Zero-to-Hero Dom Clobbering

Agenda

• What is Dom Clobbering?
• Basics of Dom Clobbering
• Writing Window Object
• Writing Document Object
• Why it’s not possible to Override Window Object and Why it is possible to override document Object
• Javascript Prototype Chain
• Clobbering More that 1 Level
• Conclusion

Prerequisites - HTML, Good Understanding of Javascript and Javascript Prototype

What is Dom Clobbering?

DOM clobbering is a technique in which you inject HTML into a page to manipulate the DOM and ultimately change the behavior of JavaScript on the page. DOM clobbering is particularly useful in cases where XSS is not possible, but you can control some HTML on a page where the attributes id or name are whitelisted by the HTML filter. The most common form of DOM clobbering uses an anchor element to overwrite a global variable, which is then used by the application in an unsafe way, such as generating a dynamic script URL. - PortSwigger

Basics of Dom Clobbering

Before getting into Dom Clobbering, we need to learn about id attribute and name attribute

ID Attribute

• The id attribute specifies its element’s unique identifier (ID) . The value must be unique amongst all the IDs in the element’s home subtree and must contain at least one character. The value must not contain any space characters
• Ex:

Name Attribute

• Basically name attribute specifies a name for an HTML element.
• The name attribute specifies a name for an HTML element. This name attribute can be used to reference the element in a JavaScript. For a <form> element, the name attribute is used as a reference when the data is submitted. For an <iframe> element, the name attribute can be used to target a form submission.
• Ex:

Basic Stuffs

Introduction to Javascript Objects

• Ex:

What is Window in Javascript?

What is document in javascript?

• Basically we can Declare Variables in javascript in 3 methods. var, let, const. Variables Declared with var are stored under window object with key as variable name and value as declared variable’s value

• We can see, myName is a Key in window Object Now.

Writing Window Objects

• Now, We have Basic understanding of idname, objects. Now Let’s have a Look in Clobbering window Elements.
• As we Seen Already, while used id attribute in HTML tags, we can get those tags by document.getElementById("<id>") or window.<id>.
• For Ex, If we have a HTML tag <h1 id="hi">Hey!</h1>, then we can get this tag by calling document.getElementById("hi") or simply call window.hi.

Exploitation:

Hidden toString function call:

• Sometimes in Javascript,when some functions like toString are called hiddenly when some functions are called. those functions use toString function internally for some purpose. Some Examples:

• In the Above Image, I innerHTMLed a Array into the body tag. But the Values are Not appended as List, But converted into String.

• Here Again, When Using template String Syntax, toString is automatically called!
• Another Example, Array.join called automatically with using Array.toString. There are tons of examples around there.

• Now, How this Hidden toString call in innerHTML going to help us?
• Well, If you found a Reflected HTMLi, then this doesn’t matter. But incase of DOM based HTMLi, then We need the Help of this hidden toString call in order to do something.
• Let’s Take a Example:

• Here, I called toString function for an h1 tag which return -> [object HTMLHeadingElement]
• So, We can’t actually Declare our own desire value, but still we can create a key under window Object.
• A Small Research can help us to find an Interesting behaviour among tags.

var allTags = ["a","img",...]
for (tag of allTags){
    temp = `<${tag} id="test">`;
    document.body.innerHTML = temp;
    try{
        if(!document.getElementById("test").toString().includes("[object")){
        console.log(tag)}
    }catch{}
}

• If we Run the About Javascript, We can find that, a tag and area tag doesn’t return [object... when toString is called. Something interestingly happing with those 2 tags.

• when we Supply a href attribute with these tags, we can see, the value of the href is returned when toString is called.

• Pretty Cool. But still we cannot control full value. protocol is messed up. But still we can use data uri, javascript uri, other cool protocols like cid and more.
• We know, every id attributes can create a key in window object. what about name attribute?
• Let’s Fuzz Again!

var allTags = ["a","img",...]
for (tag of allTags){
    temp = `<${tag} name="test">`;
    document.body.innerHTML = temp;
    try{
        if(window.test){
        console.log(tag)}
    }catch{}
}

• Turns out, embedformiframeimageimg,and object tags can create a key under window object.

