Zero-Day Attacks: Meaning, Examples, and Modern Defense Strategies

Zero-day exploits (also known as zero-day vulnerabilities or zero-day threats) are security flaws in a system or software that are unknown to the vendor or creator. Attacks on zero-day flaws are known as zero-day attacks.

Zero-days are named as they are because they have spent zero days being addressed or have zero days to be addressed. This is either because the developers have not yet discovered them or have already been exploited by malicious actors, hence the name ‘zero-day.’

In this article, we will delve deeper into zero-days and why they matter in cybersecurity. We will look at some of the most popular cases of zero-day attacks, the zero-day economy, and explore countermeasures to zero-day attacks.

What are zero-day attacks? What does “zero-day” mean?

While zero-days are usually referred to as attacks, exploits, threats, or vulnerabilities, these terms are not interchangeable. Let’s get into each of them.

• Zero-day attacks are cyberattacks carried out by threat actors exploiting zero-day vulnerabilities. They occur when a hacker deploys malware into a system to exploit a vulnerability before developers have time to fix it.
• Zero-day exploits are tactics or techniques used by malicious actors to exploit weaknesses and attack systems.
• Zero-day threats are malicious actors or harmful code that exploit zero-day vulnerabilities to harm systems.
• Zero-day vulnerabilities are unknown weaknesses in a system that threats can target using malicious software or code.

Zero-day vulnerabilities are not the only kind of time-bound vulnerabilities in cybersecurity. There are also n-day vulnerabilities and one-day vulnerabilities.

N-day vulnerabilities

These are security flaws that have already been discovered, publicly disclosed, and usually patched. However, they are still exploitable because many systems haven’t been updated, or the patch hasn't been fully implemented.

Unlike zero-day vulnerabilities that no one knows about yet, and therefore haven’t been patched, n-day vulnerabilities are known and documented, but their systems remain unpatched. The “n” in n-day simply represents the number of days since the vulnerability became public knowledge.

One-day vulnerabilities

One-day vulnerabilities refer to vulnerabilities that have been publicly disclosed very recently, typically within 24 hours; hence, they’re essentially in the same category as n-day vulnerabilities. They represent the moment a vulnerability transitions from a zero-day (unknown and unpatched) to an n-day (known and usually patched).

A one-day vulnerability is a security flaw that has just been disclosed (usually within the last day). It often already has a patch or workaround available or is currently working on one. One-day vulnerabilities are now publicly documented, meaning attackers also know about them.

History of zero-days

Zero-day vulnerabilities have existed long before the term itself came to be. Through the 1970s and 1980s, as computers became more commonplace, hackers and researchers alike found exploitable vulnerabilities in systems even before the engineers who built them discovered them.

One particularly notable instance of this occurred in 1988, when the Morris Worm exploited vulnerabilities in the Unix sendmail, finger, and rsh/rexec services. These vulnerabilities were previously unknown, and the attack affected about 10% of all internet-connected computers at the time.

The term “zero-day” did not come into existence until the 1990s, when it first sprang up in software piracy spaces. While the term was initially intended to refer to software pirated on the day of its release, it eventually evolved to its modern-day meaning.

As zero-days became increasingly popular, researchers realized that it was financially rewarding to find vulnerabilities in systems and then sell the information at auction. By the early 2000s, the commercialization of this practice led to the creation of the “vulnerability marketplace.”

Why do zero-days matter?

Every organization needs to be acutely aware of the threat that zero-day attacks can pose to its systems and infrastructure. These days, zero-days have become so popular and valuable that incentives now exist for hackers to find zero-day vulnerabilities in organizations to exploit for profit. Some of these expeditions are state-sponsored, and others are courtesy of corporations seeking to engage in corporate espionage.

Unfortunately, most organizations are not sufficiently prepared to handle or repel zero-day attacks. This usually stems from the general sentiment that software security is reactive rather than proactive. Organizations typically wait for a breach of their systems, then hurriedly begin looking for solutions to fix the problem.

Unfortunately, zero-day attacks, by definition, require proactive measures to repel. Organizations that are serious about avoiding zero-day attacks need to prioritise early detection and swift responses. Potential vulnerabilities need to be explored in advance and, if discovered, patched before they become vectors for malicious actors to exploit.

Historical examples of zero-day attacks

Over the decades, the tech world has seen many instances of zero-day attacks. These attacks typically leave millions of dollars in losses in their wake. The biggest ones leave hundreds of millions, and sometimes, billions of users’ data compromised. Here are some of the most notorious examples.

The Stuxnet attack

In June of 2010, a piece of malware called Stuxnetwas discovered. It was a computer worm, no larger than 500 kilobytes, that infected the software of more than 14 industrial sites in Iran, including a uranium-enrichment plant. This worm exploited four zero-day vulnerabilities, causing significant impact across entire government facilities.

The Chrome attacks

One of the most recent examples of a famous zero-day attack was the one carried out in early 2022. The perpetrators were North Korean hackers who exploited a zero-day vulnerability in Google Chrome. Using phishing emails, they directed users to spoofed websites, where they installed spyware and malware on their victims’ devices.

Thanks to its sophisticated cybersecurity infrastructure, Google quickly detected the breach and patched it. However, the damage was already done. Neither Google nor independent research has determined to date what data was stolen.

How zero-day attacks work

As explained earlier, zero-day attacks work by finding vulnerabilities in a system before the software developers who built the system even realise they exist. Zero-day attacks will continue to work as long as the vulnerability remains undetected and unpatched. This makes zero-days a particularly unique type of scenario.

