If someone has patched properties onto Object.prototype——这事不稀奇。某个 npm 依赖的 deepMerge 没校验 key，一段 URL 参数解析没过滤 \_\_proto\_\_。甚至你项目里某行不起眼的 tag = value-都能干这件事。

// 某个你根本不知道的角落，可能已经发生了：
Object.prototype.tagNameCheck = /.*/;其实，Object.prototype.attributeNameCheck = /.*/;老实说，

Later DOMPurify 再去读 {}.tagNameCheck——空对象自己没有。但原型链上有，读到了 /.*/b>/.

This regex means “match all tags”. DOMPurify sees a whitelist and assumes all tags are safe,so it passes attack through:

test


三行代码就能在控制台复现：XSS 逃逸成功！


// 三行代码就能在控制台复现
Object.prototype.tagNameCheck = /.*/;话说回来，Object.prototype.attributeNameCheck = /.*/;const clean = DOMPurify.sanitize tabindex= autofocus>test');document.body.innerHTML = clean;// alert 弹了


You pass nothing,no error is thrown。yet XSS sneaks past.

至于痛点，自己代码检查不到、版本已更新却仍受影响！


No obvious bug in your own code.

Your project uses latest DOMPurify .

The attacker exploits an unrelated entry point that polluted Object.prototype,rendering your runtime dirty.

You’re left as innocent victim of a “dirty environment” attack.

This feels like you inspected Redux store for hours only to find no bug—because real flaw lives outside your source tree.


Illustration of pollution flow:
|污染| P
B -->|污染| P
C -->|污染| P
P -->|DOMPurify读取脏配置| D
D --> E
style P fill:#fde2e2。stroke:#c0392b
style E fill:#fde2e2,stroke:#c0392b



*Note:* Object.prototype* is a lower-level global scope than *window*—any polluted property propagates everywhere.

This demonstrates a classic issue in JS: objects you receive might not be what you expect.

The culprit line “CUSTOM_ELEMENT_HANDLING = cfg.CUSTOM_ELEMENT_HANDLING || {}” fails when {} inherits from polluted prototype.


How to fix it?



If you can upgrade:




// In newer versions this line was changed to avoid prototype pollution:
CUSTOM_ELEMENT_HANDLING = cfg.CUSTOM_ELEMENT_HANDLING || Object.create;<\/span>



// Explicitly pass a clean config to guard against polluted prototypes:
DOMPurify.sanitize;<\/span>


*Takeaway*: The object you think is empty may contain hidden traps if its prototype chain was tainted elsewhere. Always guard against prototype pollution when trusting third‑party libraries.

附录的观点是，完整 PoC


运行 PoC







Affected versions: DOMPurify v1.x.x – v1.x.y 

Fix available in: v1.x.z+

Prerequisite: Application must have an entry point that can pollute Object.prototype 



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