Writing document Objects

• Basically document object is not like window object that stores variables, functions…
• The document object represents your web page. If you want to access any element in an HTML page, you always start with accessing the document object.
• So, How can we clobber document object? Well, Lets Do a Bit of Research.

var allTags = ["a","img",...]
for (tag of allTags){
    temp = `<${tag} id="test">`;
    document.body.innerHTML = temp;
    try{
        if(document.test){
        console.log(tag)}
    }catch{}
}

• Yeah!, We can Clobber document Object with object tag with id as attribute

• Pretty Cool right? Now lets try with name attribute

var allTags = ["a","img",...]
for (tag of allTags){
    temp = `<${tag} name="test">`;
    document.body.innerHTML = temp;
    try{
        if(document.test){
        console.log(tag)}
    }catch{}
}

• Turns out, embedformiframeimageimg,and object tags can create a key under document object.
• Well, in window clobbering, we used a tag and area tag with href attribute to return our own value. Let See if we can control the output.

var mytags = ["embed","form","iframe","image","img","object"]
for (tag of mytags){
    temp = `<${tag} name="test">`;
    document.body.innerHTML = temp;
    console.log(document.getElementsByName("test").toString())
}

Why it’s not possible to Override Window Object and Why it is possible to override document Object?

• Maybe you can have a question in your mind. why not overwrite the variables already declared?
• Well, We cannot override/overwrite window keys with id attribute or name attribute. But it is possible to override document keys.
• So, to understand this answer for this question, you must need good understanding about javascript prototype chain

Javascript Prototype Chain

• Prototype type chain is little bit huge topic. So i can’t cover that here.
• You can learn that from <a href=“https://developer.mozilla.org/en-US/docs/Web/JavaScript/Inheritance_and_the_prototype_chain">here</a>. If you kow Tamil, you can refer my OWASP session about Prototype Pollution. There I gave a Detailed View about prototype chain. you can watch by clicking <a href=“>

youtu.be

slide link: https://github.com/0xGodson/talks/blob/main/prototype%2Bpollution.pdf

youtu.be

• Now, Lets Take this example:

• we have a key name in the Object myObj. But we Don’t have a key toString in the Object myObj. where the toString comes from? It’s from prototype of the Object. As i said before, I am not going to explain this topic. you can learn from the above links.
• Cool, In Javascript, there is method/function which help us to find, if the key is really present on the object.

• We can see, hasOwnProperty("name") returned true and hasOwnProperty("toString") returned false.
• So, the name is a key in myObj, toString is a function comming from the Object.prototype
• what if we mentioned toString as a key in myObj?

• So, Did we Overwritten the toString function same from Object.prototype? No.

• You can see, Still we can access the Original toString function which printed [object Object].

• After some time, <a href=“https://twitter.com/IvarsVids">IvarVids</a> texted that, myObj.__proto__.toString is not the correct way of calling,and mentioned to use Object.prototype.toString.call(myObj) instead of this. kudos to <a href=“https://twitter.com/IvarsVids">IvarVids</a>
• Why we should call the toString like Object.prototype.toString.call(myObj)? Well, Before Getting into that We need to know little bit about toString. You can completly Skip this if you only interested in Dom Clobbering
• Most Common Data types in javascript are StringArray, Number.
• Basically Object, NumberArray are build-in Constructor Functions which can be used to Create Objects, Numbers, Arrays

• The Below Images Shows that, all three Constructor Objects have a property toString in its prototype

• This is Tricky one, If you Look Closed to the image, hasOwnProperty is coming from Object.prototype. We already Saw, How hasOwnProperty works.
• Even the hasOwnProperty coming from Object.prototype it is searching for a key toString in Object.prototype.
• To Understand Better

• Here, We Have a Nested Object. myobj2 is also a Object, so its Prototype is also equal to Object.protoptype
• Another Example with Arrays:

• Basically includes is a property comimg from its prototype [Array.prototype].inclueds will return true/false values. true of passed argument is present in the array, else it will return false.
• Even myarr.__proto__.includes is equal to myarr.includes, whiles using myarr.__proto__.includes(2), the includes in not searching inside myarr but searching inside myarr.__proto__
• Lets add a value to Array.prototype

• Now, You can clearly see, how the includes works. toString is also works similar to this. Even if we call the toString from its prototype with __proto__, to toString is not actually working with the myObj but with myObj.__proto__ when we call myObj.__proto__.toString.
• When we Call String.prototype.toString, it will return as a String, If we call Arrar.prototype.toString, then it will convert the array to string. If we call the Object.prototype.toString, then it will return [object <DATA TYPE>].

