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What is a zero-day exploit, and why are they dangerous?

Most software has had a bug at some point. But what if that bug has the potential to open backdoors for hackers to exploit? Worse still, what if the developers of the software have no idea the vulnerability exists? Criminals can abuse these so-called zero-day vulnerabilities for months or even years before someone patches them. So, what are zero-day exploits and vulnerabilities, and how much damage can they cause?

What is a zero-day exploit, and why are they dangerous?

Table of Contents

Table of Contents

What is zero day?

Zero day definition

Zero day is a broad term that includes both vulnerabilities and exploits. A zero-day vulnerability is a flaw in software or hardware which is yet to be discovered by its developers. A zero-day exploit is when cybercriminals discover the vulnerability and abuse it for their own means.

“Zero day” refers to a software or hardware vulnerability unknown to people who would be interested in fixing it, for example, vendors. This term includes both zero-day vulnerabilities and zero-day exploits.

The term zero day alludes to the amount of time — zero days — that the software developer or vendor has been aware of the problem. And since the software is already in use, they have had zero days to address and patch the vulnerability.

In the best-case scenario, when someone discovers a zero-day vulnerability, they report it to the software developers so they can fix it. Unfortunately, sometimes hackers get to the vulnerability first and exploit it before the developer can address it.

Zero-day vulnerability defined

A zero-day vulnerability is a cybersecurity vulnerability in a piece of software or hardware yet to be discovered by its developers and vendors. This means there’s currently no way to plug the hole in security. It can be any vulnerability — a bug, lack of encryption, or missing authorizations. A zero-day vulnerability can pave the way for a zero-day exploit.

Zero-day exploit defined

A zero-day exploit is a method or technique that criminals use to take advantage of a zero-day vulnerability. It’s a code, tool, or strategy that cybercriminals use to exploit the security flaw that’s yet to be discovered by software’s creators. The zero-day exploit is the means for carrying out a zero-day attack.

Zero-day attack defined

A zero-day attack is the type of cyberattack that implements the zero-day exploit. It’s the process of exploiting the vulnerability to achieve an unauthorized action — installing backdoors, injecting malware, or stealing sensitive information.

How do zero-day attacks work?

Hackers carry out zero-day attacks by detecting a security flaw in the software or hardware, then writing and implementing code to take advantage of the vulnerability before the developers have had a chance to patch it.

A successful zero-day exploit opens access to the software or system for the attackers. By extension, these attacks also endanger the cybersecurity of the software users — hackers might steal their personal information and use it for illegitimate purposes.

For example, if attackers carried out a successful zero-day IoT attack on a smart building, its occupants could lose access to the building, and the attackers could steal the information about the occupant behavior and even disrupt the critical infrastructure, like cutting off the power or water supply.

Who executes zero-day attacks?

Depending on the motivation, different groups of malicious actors can use zero-day exploit to infiltrate systems with zero-day vulnerability. The most common perpetrators include:

  • Cybercriminals – Groups or individuals interested in exposing sensitive data mostly for financial gain.
  • Hacktivists – malicious actors motivated by political and social causes who want the attack to draw attention to their wanted cause.
  • Corporate espionage hackers – hackers who spy on companies to gather relevant information, such as trade secrets and undisclosed records.
  • Cyberwarfare agents – government agencies and political actors that take advantage of cybersecurity vulnerabilities to spy or attack another country’s cyberinfrastructure.

Each of these entities exploits cyber risks such as zero-day vulnerability to gain access to unsecure systems. While some perpetrators act silently, others can sell zero-day exploits on the dark web and even share information on vulnerabilities in online cybersecurity forums and social media.

What are the most common zero-day attack targets?

Cybercriminals and governments use zero-day vulnerabilities targeting operating systems, web browsers, office applications, hardware, firmware, and even Internet of Things (IoT) systems. The wide array of targets makes the list of targets respectively long:

  • Large businesses and organizations.
  • Government agencies.
  • Critical infrastructure.
  • Research institutions and universities.
  • High-profile individuals, especially those possessing valuable, sensitive information or who have access to vulnerable systems.
  • Political targets and national security threats.

You can also categorize zero-day attack targets based on whether the zero-day attack is targeted or not:

  • Targeted zero-day attacks focus on potentially valuable targets, such as large corporations, high-profile individuals, and government agencies.
  • Non-targeted zero-day attacks are commonly used against users of vulnerable systems, such as browsers or operating systems.

Examples of zero-day attacks

Many infamous zero-day attack instances have occurred throughout modern history. Let’s take a look at some of the most notorious incidents.


Stuxnet was a computer worm that used different Windows zero-day vulnerabilities to target supervisory control and data acquisition (SCADA) systems.

The worm caused enormous damage to Iran’s nuclear program. It destroyed nearly a fifth of Iran’s nuclear centrifuges and infected a staggering 200,000 computers. It’s often described as one of the first cyber weapons because the perpetrators behind the worm are thought to be the United States and Israel.

Sony hack

The Sony hack in 2014 also tops the list as one of the most famous zero-day exploits. During the Sony Pictures hack, criminals utilized a zero-day vulnerability to break into the company’s network and steal data.

Hackers later released the incredibly sensitive information, including copies of upcoming movies, the company’s plans for the future, business deals, and emails from Sony’s top management. What specific exploit the hackers used remains a mystery to this day.


Back in 2017, hackers found a vulnerability in Microsoft Word and developed the Dridex malware, which they then hid in MS Word attachments. Those who downloaded these infected files would activate the Dridex trojan. This dangerous bank fraud malware spread to millions of users worldwide. Word wasn’t the only Microsoft product attacked — the company suffered a Microsoft Exchange zero-day vulnerability in 2021.

