CVE-2026-115897

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.

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.

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:

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.

Patches

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.

Workarounds

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.

Credits

Reported by ha_kr25 via the Yahoo bug bounty program.

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.

serialize-javascript from 7.0.1 before 7.1.3 contains a cross-site scripting vulnerability that allows attackers to break out of HTML script elements by ending serialized function source text with an unescaped </script. Attackers controlling a function's source via a spoofed toString() can combine surrounding output delimiters and later values to inject live markup such as img onerror handlers.

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.