Zero-days are a sort of cybersecurity catch-22. Organizations cannot block zero-day exploits until they are exploited, and without being exploited, they cannot be aware of their existence. This truth forms the very essence of zero-day vulnerabilities.

Zero-days typically follow a predictable cyber kill chain. This is a repeatable, structured sequence of steps an attacker follows to compromise a target. A predictable kill chain usually follows a 7-step sequence:

1. Reconnaissance: The attacker gathers information about the target (e.g., through network scans or social engineering).
2. Weaponization: Creating or choosing the malware/exploit needed.
3. Delivery: Sending the malicious file, link, or payload (e.g., phishing email).
4. Exploitation: Triggering the vulnerability.
5. Installation: Installing malware or gaining a foothold.
6. Command & Control (C2): Connecting back to the attacker.
7. Actions on Objectives: Stealing data, encrypting files, moving laterally, etc.

Countermeasures: how to defend against zero-day attacks

Just because zero-day attacks cannot entirely be prevented doesn’t mean there are absolutely no means to protect your organization from them. Some of the most effective countermeasures against zero-day attacks are the following:

Zero-trust network architecture (ZTNA)

Zero-trust assumes every device, user, and connection is potentially compromised. By requiring continuous verification and restricting lateral movement, ZTNA ensures that even if a zero-day exploit grants an attacker initial access, it doesn’t grant them the keys to the entire network.

Zero-trust is becoming the go-to model, especially for remote and distributed enterprises.

Advanced threat detection and behavioral analytics

Traditional signature-based tools fail against zero-day threats, but behavioral analytics can detect anomalies in user activity, network traffic, and endpoint behavior. Solutions like EDR, XDR, and SIEM platforms use machine learning to flag deviations — such as unexpected privilege escalation, unusual data flows, or unrecognized process executions — enabling defenders to respond before significant damage occurs.

Strong endpoint security and isolation

Segmenting networks, sandboxing applications, and enforcing strict endpoint policies reduce the potential impact of a compromise. If a malicious payload executes, isolation prevents it from spreading beyond its initial entry point.

Continuous patch management

While zero-days are, by definition, unpatched, many attacks rely on chaining unknown vulnerabilities with known ones. An effective measure against this is patch management. Regular updates, timely patching, and automated vulnerability management tools drastically reduce an attacker’s ability to escalate privileges or maintain persistence.

Strict access controls and least-privilege policies

Restricting access rights ensures that even successful attacks have a limited scope. Least-privilege enforcement, MFA, and periodic credential audits help minimize opportunities for unauthorized actions.

Regular threat hunting and red team exercises

Proactive threat hunting helps identify suspicious patterns before attackers fully execute their kill chain. Red teaming reveals blind spots an organization may overlook, strengthening its defensive posture and preparing it to respond to unexpected exploits.

Robust incident response plan

Preparation is everything. An updated, rehearsed incident response plan ensures organizations can quickly contain, mitigate, and recover from zero-day attacks. Clear workflows, communication protocols, and defined responsibilities reduce downtime and data loss.

Zero Day with AI: How AI changes zero-day security

Artificial intelligence has been a game-changer in every aspect of information and systems technology. Zero-days are not immune to the impact of AI on both the attackers' and the victims' sides. AI has helped change the world of zero-days in the following ways:

AI accelerates zero-day discovery for attackers

Offensively, AI can rapidly scan codebases, binaries, and network behaviors to identify potential vulnerabilities that human researchers might overlook. Machine learning models can spot unusual patterns in software logic, predict weak points, and even generate working exploits with minimal human involvement. What once took weeks of manual reverse-engineering can now be compressed into hours.

Automated exploitation makes attacks more scalable

AI-driven exploitation frameworks can adapt in real time to different environments, privilege levels, or security configurations. Instead of relying on rigid scripts, attackers use reinforcement learning models that “test and learn” inside the target system, refining their tactics automatically. This makes zero-day attacks more scalable and more challenging to predict.

Defenders gain predictive detection and real-time insight

On the defensive side, AI excels at spotting the subtle signals of zero-day activity. Since zero-day exploits lack known signatures, defensive AI systems rely on anomaly detection by monitoring process behavior, system calls, and network patterns for signs of compromise. These models learn what “normal” looks like and flag deviations instantly, often identifying an attack before it fully executes.

AI enhances threat intelligence sharing

AI streamlines correlation across millions of global events, contextualizing threat signals faster than any human team. It can detect relationships between seemingly unrelated incidents, predict attacker motives, and surface hidden zero-day campaigns. This collective intelligence shortens response time and limits attackers' dwell time.

Offensive AI forces the rise of autonomous defense

As attackers automate, defenders must respond in kind. Autonomous defense platforms use AI to block suspicious actions as they occur by isolating endpoints, shutting down malicious processes, or quarantining files without waiting for human approval. This machine-speed response is critical when dealing with fast-moving zero-day exploits.

AI blurs the line between known and unknown threats

Traditionally, security relied on reactive signatures. AI flips this model by focusing on behavior rather than identity. This reduces the gap between known and unknown threats, making zero-days less uniquely dangerous. The same behavioral indicators that detect malware variants can also catch brand-new exploits.

The future of zero-day defense

The future of zero-day defense won’t rely on a single technology or strategy. Instead, it will be defined by how well organizations combine automation, intelligence, and resilient design. As attackers increasingly rely on AI to uncover and weaponize unknown vulnerabilities, defenders must move beyond reactive security models and embrace proactive, behaviour-driven protection. The emerging reality is clear: speed and adaptability are now as important as accuracy.

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