Key Takeaways

Zero-day vulnerabilities are unknown and unpatched flaws exploited by attackers before vendors can fix them.
Attacks follow a lifecycle, from vulnerability discovery and exploit development to delivery and execution.
Detection requires diverse methods, including vulnerability scanning, performance monitoring, and user reports.
High-value targets like government, financial, and IT organizations are frequently attacked, but any entity holding valuable data is at risk.
Prevention involves timely patching, robust security software, user access controls, and proactive threat hunting.

Imagine a shadow lurking inside your software, an invisible crack in the foundation, ready to be triggered before anyone even knows it exists. That’s the real danger of zero-day vulnerabilities, previously unknown flaws in protocols, software, or applications that leave no time for defense. By definition, there’s no patch and no warning; hackers exploit them in the wild while developers and users remain oblivious, turning the unknown into a weapon.

According to Google’s Threat Intelligence Group, in 2024, attackers exploited 75 zero-day vulnerabilities, down from 98 in 2023, but still significantly more than the 63 reported in 2022. Notably, 44% of those zero-days targeted enterprise platforms, up from 37% in 2023, with nearly two-thirds of enterprise zero-days hitting security and networking products. Meanwhile, exploitation of browsers and mobile devices fell sharply: browser zero-days dropped by roughly one-third, and mobile by nearly half year-over-year.

But what exactly is a zero-day vulnerability? That’s what you’ll learn in this guide. Keep on reading!

What Is a Zero Day Vulnerability?

A zero-day vulnerability is a hidden flaw in software, hardware, or protocols that has not yet been discovered or patched by developers. Since no fix is available, attackers have a “zero-day” window to exploit the weakness before it becomes publicly known. These exploits often come packaged with malicious code, sometimes referred to as zero-day attacks or day-0 exploits.

Zero-day vulnerabilities are dangerous because they give attackers the upper hand: organizations are unaware of the flaw, no security updates exist, and traditional defenses may fail to detect the threat.

The terms zero-day vulnerability, zero-day exploit, and zero-day attack are often used interchangeably, but they don’t mean the same thing. Each represents a different stage in the lifecycle of a security flaw, from the hidden weakness itself to the tools that take advantage of it to the real-world attack that causes damage:

Zero-day vulnerability → the undiscovered flaw in the system.
Zero-day exploit → the method or code hackers use to take advantage of the flaw.
Zero-day attack → the actual cyberattack carried out using the exploit.

Once a vulnerability is discovered and patched, it no longer qualifies as a zero-day.

Key examples

Zero-day vulnerabilities have been behind some of the most damaging cyberattacks in history. These flaws, often unnoticed for years, give attackers a critical window to steal data, disrupt services, or install malware before a fix is available.

Here are some notable examples:

Heartbleed (2014): A flaw in OpenSSL that let attackers steal sensitive data like private keys directly from server memory.
Shellshock (2014): A vulnerability in the Bash shell that allowed remote attackers to execute arbitrary commands on Linux and macOS systems.
Equifax Breach (2017): Hackers exploited an Apache Struts vulnerability to steal data of 145 million people, including Social Security numbers.
WannaCry (2017): A ransomware worm leveraging a Windows SMB flaw (EternalBlue) that infected 300,000+ systems worldwide.
Hospital Malware Attacks: Healthcare providers like Hollywood Presbyterian Medical Center were hit with ransomware and phishing campaigns, often fueled by zero-day exploits.

Common targets

A zero-day exploit can target any individual or organization that can bring them profits. Common targets include:

High-value targets, including government agencies, financial institutions, and healthcare facilities.
Companies with poor cybersecurity.
Companies that record users’ data like names, contact details, financial details, addresses, social security numbers, medical details, etc.
Companies that handle confidential data.
Companies that develop software and hardware for customers.
Companies that work for the defense sector.

