What is a zero-day cybersecurity vulnerability?

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At first glance, a zero-day vulnerability can sound like a good thing. After all, we all want zero days without security issues! However, this type of vulnerability is one of the most dangerous. 

So, what is a zero-day cybersecurity vulnerability

It’s a flaw in software that hackers discover, and developers don’t know about, or one that they know about but haven’t been able to fix yet. Hackers love to find and take advantage of these areas, and when they do, it’s called a zero-day attack. 

This article covers the basics of these vulnerabilities, the problems they create, and the best ways to identify and fix them.

Zero-Day Definition—Why is it called a zero-day vulnerability?

These weaknesses are called “zero-day” because developers and coders literally have “zero days” to fix these vulnerabilities. These are not minor issues that can wait weeks or be fully solved by a quick patch. Once a hacker discovers a zero-day vulnerability, they will exploit it right away.

Examples of zero-day attacks

Because of its broad definition, a zero-day attack can happen to a wide variety of online users. 

It can target operating systems, web browsers, hardware, and almost anything else that runs on computer code—anything could have a vulnerability. 

Attacks aren’t limited to small-time companies either. Over the last few years, big-name providers such as Google, Microsoft Word, Apple, and Sony have all been targets of zero-day attacks, falling prey to hackers’ schemes through vulnerabilities they never knew about.

A recent high-profile example of a zero-day attack targeted motion picture giant Sony Pictures Entertainment. 

In September of 2015 a group of hackers calling themselves the Guardians of Peace discovered an undetected vulnerability in Sony’s code. They gained access to Sony’s systems, setting up undetected malware before Sony was scheduled to release some new films. The attack also affected emails sent from Sony executives and other sensitive studio information.

While the exact method the hackers used is unknown, it’s likely that a phishing scam was involved. Sony shut down its entire network for a few days in response to the gigantic attack, proving that no target is too big for hackers looking to exploit zero-day vulnerabilities. 

How are zero-days found?

Usually, hackers initially discover a zero-day vulnerability through constant probing. 

They’ll take advantage of the weakness by writing new code that infects the system itself or the browsers of unsuspecting users. Good developers–and sound security systems—are always on the lookout for suspicious behavior from the program or code.

In addition, these systems monitor strange emails or texts that look like they could be from a hacker trying to exploit a vulnerability. Without this careful monitoring, hackers can maintain access for days, weeks, or even months without developers realizing the flaw.

Zero-day protection and prevention

The best way to stop a zero-day attack is to make sure it never happens in the first place. 

Developers should be vigilant about testing updates, examining code, and even doing penetration testing (ethical hacking) to discover possible vulnerabilities before a hacker can. 

Once a vulnerability is found, it needs to be fixed as soon as possible. Remember, a zero-day attack can even target a flaw that coders know about, as long as they haven’t fixed it yet. 

You can also guard against zero-day attacks by keeping all your applications and software up to date. Developers may have discovered and patched vulnerabilities, but if you don’t have the latest version, your software is still vulnerable. 

Also, be sure to limit the number of applications you use to decrease opportunities for hackers.


 A zero-day cybersecurity vulnerability is bad news for any company, exposing it to attacks by hackers. Fortunately, preparation goes a long way toward preventing zero-day attacks. 

Stay on top of your security systems to ensure you’re not giving hackers a day to themselves.

If your business needs help assessing security vulnerabilities, contact us today to see how our team of cybersecurity professionals can help your business stay secure and exploit-free.