JavaScript drives nearly everything on the web today, from simple interactions to large-scale single-page applications. With that power comes responsibility: insecure code can expose sensitive data, break logic, or even let attackers take control of an application.
Most developers already know about cross-site scripting (XSS) or cross-site request forgery (CSRF). But some less common vulnerabilities often fly under the radar. They may not make headlines as often, yet they can be just as damaging if ignored.
In this article, we will explore seven uncommon JavaScript security vulnerabilities. Each section explains how the attack works, shows a practical example, and ends with actionable defenses.
1. Prototype Pollution
What it is
Prototype pollution happens when untrusted input modifies JavaScript’s Object.prototype. Since most objects inherit from it, this change spreads across the application.
Example
function merge(target, source) {
for (let key in source) {
target[key] = source[key];
}
}
let userInput = JSON.parse('{"__proto__": {"isAdmin": true}}');
let config = {};
merge(config, userInput);
console.log(config.isAdmin); // undefined
console.log({}.isAdmin); // true (polluted prototype)
An attacker has polluted all objects with isAdmin. This could bypass access checks or change application logic.
Defense
Avoid deep merging of user input.
Use safe libraries and keep them patched.
Create objects with Object.create(null) When inheritance is unnecessary.
2. DOM Clobbering
What it is
Browsers sometimes place DOM elements into the global scope by their id or name. Attackers can use this to override variables or break scripts.
Example
<form id="loginForm" action="/auth">
<input type="text" name="username">
<input type="password" name="password">
</form>
<script>
let loginForm = document.getElementById("loginForm");
if (loginForm) {
console.log("Form exists");
}
</script>
If an attacker injects:
<input id="loginForm" value="malicious">
The variable loginForm may now point to the injected input, not the form.
Defense
Do not rely on global DOM variables.
Always query elements explicitly with document.getElementById or querySelector.
Use const and let to keep the scope strict.
3. JSONP Injection
What it is
Legacy APIs that use JSONP can be abused if the callback function is not validated. This allows arbitrary JavaScript execution.
Example
// Request
https://api.example.com/data?callback=display
// Response
display({"user":"Alice"});
If the callback is unchecked:
https://api.example.com/data?callback=alert('Hacked!')
Response
alert('Hacked!')({"user":"Alice"});
The attacker executes code on the client.
Defense
4. Regular Expression Denial of Service (ReDoS)
What it is
Badly designed regular expressions can consume massive CPU resources when attackers send crafted input, making the app hang.
Example
let regex = /(a+)+$/;
let input = "a".repeat(20000) + "!";
regex.test(input); // Freezes the app
Defense
Avoid regex patterns with nested quantifiers.
Use tools like safe-regex to audit expressions.
For complex parsing, prefer dedicated parsers over regex.
5. Insecure postMessage Handling
What it is
The window.postMessage API enables cross-window communication. If origin checks are skipped, attackers can send harmful messages.
Example
window.addEventListener("message", (event) => {
// Unsafe: trusting all origins
eval(event.data);
});
Any page or iframe can send malicious input here.
Defense
Always check event.origin against trusted domains.
Never use eval to process incoming data.
Treat all messages as untrusted input.
6. Client-Side Template Injection
What it is
If untrusted input is placed inside client-side templates, attackers may inject expressions that execute code.
Example
let template = "<div>{{userInput}}</div>";
let userInput = "{{#with this.constructor.constructor('alert(1)')()}}{{/with}}";
let compiled = Handlebars.compile(template);
document.body.innerHTML = compiled({ userInput });
This injection runs arbitrary JavaScript.
Defense
Escape all user inputs by default.
Disable unsafe template features if not needed.
Sanitize content before rendering.
7. Logic Flaws with Type Coercion
What it is
Loose equality (==) and JavaScript’s type coercion can create unexpected security gaps.
Example
let isAdmin = "false";
if (isAdmin == true) {
console.log("Access granted");
}
An attacker could exploit type quirks to bypass logic.
Defense
Always use strict equality (===).
Validate input types before processing.
Reject ambiguous or unexpected values early.
Final Thoughts
Uncommon vulnerabilities like prototype pollution, DOM clobbering, or client-side template injection are easy to overlook but dangerous in practice. Unlike XSS or CSRF, they often hide in plain sight and exploit quirks of the JavaScript language or browser behavior.
The defenses share a common theme: never trust input, use strict validation, and avoid risky features such as eval or unsafe template expressions. By adding these protections to your development workflow, you close gaps that attackers rely on and make your applications significantly more secure.