This strategic targeting can lengthen the duration of the attack and decrease the likelihood that the victim will find a vulnerability. For example, cloud computing giant Rackspace made a public announcement that hackers accessed the personal data of 27 customers during a ransomware attack that leveraged a zero-day exploit.

Why Are Zero Day Vulnerability So Dangerous?

Zero-day vulnerabilities are uniquely dangerous because they exist in a gap between discovery and defense. At this stage, the flaw is unknown to the software vendor, undetected by security systems, and unpatched by users. This makes it an open opportunity for attackers to strike before defenses can be prepared.

Core dangers of zero-day exploits:

No patch exists: Since the flaw is undiscovered, vendors haven’t released a fix. Organizations remain vulnerable until a patch is developed and deployed.
High probability of success: Traditional defenses like antivirus or intrusion detection rely on known threat signatures. Zero-day bypasses these, giving attackers a direct path inside.
Reactive vs. proactive defense: Defenders often don’t know a zero-day exists until it’s actively exploited. By then, attackers may already have stolen data, installed malware, or disrupted operations.
Strategic value to hackers: Advanced cybercriminal groups often save zero-days for high-value targets, such as governments, enterprises, or critical infrastructure, maximizing damage and impact.

Because of these characteristics, zero-day exploits frequently lead to data breaches, financial losses, reputational damage, and prolonged recovery times. The danger lies in the fact that defenders have no head start and the race to respond only begins once the attack is already underway.

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The Lifecycle of a Zero Day Exploit

A zero-day exploit doesn’t appear overnight. This is a process that follows a lifecycle that determines how long attackers can weaponize the flaw before defenders catch up. Each stage represents a critical point in the timeline where the balance of power shifts between attackers and security teams.

Stage 1: Discovery

The lifecycle begins when a flaw is first uncovered. This can happen in two primary ways:

Malicious discovery: Threat actors actively scan and test software, hardware, or protocols, looking for weaknesses. They may use fuzzing tools, reverse engineering, or brute-force methods to trigger unexpected behavior.
Benign discovery: Security researchers or ethical hackers identify vulnerabilities during audits, penetration testing, or bug bounty programs.

At this point, the discoverer decides what to do:

Report responsibly to the vendor so a fix can be developed.
Exploit directly for personal gain or sabotage.
Sell the vulnerability on dark web marketplaces, where zero-days can fetch hundreds of thousands, or even millions, of dollars depending on the target (e.g., iOS, enterprise software, or critical infrastructure).

Stage 2: Exploit Creation

Once the flaw is known, attackers begin crafting an exploit, malicious code designed to take advantage of the vulnerability. This is the weaponization stage:

The exploit is written to target the flaw precisely, whether by injecting code, bypassing security checks, or executing unauthorized commands.
More advanced attackers may chain multiple zero-days together for a multi-layered attack, significantly increasing the impact.

At this stage, the vulnerability has transformed from an unknown bug into an operational threat.

Stage 3: Infiltration

With the exploit ready, attackers need a way to deliver it to the target environment. Common delivery vectors include:

Phishing and spear-phishing emails with infected attachments or malicious links.
Drive-by downloads on compromised websites, where simply visiting the page triggers the exploit.
Trojanized software or updates, where legitimate-looking applications are bundled with hidden exploits.
Removable media (USB drives, etc.), especially in targeted attacks against air-gapped systems.

The infiltration stage determines whether the exploit reaches a wide audience (mass campaigns) or a specific high-value target (espionage or sabotage).

Stage 4: Exploitation and Execution

Once delivered, the exploit is executed on the target system. This is where the attack becomes visible, though often too late. Depending on the attacker’s intent, the exploit may:

Install malware or ransomware to encrypt files and demand payment.
Create backdoors for persistent remote access.
Escalate privileges, giving attackers full control of the system.
Exfiltrate sensitive data such as intellectual property, financial records, or personal information.
Disrupt operations through denial-of-service or system manipulation.

At this point, the zero-day exploit will be actively causing damage.

How to Detect a Zero-day Vulnerability?

