Serialize JavaScript serializes JavaScript values to a superset of JSON that includes regular expressions and functions. Prior to 7.1.3, the function-source escaping logic does not neutralize a bare closing-script prefix when attacker-influenced function source text ends exactly at that prefix. Because each function body is escaped independently, a delimiter supplied by surrounding serialized output can terminate an HTML script element, and a later serialized value supplies injected markup that becomes a live element in the page. Exploitation requires control of a serialized function's source text, such as through a spoofed toString(). This issue is fixed in version 7.1.3.

Affected: npm: serialize-javascript

### Impact

`serialize-javascript` escapes its output so it is safe to embed inside a `<script>` element. Before 7.1.3 that guarantee did not hold for a **function value whose source ends in a bare `</script`**.

Each function body is escaped on its own, and a bare `</script` was only neutralized when followed — within the same body — by a character the HTML tokenizer treats as ending the tag name (TAB, LF, FF, CR, SPACE, `/` or `>`). A body ending exactly at `</script` has no following character, so it was emitted unescaped. The delimiter is then supplied by the surrounding output (for example the newline the `space` option emits after the value), and a later serialized value can supply the closing `>` followed by markup:

```js const serialize = require('serialize-javascript'); function f() {} f.toString = () => 'function f(){ return 1 }//</script'; function g() {} g.toString = () => 'function g(x){ return x } > <img src=x onerror=alert(1)>'; serialize({ a: { f }, g }, { space: 2 }); // { // "a": { // "f": function f(){ return 1 }//</script // }, // "g": function g(x){ return x } > <img src=x onerror=alert(1)> // } ```

Embedded in a `<script>` element, the script ends at the injected tag and the `<img>` becomes a live element in the page.

Exploitation requires an attacker to control the **source text** of a serialized function, in practice through a spoofed `toString()`. Real functions always end in `}`, so ordinary function values are not affected, and non-fun…