In AngularJS, $eval() is a powerful method that allows you to evaluate an expression in the context of the current scope. This method can be used for evaluating expressions or executing code dynamically in your AngularJS application. While $eval() is useful in certain scenarios, it also poses significant security risks, especially if the expression being evaluated contains user input or untrusted data.
Understanding the security risks and the best practices for using $eval() securely is crucial to protect your AngularJS applications from vulnerabilities such as Cross-Site Scripting (XSS) and code injection attacks.
What is $eval()?
In AngularJS, $eval() is a method that evaluates an AngularJS expression in the current scope context and returns the result of the expression. This method can execute expressions like:
$scope.$eval('2 + 3'); // Returns 5
$scope.$eval('user.name'); // Returns the value of user.name in the current scope
You can also use $eval() to invoke functions in the scope:
$scope.$eval('greetUser("John")'); // Calls greetUser() function in the current scope
Although $eval() can be very powerful, it also opens up risks if the expression being evaluated contains untrusted content.
Security Risks of Using $eval()
The primary risk associated with $eval() is the execution of arbitrary code, which could be exploited by attackers to inject malicious code into your application.
1. Cross-Site Scripting (XSS) Attacks
If an attacker manages to inject malicious scripts into the $eval() expression, the attacker can execute arbitrary JavaScript in the context of your application. This can lead to various security issues, such as:
- Stealing sensitive user data (cookies, tokens, etc.)
- Performing actions on behalf of the user without their consent (such as submitting forms, making API requests, etc.)
- Defacing the web page or changing the behavior of your application
For example, if you pass untrusted user input directly to $eval():
$scope.$eval(userInput);
An attacker could provide a payload such as:
$scope.$eval('alert("XSS Attack")');
This would execute alert("XSS Attack") in the browser, demonstrating a simple XSS attack. If the expression is more malicious, it could compromise the entire application.
2. Code Injection
Another potential issue is code injection, where an attacker injects harmful JavaScript code into an AngularJS expression. This can happen if user input is evaluated directly using $eval().
$scope.$eval(userInput);
If userInput contains malicious JavaScript, it could cause unexpected behavior, potentially compromising the integrity of your app or your users.
Best Practices for Avoiding Security Risks with $eval()
While $eval() can be useful in certain scenarios, it should be used with caution. Here are some best practices to minimize security risks and prevent vulnerabilities:
1. Avoid Using $eval() with Untrusted Data
The most important rule is to never use $eval() with untrusted data, such as user input, unless it has been sanitized or validated. User input can come from various sources like query parameters, form inputs, or URLs, and these sources should be considered untrusted.
Example of Unsafe $eval() Usage:
$scope.$eval(userInput); // Risky, as userInput can contain untrusted data
Instead, always ensure that any data passed to $eval() is safe and does not contain any malicious code.
Safe Approach:
If you absolutely must evaluate expressions dynamically, make sure that the input is sanitized or restricted to known, safe inputs:
if (userInput === 'safeExpression') {
$scope.$eval(userInput);
}
You can also use a whitelist of allowed expressions to prevent any unexpected code execution.
2. Use $evalAsync() for Safer Context Evaluation
If you need to evaluate an expression asynchronously, you can use $evalAsync() instead of $eval(). $evalAsync() evaluates expressions in the AngularJS context but in a safer, deferred way, allowing for better control over timing and scope.
$scope.$evalAsync('someExpression');
While $evalAsync() doesn’t completely eliminate the security risks, it gives you more control over when the expression is evaluated and helps you avoid immediate execution during the current digest cycle, which can sometimes lead to race conditions or other issues.
3. Use ng-bind and ng-bind-html for Binding Content
Rather than using $eval() to evaluate expressions that will update the view, it’s safer and more efficient to use AngularJS’s built-in binding mechanisms like ng-bind or ng-bind-html.
For example:
<div ng-bind="user.name"></div> <!-- Safer than evaluating user.name with $eval() -->
AngularJS automatically escapes any user-generated content, which helps prevent XSS attacks.
If you’re working with HTML content that may contain user input, prefer using ng-bind-html with proper sanitization:
<div ng-bind-html="userProfileHtml | trustedHtml"></div>
You can use $sanitize to sanitize HTML before rendering it, preventing malicious scripts from being executed.
4. Use $scope.$watch() Instead of $eval()
If your goal is to dynamically evaluate a model or scope expression, consider using $watch() instead of $eval(). $watch() allows you to observe changes to a specific expression in the scope and trigger a callback when it changes, making it safer and more manageable.
$scope.$watch('user.name', function(newValue, oldValue) {
console.log('User name changed:', newValue);
});
This method doesn’t require evaluating arbitrary expressions and ensures that you are only working with safe, predefined expressions.
5. Limit Dynamic Evaluation to Trusted Code
If your application requires the dynamic evaluation of expressions (e.g., through an expression language), ensure that only trusted code or predefined expressions are evaluated. You can use a whitelist approach to prevent execution of potentially dangerous expressions.
For example, only allow specific predefined expressions or function calls to be evaluated:
var allowedExpressions = ['expression1', 'expression2'];
if (allowedExpressions.includes(userInput)) {
$scope.$eval(userInput);
}
6. Sanitize User Input
Before using user input in any context that might involve $eval(), it is essential to sanitize the input to prevent malicious scripts from executing. Use a sanitizer like $sanitize or an external library like DOMPurify to clean HTML and JavaScript content.
Example:
var sanitizedInput = $sanitize(userInput);
$scope.$eval(sanitizedInput); // Only use sanitized input
7. Limit Scope of $eval() Usage
Where possible, avoid the use of $eval() entirely. For most cases, AngularJS’s binding system, controllers, and services can provide safer alternatives to dynamic evaluation.
If you must use $eval(), ensure that it is limited to trusted data and used with caution. Keep the scope of its usage as narrow as possible.