diff --git a/projects/packages/waf/changelog/fix-waf-undefined-entrypoint-in-bootstrap b/projects/packages/waf/changelog/fix-waf-undefined-entrypoint-in-bootstrap
new file mode 100644
index 0000000000000..9c6e1ae4648d4
--- /dev/null
+++ b/projects/packages/waf/changelog/fix-waf-undefined-entrypoint-in-bootstrap
@@ -0,0 +1,4 @@
+Significance: patch
+Type: fixed
+
+Firewall: improve backwards compatibility for sites running outdated bootstrap scripts via standalone mode.
diff --git a/projects/packages/waf/src/class-waf-standalone-bootstrap.php b/projects/packages/waf/src/class-waf-standalone-bootstrap.php
index a272b1492cf0a..61a930b2a948d 100644
--- a/projects/packages/waf/src/class-waf-standalone-bootstrap.php
+++ b/projects/packages/waf/src/class-waf-standalone-bootstrap.php
@@ -120,6 +120,15 @@ public function get_bootstrap_file_path() {
 		return trailingslashit( JETPACK_WAF_DIR ) . 'bootstrap.php';
 	}
 
+	/**
+	 * Gets the entrypoint file.
+	 *
+	 * @return string The entrypoint file.
+	 */
+	private function get_entrypoint() {
+		return defined( 'JETPACK_WAF_ENTRYPOINT' ) ? JETPACK_WAF_ENTRYPOINT : 'rules/rules.php';
+	}
+
 	/**
 	 * Generates the bootstrap file.
 	 *
@@ -141,6 +150,7 @@ public function generate() {
 		$autoloader_file = $this->locate_autoloader_file();
 
 		$bootstrap_file          = $this->get_bootstrap_file_path();
+		$entrypoint              = $this->get_entrypoint();
 		$mode_option             = get_option( Waf_Runner::MODE_OPTION_NAME, false );
 		$share_data_option       = get_option( Waf_Runner::SHARE_DATA_OPTION_NAME, false );
 		$share_debug_data_option = get_option( Waf_Runner::SHARE_DEBUG_DATA_OPTION_NAME, false );
@@ -154,7 +164,7 @@ public function generate() {
 			. sprintf( "define( 'JETPACK_WAF_SHARE_DEBUG_DATA', %s );\n", var_export( $share_debug_data_option, true ) )
 			. sprintf( "define( 'JETPACK_WAF_DIR', %s );\n", var_export( JETPACK_WAF_DIR, true ) )
 			. sprintf( "define( 'JETPACK_WAF_WPCONFIG', %s );\n", var_export( JETPACK_WAF_WPCONFIG, true ) )
-			. sprintf( "define( 'JETPACK_WAF_ENTRYPOINT', %s );\n", var_export( JETPACK_WAF_ENTRYPOINT, true ) )
+			. sprintf( "define( 'JETPACK_WAF_ENTRYPOINT', %s );\n", var_export( $entrypoint, true ) )
 			. 'require_once ' . var_export( $autoloader_file, true ) . ";\n"
 			. "Automattic\Jetpack\Waf\Waf_Runner::initialize();\n";
 		// phpcs:enable