Firefox zero-day

In 2020, Firefox had a vulnerability that allowed hackers to place and execute code inside Firefox’s memory. This enabled criminals to run malicious code on any of their victims’ devices. The developers released an emergency patch, but not before some hackers managed to exploit it.

Zoom zero-day threats

In 2020, Zoom faced two serious zero-day vulnerabilities. One allowed potential credential theft through a malicious link in Zoom chat on Windows. The other affected Macs — the vulnerability enabled attackers to gain root access and control of the user’s microphone and camera. The Zoom zero-day exploits were quickly fixed by releasing relevant patches.

Google Chrome zero-day vulnerabilities

2021 wasn’t great for Chrome in regard to zero-day exploits. The browser had to issue three emergency patches for zero-day vulnerabilities that year. One of the flaws could enable remote code execution and DDoS attacks on affected systems. 2022 saw another bout of Google Chrome zero-day attacks, but the vulnerabilities have been patched since.


In 2021, the US software provider Kaseya suffered a zero-day cyberattack that targeted its VSA software. The attackers exploited a zero-day vulnerability in the VSA software to distribute ransomware to numerous managed service providers (MSPs) and their customers. This attack resulted in widespread disruption, with thousands of organizations affected globally.


The Log4Shell zero-day vulnerability was a critical security flaw found in 2021 in the Apache Log4j logging library, which is widely used in Java-based applications. It allowed remote attackers to execute arbitrary code on vulnerable Java servers. Due to the widespread use of Log4j, the vulnerability posed a significant threat to a vast number of web applications and services globally, which led to various security risks, including data breaches, server hijacking, and unauthorized access to sensitive information.


Shellshock was a critical zero-day vulnerability discovered in September 2014. It affected the Bash shell, a widely used command-line interpreter in Unix-based operating systems (such as Linux and MacOS). This vulnerability allowed attackers to execute arbitrary commands on vulnerable systems remotely through network services such as web servers and DHCP clients. The widespread use of Unix-based systems made Shellshock a significant threat, leading to widespread patching and mitigation efforts by system administrators and software vendors.

Petya and NotPetya

Discovered in 2016 and 2017 respectively, Petya and NotPetya exploited vulnerabilities in the Windows operating system. Both zero-day exploits used multiple methods for distribution, including phishing emails and malware kits, rendering the system unusable until a ransom was paid.

While similar to its predecessor, NotPetya’s main objective seemed to be to cause disruption rather than generate ransom payments. The exploits resulted in widespread damage to organizations globally, affecting critical infrastructure, businesses, and government agencies.


WannaCry was a ransomware attack that occurred in May 2017. It targeted computers running the Microsoft Windows operating system by exploiting a vulnerability in the Server Message Block (SMB) protocol.

The exploit, also known as EternalBlue, was allegedly developed by the United States National Security Agency (NSA) but was leaked by a group called the Shadow Brokers. It infected thousands of computers in over 150 countries within hours, encrypting files and enabling attackers to demand a ransom payment in Bitcoin for their release.

How to know if you have been a victim of a zero-day attack

The term zero-day attack suggests that there is no way to know that you’re being attacked until it’s too late. However, you can mitigate potential damage if you keep an eye on and react to these warning signs:

  • Unusual behavior. Unexpected system crashes or slowdowns could be a sign of a potential cyberattack.
  • Unexplained network activity. If you notice suspicious connections or domains in your network logs, it could mean your system has uninvited guests.
  • Security alerts. Antivirus software and intrusion detection system alerts can be an indication of an attempted or successful cyberattack.
  • Unexplained file changes. Surprise changes to files or directories on your system, such as files being encrypted or modified without your permission, are another red flag for potential cyberattacks.
  • Unexplained account activity. Logins from unfamiliar locations or changes to account settings are a strong indication of breached security.
  • Unexplained data loss. Missing data and corrupted files (especially if you can’t recover them) could be the result of a cyberattack.

How to protect yourself from zero-day attacks and exploits

So how do you protect yourself from a threat you don’t know about? Sometimes, hackers use zero-day vulnerabilities alongside other attack methods, such as social engineering attacks. Here’s how to lower your risk of falling victim to a zero-day attack:

  • Update your software ASAP. Software updates often contain patches for critical vulnerabilities.
  • Stay informed. Vulnerability databases and bug bounty programs are vital in detecting flaws in your software.
  • Be wary of phishing scams. Some zero-day attacks only work when combined with other attacks. Don’t click on unknown links or email attachments — you may end up providing sensitive data to criminals.

Make sure you’re using a VPN and antivirus software to protect your device from potential cyber threats, such as malware, that could open up a backdoor to your system. A VPN will help protect your company’s network and even block potential phishing sites. Here’s how:

  • A VPN protects your company network. A VPN encrypts all online traffic for every device connected to your network. If your staff are using their own unencrypted apps to send sensitive information, it could easily be intercepted and stolen by criminals. Make sure your employees are aware of split tunneling security risks to discourage them from letting apps access the internet directly on work devices.
  • A VPN blocks phishing sites. NordVPN includes the Threat Protection feature, which blocks malicious malware-loaded sites. It also works to stop pop-up ads, which are notorious for spreading spyware and other file-stealing malware. Threat Protection scans the files you download and identifies malware-ridden ones.

Most organizations’ responses to cybersecurity incidents tend to be reactionary — responding to previously known threats. However, the problem with zero-day vulnerability is that by the time you know what happened, it’s already too late.

The key to zero-day protection is a proactive approach. Detection, data, and activity monitoring are some of the first steps in avoiding zero-day attacks.

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