My webpage

JavaScript in HTML

JavaScript is a programming language commonly used in websites to perform functions that the HTML cannot do. It can be used for validating forms, detecting browsers, adding dynamic functionality, and more!

It is beyond the scope of this guide to teach you JavaScript, but below you can learn how to embed or integrate JavaScripts into your website using the tag.

Embedding Scripts

External Files Probably the most common way, and often preferred way, is to define the scripts in a separate file and link to them using the src attribute of the script tag. The type must be set to text\javascript , and optionally the language attribute set to JavaScript1.2 or the version of JavaScript required for you script. Between SCRIPT Tags Instead of specifying a src attribute, you can write javascript between the tags. The type and language attributes are still used the same as when specifying external scripts.

 var myVar="hello"; function showAlert() 

Events

Some html elements have event attributes, that can be used to call javascript functions when an event is triggered. There are many events available, but these are the ones you are most likely to use:

OnClick As the name suggests, the onClick attribute is executed when an element, such as a link or image, is clicked on. OnMouseOver Use the OnMouseOver attribute to call javascript functions that you want to run when a user moves the mouse onto and hovers over an element. OnMouseOut Another mouse event handler, except OnMouseOut is the opposite to OnMouseOver , and will be called when the users’ mouse is moved back off an element.

Example:

The following example has two JavaScript functions defined in the header, which are activated by clicking on the links. Note that each link shows a different way of calling the function, you would only need to pick the one that suits you.

   function functionOne() < alert('You clicked the top text'); >function functionTwo() 
Top Text
 
Bottom Text
  

Example:

Using inline javascript to change the CSS class of an image when rolling the mouse over it.

 .red < border: 2px solid red; >.blue 

Notes & Warnings

JavaScript can be a powerfull tool when used properly, or a big mess when used incorrectly. Good scripts can make websites feel professional and easier to use. Bad scripts, or badly implemented scripts, can make a site annoying or prevent it from working alltogether. Some scripts can also be dangerous, exposing sites to script vulnerabilities and hacks. Some older browsers may not support Javascript, or have it disabled, so you may want to offer a non-script alternative.

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JavaScript in an HREF or SRC Attribute

In the above example, notice that the HREF is set with a value starting with “javascript:”. This identifier tells the browser to execute the code following that prefix. For those that are security savvy, you might be thinking about cross-site scripting when you hear about executing JavaScript within the browser. For those of you that are new to security, cross-site scripting refers to the ability for an attacker to execute unintended JavaScript in the context of your application (https://www.owasp.org/index.php/Cross-site_Scripting_(XSS)).

I want to walk through a simple scenario of where this could be abused. In this scenario, the application will attempt to track the page the user came from to set up where the Cancel button will redirect to. Imagine you have a list page that allows you to view details of a specific item. When you click the item it takes you to that item page and passes a BackUrl in the query string. So the link may look like:

On the page, there is a hyperlink created that sets the HREF to the backUrl property, like below:

When the page executes as expected you should get an output like this:

There is a big problem though. The application is not performing any type of output encoding to protect against cross-site scripting. If we instead pass in backUrl=”%20onclick=”alert(10); we will get the following output:

In the instance above, we have successfully inserted the onclick event by breaking out of the HREF attribute. The bold section identifies the malicious string we added. When this link is clicked it will prompt an alert box with the number 10.

To remedy this, we could (or typically) use output encoding to block the escape from the HREF attribute. For example, if we can escape the double quotes (” -> " then we cannot get out of the HREF attribute. We can do this (in PHP as an example) using htmlentities() like this:

When the value is rendered the quotes will be escapes like the following:

Notice in this example, the HREF actually has the entire input (in bold), rather than an onclick event actually being added. When the user clicks the link it will try to go to https://www.developsec.com/” onclick=”alert(10); rather than execute the JavaScript.

But Wait… JavaScript

It looks like we have solved the XSS problem, but there is a piece still missing. Remember at the beginning of the post how we mentioned the HREF supports the javascript: prefix? That will allow us to bypass the current encodings we have performed. This is because with using the javascript: prefix, we are not trying to break out of the HREF attribute. We don’t need to break out of the double quotes to create another attribute. This time we will set backUrl=javascript:alert(11); and we can see how it looks in the response:

When the user clicks on the link, the alert will trigger and display on the page. We have successfully bypassed the XSS protection initially put in place.

Mitigating the Issue

There are a few steps we can take to mitigate this issue. Each has its pros and many can be used in conjunction with each other. Pick the options that work best for your environment.

• URL Encoding – Since the HREF is meant to be a URL, you could perform URL encoding. URL encoding will render the javascript benign in the above instances because the colon (:) will get encoded. You should be using URL encoding for URLs anyway, right?
• Implement Content Security Policy (CSP) – CSP can help limit the ability for inline scripts to be executed. In this case, it is an inline script so something as simple as ‘Content-Security-Policy:default-src ‘self’ could be sufficient. Of course, implementing CSP requires research and great care to get it right for your application.
• Validate the URL – It is a good idea to validate that the URL used is well formed and pointing to a relative path. If the system is unable to parse the URL then it should not be used and a default back URL can be substituted.
• URL White Listing – Creating a white list of valid URLs for the back link can be effective at limiting what input is used by the end user. This can cut down on the values that are actually returned blocking any malicious scripts.
• Remove javascript: – This really isn’t recommended as different encodings can make it difficult to effectively remove the string. The other techniques listed above are much more effective.