Detecting zero-day vulnerabilities is one of the most complicated challenges in cybersecurity because, by definition, these flaws are unknown to vendors and traditional security tools.

Detection typically falls into two approaches: proactive discovery, where organizations actively hunt for hidden flaws before they’re exploited, and reactive detection, where defenders identify suspicious activity or evidence of an ongoing attack.

Proactive discovery

Proactive methods aim to uncover vulnerabilities before attackers can weaponize them:

Fuzzing: Feeding unexpected or random inputs into software to trigger crashes or abnormal behavior that may reveal unknown flaws.
Anomaly-based scanning: Using advanced scanning tools to detect unusual patterns or system responses that don’t match expected behavior.
Reverse engineering: Breaking down software or malware code to uncover hidden vulnerabilities or to understand how an exploit works.

Reactive detection

When a zero-day slips past proactive measures, reactive techniques help uncover them after exploitation has begun:

Behavior-based monitoring: Tracking unusual system or network activity, like unexplained traffic spikes, privilege escalations, or process anomalies, that may indicate exploitation.
Retro hunting: Searching through historical logs or threat intelligence data to identify signs that a zero-day exploit was previously active.
Analyzing user reports: Collecting and investigating user complaints, such as frequent crashes or abnormal errors, that may signal an undiscovered flaw being exploited.

How to Prevent Zero-Day Exploits

While completely preventing zero-day attacks is impossible due to their nature, several best practices can significantly reduce the risk and impact:

Keep Software and Systems Updated: Apply patches and updates promptly. While this doesn’t prevent zero-day attacks (as the patch doesn’t exist yet), it closes known vulnerabilities that attackers might chain together with a zero-day exploit. Updated versions also fix minor bugs that might be exploitable.
Use Comprehensive Security Software: Employ multi-layered security solutions, including next-generation antivirus (NGAV), endpoint detection and response (EDR), firewalls, and intrusion prevention systems (IPS). These tools often use behavior-based detection and heuristics that can sometimes identify or block zero-day exploit activity even without a specific signature.
Restrict User Access and Privileges: Implement the principle of least privilege. Limiting user permissions ensures that even if an account is compromised via a zero-day exploit, the attacker’s access and potential damage are restricted. Use allowlisting or blocklisting to control application execution.
Network Segmentation: Divide your network into smaller, isolated segments. This can contain the spread of malware introduced via a zero-day exploit, limiting the attack’s scope.
Web Application Firewalls (WAFs): For web-facing applications, WAFs can filter, monitor, and block malicious HTTP/S traffic, potentially mitigating web-based zero-day exploits.
Regular Backups: Maintain regular, tested backups of critical data. This won’t prevent an attack but is crucial for recovery, especially from ransomware deployed via zero-day exploits.
Security Awareness Training: Educate users about phishing, social engineering, and safe browsing habits to reduce the chance of successful exploit delivery.

Final Words

Zero-day vulnerabilities represent one of the most dangerous threats in cybersecurity because they exploit flaws that no one yet knows about, leaving organizations without patches, defenses, or warnings. From discovery to exploitation, attackers hold the advantage, and traditional security tools often fall short.

The key to mitigating this risk lies in layered, proactive defense: combining vulnerability discovery, real-time monitoring, threat intelligence, and rapid patch management. While no single solution can block every zero-day exploit, building a strong security posture significantly reduces exposure and improves resilience.

Protect your organization from email-based zero-day threats like phishing, spoofing, and impersonation. Contact PowerDMARC today to learn how to lock down your email domain with DMARC, SPF, and DKIM.

Frequently Asked Questions

Who finds zero-day vulnerabilities?

They may be discovered by hackers, security researchers, or even state-sponsored groups.

How many zero-day vulnerabilities are there?

Exact numbers are unknown, but Google tracked 75 in 2024, following 98 in 2023 and 63 in 2022.

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About
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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.

Zero Day Vulnerability: Examples, Detection, and Prevention