serialize-javascript: Serialize JavaScript: Cross-site scripting (XSS) via unescaped </script> in serialized function bodies

Impact

serialize-javascript escapes its output so it is safe to embed inside a <script> element. In 7.1.1 that guarantee does not hold for function values: a crafted function body can carry a literal, unescaped </script> into the output, terminating the script element early so the remainder is parsed as HTML.

SCRIPT_CLOSE_REGEXP used <\/script[^>]*> as its first alternative. The character class excludes only >, so a single match could run from one </script all the way to the next > anywhere in the source — swallowing a second, complete </script> along the way. Only one replacement is emitted per match, and the plain-code branch neutralizes just the leading < ('< ' + match.slice(1)), so the swallowed tag was re-emitted verbatim.

Reaching that shape requires </script in code position, which is legal JavaScript: x</script=+/ parses as x < /script=+/, a comparison against a regex literal.

const serialize = require('serialize-javascript');
const src = "function f(x){ return x</script=+/ + '</script><img src=x onerror=alert(1)>' }";
const out = serialize({ h: new Function('return ' + src)() });
// {"h":function f(x){ return x< /script=+/ + '</script><img src=x onerror=alert(1)>' }}

Embedded as the README documents (<script>window.S = <%= serialize(state) %></script>) and parsed by Chromium, the script element ends at the injected tag and the <img> becomes a live DOM node with its onerror handler executing in the page origin.

Only the function path is affected. The same payload passed as data is escaped correctly, and options.isJSON / non-function values are unaffected.

Patches

Fixed in 7.1.2. The wildcard now excludes < as well as > ([^<>]*), so a match can never reach past a second <. Every </script in the source therefore either begins its own match or is followed by a character the HTML tokenizer does not accept as ending a tag name — it ends the tag name only on TAB, LF, FF, CR, SPACE, / or >, and emits anything else as text.

Workarounds

Upgrade to 7.1.2. If you cannot upgrade, 7.1.0 and earlier are unaffected, or avoid serializing functions whose source text is attacker-influenced.

Regression note

This is a regression specific to 7.1.1, not a long-standing issue. 7.1.0 and earlier applied the same wildcard but escaped the entire match, so no tag survived. Downstream scanners defaulting to a >= 7.1.0 range would be overly broad.