[fix] harden attribute escaping during ssr

src/runtime/internal/ssr.ts

@@ -85,18 +85,20 @@ export function escape(value: unknown, is_attr = false) {
 	let escaped = '';
 	let last = 0;
 
-  while (pattern.test(str)) {
-    const i = pattern.lastIndex - 1;
-    const ch = str[i];
-    escaped += str.substring(last, i) + (ch === '&' ? '&' : (ch === '"' ? '"' : '<'));
-    last = i + 1;
-  }
+	while (pattern.test(str)) {
+		const i = pattern.lastIndex - 1;
+		const ch = str[i];
+		escaped += str.substring(last, i) + (ch === '&' ? '&' : (ch === '"' ? '"' : '<'));
+		last = i + 1;
+	}
 
 	return escaped + str.substring(last);
 }
 
 export function escape_attribute_value(value) {
-	return typeof value === 'string' ? escape(value, true) : value;
+	// keep booleans, null, and undefined for the sake of `spread`
+	const should_escape = typeof value === 'string' || (value && typeof value === 'object');
+	return should_escape ? escape(value, true) : value;
 }
 
 export function escape_object(obj) {
@@ -192,7 +194,7 @@ export function create_ssr_component(fn) {
 
 export function add_attribute(name, value, boolean) {
 	if (value == null || (boolean && !value)) return '';
-	const assignment = (boolean && value === true) ? '' : `="${escape_attribute_value(value.toString())}"`;
+	const assignment = (boolean && value === true) ? '' : `="${escape(value, true)}"`;
 	return ` ${name}${assignment}`;
 }
 

test/server-side-rendering/samples/attribute-escaped-quotes-spread/_expected.html

@@ -2,4 +2,10 @@
 	bar="'></div><script>alert(42)</script>"
 	foo="&quot;></div><script>alert(42)</script>"
 	qux="&amp;&amp;&amp;"
-></div>
+	quux="&quot;><script>alert(42)</script>"
+></div>
+
+<div
+	foo="foo"
+	unsafe="&quot;><script>alert(42)</script>"
+></div>

test/server-side-rendering/samples/attribute-escaped-quotes-spread/main.svelte

@@ -1,10 +1,15 @@
 <script>
+	const safe = { foo: 'foo' };
+	const unsafe = { toString: () => '"><script>alert(42)<\/script>' };
+
 	export let props = {
 		foo: '"></div><script>alert(42)</' + 'script>',
 		bar: "'></div><script>alert(42)</" + 'script>',
 		['"></div><script>alert(42)</' + 'script>']: 'baz',
 		qux: '&&&',
+		quux: unsafe
 	};
 </script>
 
-<div {...props}></div>
+<div {...props}></div>
+<div {...safe} {unsafe}></div>