• So, when we call myObj.__proto__.toString() we are not actually working with myObj but working with myObj.__proto__. myObj.__proto__.toString() == Object.prototype.toString.call(myObj.__proto__) —
• Lets Get Back to the Question…

Why it’s not possible to Override Window Object and Why it is possible to override document Object?

Why we can’t override window clobbering?

• To Understand, Why it is not possible to override window object clobbering, we need to know where and how the elements and variables are stored in the window Object.

• So, if the h1 tag id attribute whoami is not stored in window object, then how is it possible to access window.whoami?
• well, that’s called prototype chain. While we try to get window.whoami, In background, Javascript will follow its prototype chain to find the whoami key.

• Now, the tags with id are stored into it’s prototype. it is not stored directly in the window object.
• We already saw, variables/functions are stored in the window object itself. it is not stored under its prototype. So, when there is a variable already declared, then we can’t override that. but still we can access through prototype chain

• while javascript can’t find the whoami from first chain __proto__, then it will automatically move to next chain untill the __proto__ hit null.
• This is the Reason why we can’t override window clobbering

Why we can override document clobbering?

• we can use any one of the above tags found to be helpful in document clobbering. for now, we are going to use img tag with name attribute.
• There is a another method in javascript which help us to view the keys from a object. methods/functions from the prototype will not be returned. Object.getOwnPropertyNames(<Object>)

• Surprisingly, only location, cookie are it own properties (keys).
• So, where the document.domain, document.getElementById,…. are comming from? yes its from its prototype!
• So, If we can add domain as key to document object, when we call document.domain, instead of the real document.domain, our value will be returned.
• So, let now see where and how the values we inject with img tag with name attributes are stored.

• Now, we can see out keys are directly stored into document as it own property.
• So, According to prototype chain, our tag overrided the orginal property from its prototype.
• Except document.location, we can clobber any thing in document Object. This mostly help us to bypass IF Conditions like if(document.domain != "..."){} and maybe we can break the functionalities by overwriting over used tags!
• Example:

Clobbering More that 1 Level

• Till now, We saw dom clobbering level 1. Like clobbering single property. But actually we can clobber multiple levels like a Object.
• What happen if we used same id 2 times for 2 different tags?

• So, basically when we used 2 times same id, it create a HTMLCollection. We can combine id and name attributes to build more complex HTMLCollection.
• For Example we can use form tags with input/output tags
• Why form tags? basically they are nested HTML tags. [Dom Tree Struct]

• we can’t call our own attribute which we created while injecting HTML. This method of Calling out Own attributes are Exploited in the Past By Legends in IE. But it is not possible now.
• Later Terjanq came up a trick which can clobber 4 levels with iframe tag and name and id attributes with nested iframes with srcdoc attribute.

• Basically using name attribute with a as value creates reference in window as window.a and srcdoc attribute can be used to inject html contents.
• In the Nested iframe we used name attribute and set that to b. So we can access that through window.a.b.Inside the nested frame we using another srcdoc and we using 2 id attributes with same value as c which creates a HTMLCollection.
• Now, we can access window.a.b.c, adding a name attribute inside a tag, we achive 4 level clobbering .window.a.b.c.d

Conclusion

• If you want to pratice this, you can check my challenge at https://0xgodson.me/my-ctfs/chal1/ and postswigger Labs too!

Fix

• Check that objects and functions are legitimate. If you are filtering the DOM, make sure you check that the object or function is not a DOM node.
• Avoid bad code patterns. Using global variables in conjunction with the logical OR operator should be avoided.
• Use a well-tested library, such as DOMPurify, that accounts for DOM-clobbering vulnerabilities.

Resources