Zero-day Vulnerability: Definition and Examples?

Zero-day vulnerabilities are protocol, software, and application vulnerabilities not yet known to the general public or the product developers where the vulnerability exists. Since a zero-day vulnerability is unknown to the public or the developers, patches are unavailable.

According to GPZ research, half of the 18 zero-day vulnerabilities exploited by hackers in the first half of 2022 before a software update was made available could have been prevented had software vendors conducted more thorough testing and created more comprehensive patches. Surprisingly, at least four of this year’s zero-day vulnerabilities were variations from 2021.

But what exactly is a zero-day vulnerability? That’s what you’ll learn in this guide. But to fully understand the definition, we first must define a few other things.

What Is a Zero-day Exploit?

A zero-day exploit is a security vulnerability that has not been publicly disclosed or fixed. The term refers to both the exploit itself and the package of code that includes the exploit and related tools.

Attackers often use zero-day exploits to deploy malware on systems  and networks that have not been patched. Defenders can also use them to conduct penetration testing to detect network vulnerabilities.

You could hear the terms “zero-day vulnerabilities,” “zero-day exploits,” or “zero-day attacks” when learning about zero-day exploits. These terms have a crucial difference:

• The way hackers employ to target software is known as a “zero-day exploit,” 
• The defect in your system is known as a “zero-day vulnerability.”
• “Zero-day attacks” are what hackers do when they exploit a vulnerability to infiltrate your system.

When talking about zero-day vulnerabilities, the word “undiscovered” is essential because to be called a “zero-day vulnerability,” a flaw must be unknown to the system’s designers. When a security flaw is discovered, and a fix is made available, it ceases to be a “zero-day vulnerability.”

Zero-day exploits can be used by attackers in various ways, including:

• To exploit unpatched systems (i.e., without applying security updates) to install malware or take control of computers remotely;
• To conduct phishing campaigns (i.e., sending emails trying to trick recipients into clicking on links or attachments) using malicious attachments or links leading to website hosting exploits; or
• To perform denial-of-service attacks (i.e., flooding servers with requests so legitimate requests cannot get through).

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What Unique Characteristics of Zero-day Exploits Make Them So Dangerous?

There are two categories of zero-day vulnerabilities:

Undiscovered:The software vendor has yet to learn about the flaw. This type is extremely rare because most large companies have dedicated teams working full-time to find and fix their software’s flaws before hackers or malicious users discover them.

Undetected:The flaw has been found and fixed by the software developer — but no one has reported it yet because they haven’t noticed anything wrong with their system. This vulnerability can be very valuable if you’re looking to launch an attack against someone else’s system and don’t want them to know what’s going on until after it’s done!

Zero-day exploits are particularly risky since they have a higher chance of success than assaults on known flaws. When a vulnerability is made public on day zero, businesses still need to patch it, which makes an attack conceivable.

The fact that certain sophisticated cybercriminal organizations deploy zero-day exploits strategically makes them much riskier. These firms save zero-day exploits for high-value targets, including government agencies, financial institutions, and healthcare facilities. This can lengthen the duration of the attack and decrease the likelihood that the victim will find a vulnerability.

Users must continue to upgrade their systems even after a patch has been created. If they don’t, until the system is patched, attackers can still use a zero-day exploit.

How To Identify a Zero-day Vulnerability?

The most common way to identify a zero-day vulnerability is using a scanner such as Nessus or OpenVAS. These tools scan your computer for vulnerabilities using signatures (known bad files). If a signature matches, the scanner can tell you what file it matches against.

However, this type of scanning often misses many vulnerabilities because signatures are only sometimes available or updated frequently enough to catch all new threats as they emerge.

Another method of identifying zero days is reverse engineering software binaries (executable files). This method can be very difficult but is usually unnecessary for most people because plenty of free option scanners online do not require any technical knowledge or expertise to use effectively.

Examples of Zero-day Vulnerabilities

Some examples of zero-day vulnerabilities include:

Heartbleed — This vulnerability, discovered in 2014, allowed attackers to extract information from servers that use OpenSSL encryption libraries. The vulnerability was introduced in 2011 but wasn’t discovered until 2 years later when researchers found that certain versions of OpenSSL were susceptible to heartbeats sent by attackers. Hackers could then obtain private keys from servers using this encryption library, allowing them to decrypt data being transmitted by users.

Shellshock — This vulnerability was discovered in 2014 and allowed attackers to gain access to systems running an operating system vulnerable to attack through the Bash shell environment. Shellshock affects all Linux distributions and Mac OS X 10.4 and earlier versions. Although patches have been released for these operating systems, some devices haven’t yet patched against this exploit.

Equifax Data Breach – The Equifax data breach was a major cyberattack in 2017. The attack was perpetrated by an unknown group of hackers who breached Equifax’s website and stole approximately 145 million customers’ personal information, including Social Security numbers and birth dates.

WannaCry Ransomware – WannaCry is a ransomware virus that targets Microsoft Windows operating systems; it encrypts users’ files and demands a ransom payment through Bitcoin to decrypt them. It spreads through networks using EternalBlue. A Windows exploit leaked from the NSA in April 2017. The worm has affected over 300,000 computers worldwide since its release on May 12th, 2017.

Malware Attacks on Hospitals – Malware attacks have become increasingly common in recent years as hackers target healthcare organizations for personal gain or political reasons. One such attack involved hackers gaining access to patient records at Hollywood Presbyterian Medical Center via phishing emails sent from the hospital’s administration. 

Final Words

A zero-day vulnerability is a software bug that has been identified but has not yet been disclosed to the software vendor. It’s “zero days” from being known, at least by the public. In other words, it’s an exploit in the wild that no one quite knows about—except for whoever discovered and reported it first.

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Ahona Rudra

Domain & Email Security Expert at PowerDMARC

Ahona is the Marketing Manager at PowerDMARC, with 5+ years of experience in writing about cybersecurity topics, specializing in domain and email security. Ahona holds a post-graduation degree in Journalism and Communications, solidifying her career in the security sector since 2019.

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