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:

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-function values (including strings containing </script) are escaped correctly.

Fixed in 7.1.3 (#235). The bare </script form is now also matched at the end of the function body, so it is escaped regardless of what follows it in the output.

Upgrade to 7.1.3. If you cannot upgrade, do not serialize function values whose source text (or toString()) can be influenced by an attacker.

Reported by ha kr25 via the Yahoo bug bounty program.