in the root and in subdirectories should match.
+		$this->assertTrue( $folder_exclusion->is_match( 'core.*', 'core.12345' ) );
+		$this->assertTrue( $folder_exclusion->is_match( 'core.*', 'wp-content/uploads/core.98765' ) );
+		$this->assertTrue( $folder_exclusion->is_match( 'core.*', 'core.' ) );
+
+		// Unrelated files that merely contain "core" should not match.
+		$this->assertFalse( $folder_exclusion->is_match( 'core.*', 'wp-content/plugins/core-plugin/file.php' ) );
+		$this->assertFalse( $folder_exclusion->is_match( 'core.*', 'wp-content/plugins/mycore.php' ) );
+		$this->assertFalse( $folder_exclusion->is_match( 'core.*', 'wp-includes/index.php' ) );
+	}
+
+	/**
+	 * Test that a core dump file is excluded when using the default settings.
+	 *
+	 * @since SINCEVERSION
+	 */
+	public function test_core_dump_excluded_by_default() {
+		$folder_exclusion = $this->get_folder_exclusion();
+
+		// Simulate default include/exclude settings.
+		$folder_exclusion->include = '*';
+		$folder_exclusion->exclude = $folder_exclusion->default_exclude;
+
+		$this->assertFalse(
+			$folder_exclusion->allow_file( 'wp-content/uploads/core.12345' ),
+			'A Linux core dump should be excluded when using the default exclude settings.'
+		);
+
+		// A normal file should still be allowed.
+		$this->assertTrue(
+			$folder_exclusion->allow_file( 'wp-content/uploads/photo.png' ),
+			'A normal file should be included when using the default settings.'
+		);
+	}
+
+	/**
+	 * Test that users can override the default and keep core dumps in their backup.
+	 *
+	 * @since SINCEVERSION
+	 */
+	public function test_core_dump_included_when_default_overridden() {
+		$folder_exclusion = $this->get_folder_exclusion();
+
+		// User removed core.* from their exclude list.
+		$folder_exclusion->include = '*';
+		$folder_exclusion->exclude = '.git,node_modules,wp-content/cache';
+
+		$this->assertTrue(
+			$folder_exclusion->allow_file( 'wp-content/uploads/core.12345' ),
+			'A Linux core dump should be included when the user overrides the default exclude settings.'
+		);
+	}
+
+	/**
+	 * Test that the default exclude value can be filtered.
+	 *
+	 * @since SINCEVERSION
+	 */
+	public function test_default_exclude_is_filterable() {
+		$callback = function () {
+			return '.git,node_modules';
+		};
+
+		add_filter( 'boldgrid_backup_default_folder_exclude', $callback );
+
+		$folder_exclusion = $this->get_folder_exclusion();
+
+		$this->assertEquals( '.git,node_modules', $folder_exclusion->default_exclude );
+
+		remove_filter( 'boldgrid_backup_default_folder_exclude', $callback );
+	}
+}