I hadn’t heard of nulled plugins and themes, as a phrase. I do sometimes look at plugins and themes beyond those offered on WordPress.org. I like shopping for plugins. I especially like trying them out and seeing if they magically make blogging easier in some way. Most don’t do what I hoped they would do. Some don’t even start once they are activated. Some give errors and won’t even load. Some load and activate but then I can’t find the plugin settings on my dashboard.
Some aren’t free.
Some plugins and themes come with a catch, hidden at times. This is what is meant about the risk in using nulled plugins or themes. You may be importing extra code to your site which is active behind the scenes. You may be advertising another site, for free. Or, you could be running a harmful code which will seem to originate from your site (and it may because you took a risk on a plugin or theme).
I will not say you should stick to the WordPress Plugin Directory 100% of the time. I don’t plan to. But, know there is a risk and don’t load up any theme or plugin without finding out how it works, what it does and what others have said about it.
It is really hard to resist when you see those premium plugins and themes available for free. The catch is that those freely available nulled premium plugins and themes are not actually free; they take your website as the payment. To put it simply, these nulled plugins and themes often contain malicious code which can do all sorts of things like redirecting users to other website(s), injecting links, creating backdoors, spamming other users, etc. In the worst case scenario, this malicious code in the nulled plugins or themes can effectively take down your website or blog.
The moral is, never use nulled plugins and themes. If you want to install a plugin or theme (be it free or premium), only download and install that plugin or theme from a reputable source like the developer’s website or WordPress repository. If you have already installed a couple of plugins or themes and want to test them for any malicious code, you can use Theme Authenticity Checker to check the plugins and themes.
Source: Make Tech Easier