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Reading this XSS cheat sheet, I noticed a special usage I have never seen:
<img src="/" =_=" title="onerror='prompt(1)'">
What does "=_=" mean? It's below the sentence "On Mouse Over".
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Updated on July 03, 2019. The dollar sign ($) and the underscore (_) characters are JavaScript identifiers, which just means that they identify an object in the same way a name would. The objects they identify include things such as variables, functions, properties, events, and objects.
The JavaScript in operator is used to check if a specified property exists in an object or in its inherited properties (in other words, its prototype chain). The in operator returns true if the specified property exists. Anatomy of a simple JavaScript object.
-1 means the first goes before the second, 1 means it goes after, and 0 means they're equivalent.
What is = in JavaScript? Equal to (=) is an assignment operator, which sets the variable on the left of the = to the value of the expression that is on its right. This operator assigns lvalue to rvalue. For example, Writing a=10 is fine.
= in JavaScript is used for assigning values to a variable. == in JavaScript is used for comparing two variables, but it ignores the datatype of variable. === is used for comparing two variables, but this operator also checks datatype and compares two values.
The “OR” operator is represented with two vertical line symbols: In classical programming, the logical OR is meant to manipulate boolean values only. If any of its arguments are true, it returns true, otherwise it returns false. In JavaScript, the operator is a little bit trickier and more powerful.
In classical programming, logical OR is meant to manipulate boolean values only. If any of its arguments are true, then it returns true, otherwise it returns false. In JavaScript the operator is a little bit more tricky and powerful.
It's just an attribute on the element. It doesn't have any meaning by itself, so it may be present simply as a red herring.
Prettified, the code is:
<img
src="/"
=_=" title="
onerror='prompt(1)'"
>
In HTML, = in an attribute specifies the delimiter between the attribute name and the attribute value, so it's:
=_=" title="
^^ attribute name
=_=" title="
^ delimiter between attribute name and attribute value
=_=" title="
^ attribute value contents delimiter
=_=" title="
^^^^^^^ attribute value
=_=" title="
^ attribute value contents delimiter
And you could retrieve the attribute value if you wanted.
const img = document.querySelector('img');
console.log(img.getAttribute('=_'));<img
src="/"
=_=" title="
onerror='prompt(1)'"
>Note that the attribute name is =_, not =_= - the final = is the delimiter, not part of the attribute name.
The "XSS" is caused only by the src and the onerror, not by anything else. Wherever you're encountering this, the =_ probably doesn't do anything at all. It could, but it probably doesn't.
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