JavaScript is becoming ubiquitous and more popular now. However, it has some security issues if not used properly. Two of the most commonly known examples are cross-site request forgery (CSRF) and cross-site scripting. I'll touch very briefly upon these two topics as a way to prepare you for the remainder of the book.
I decided to start off with this topic as it is generally easier to explain and understand. To put it simply, cross-site request forgery refers to a type of malicious exploitation of a website where unauthorized commands are transmitted from an unknowing user that the website trusts.
The following straightforward example involves Ajax requests: go back to earlier sections where we talked about POST requests. Imagine that your server endpoint does not defend itself against an Ajax request made outside of your domain name, and somehow, malicious POST requests are made. That particular request can somehow be made to alter your database information and more.
You may argue that we can make use of CSRF tokens (a common technique to prevent cross-domain requests and a way to provide greater security to the site) as a security measure, but it is not entirely safe. For instance, the script that is performing the attack could be residing in the website itself; the site could have been hijacked with malicious script in the first place.
In addition, if some of the following conditions are met, CSRF can be achieved:
The defending websites do not check the referrer header
The attacker will need to:
Find a form submission endpoint (that typically has important side effects, such as monetary exchange or exchange of highly personal information)
Guess the right values for the form inputs in order to carry out the attack
Cross-site scripting (XSS) enables attackers to inject a client-side script (usually JavaScript) into web pages that are used by users. The general idea is that attackers use the known vulnerabilities of web-based applications, servers, plugin systems (such as WordPress), or even third-party JavaScript plugins to serve malicious scripts or content from the compromised site. The end result is that the compromised site ends up sending content that contains the malicious content/script.
If the content happens to be a piece of malicious JavaScript, then the results can be disastrous: since we know that JavaScript has global access to the web page, such as the DOM, and given the fact that that piece of JavaScript can have access to the cookies issued by the site (thus allowing the attacker to gain access to potentially useful information), that piece of JavaScript can do the following:
Make changes on the DOM so that it creates links, malicious content, and more
Perform actions on behalf of the user, such as performing web form submissions or Ajax operations straight from the site
If you are new to all this, all you need to remember at this point in time is that such security flaws come from both server-side and client-side weaknesses. We'll be touching upon them in the next chapter.