The above list is not exhaustive, but does give an idea of ways to help reduce the risk of JavaScript within the HREF attribute of a hyper link.

It is important to note that this situation also applies to the IFRAME SRC attribute. it is possible to set the SRC of an IFRAME using the javascript: notation. In doing so, the javascript executes when the page is loaded.

When developing applications, make sure you take this use case into consideration if you are taking URLs from user supplied input and setting that in an anchor tag or IFrame SRC.

If you are responsible for testing applications, take note when you identify URLs in the parameters. Investigate where that data is used. If you see it is used in an anchor tag, look to see if it is possible to insert JavaScript in this manner.

For those performing static analysis or code review, look for areas where the HREF or SRC attributes are set with untrusted data and make sure proper encoding has been applied. This is less of a concern if the base path of the URL has been hard-coded and the untrusted input only makes up parameters of the URL. These should still be properly encoded.

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How to Use Inline JavaScript in HTML: A Comprehensive Guide to Best Practices and Tips

Learn how to add, interpret, and call inline JavaScript in HTML with our comprehensive guide. Discover the benefits and best practices to optimize performance and maintainability.

• Adding Inline JavaScript to HTML
• Interpreting Inline JavaScript in HTML
• 1. JavaScript Programming: Introduction to Inline Scripts and Functions
• Calling Inline JavaScript from HTML
• Placing the Script Tag in HTML
• Best Practices for Using Inline JavaScript
• Other code snippets demonstrating how to use inline JavaScript in HTML
• Conclusion
• How to add JavaScript inline to HTML?
• How to use internal JS in HTML?
• Which HTML tag is used to write inline JavaScript?
• Can you inject JavaScript into HTML?

As web developers, we often use JavaScript to create dynamic and interactive web pages. One common way of using JavaScript in HTML is through inline scripts. In this comprehensive guide, we will discuss best practices and tips for using inline javascript in html .

Adding Inline JavaScript to HTML

Inline JavaScript is added to HTML using the tag. The tag can be placed anywhere within the or sections of an HTML document. However, it is recommended to place the tag at the bottom of the section to improve page loading times.

 
Welcome to my webpage
   

Inline JavaScript is useful for small code snippets because it eliminates the need to create a separate JavaScript file . However, for larger code, it is better to use external JavaScript files as they are easier to maintain and debug.

Interpreting Inline JavaScript in HTML

The code written between the tags is interpreted as JavaScript by the browser. The common syntax used in inline JavaScript is the same as in external JavaScript files.

To write clean and maintainable code , it is important to follow best practices. These include keeping the code minimal and optimizing it for performance. Avoid using global variables and function declarations inside inline scripts.

Here is an example of best practices for writing inline JavaScript:

 
Welcome to my webpage
   

1. JavaScript Programming: Introduction to Inline Scripts and Functions

It also demonstrates how to write and call a simple JavaScript It is assumed you already have Duration: 13:23

Calling Inline JavaScript from HTML

Inline JavaScript can be called from HTML using event handlers such as onclick , onload , and onsubmit . Event handlers are attributes on HTML elements that define a JavaScript function to be executed when the event occurs.

Here is an example of calling inline JavaScript from HTML:

While inline JavaScript can be convenient for small code snippets, it has some disadvantages. For example, it can make the HTML code harder to read and maintain. It can also lead to code duplication and decreased performance.

Placing the Script Tag in HTML

The placement of the tag in HTML can affect page loading times. The recommended practice is to place the tag at the bottom of the section to allow the HTML content to load first.

However, in some cases, it might be necessary to place the tag in the section. For example, if the JavaScript code needs to be executed before the HTML content is loaded.

Here is an example of placing the tag in the section:

   
Welcome to my webpage
  

Best Practices for Using Inline JavaScript

To ensure that inline JavaScript is clean and maintainable, it is important to follow best practices. These include:

• Keeping the code minimal
• Avoiding global variables and function declarations
• Using event handlers instead of inline JavaScript where possible
• Using external JavaScript files for larger code snippets
• optimizing the code for performance

Here is an example of best practices for using inline JavaScript:

By following best practices, you can ensure that your inline JavaScript code is clean, maintainable, and optimized for performance.

Other code snippets demonstrating how to use inline JavaScript in HTML

In Html case in point, Simple example of using inline javascript in html code sample

      
Click Me 
 
in this example we saw how to use inline JavaScript or directly in an HTML tag.
  

Conclusion

Inline JavaScript is a useful tool for adding small code snippets to HTML. However, it is important to follow best practices to ensure that the code is clean, maintainable, and optimized for performance.

In general, it is recommended to use external JavaScript files for larger code snippets. This makes the code easier to maintain and debug.

By following the best practices outlined in this guide, you can ensure that your inline JavaScript code is of high quality and provides a great user experience.

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