Zero-Day Vulnerabilities, Exploits, and Attacks Explained

What Is a Zero-Day Vulnerability?

Zero-day vulnerabilities are security flaws in IT assets—software, hardware, or firmware—that are unknown to the assets' developers, and for which no patch is available. (Hence, "zero day": an issue the developers have had zero days to address.) Threat actors who find these gaps in a target's defenses often use them to secretly lay groundwork for attacks.

Zero-day vulnerabilities often stem from human mistakes or failure to prioritize secure design practices, such as testing and peer review, during development. The most common causes are:

• Coding errors in syntax, logic, or assumptions (e.g., failing to validate user input, allowing injection attacks). Tight deadlines and skipped testing often contribute to these errors.
• Design flaws in architecture or functionality (e.g., weak privilege escalation controls allowing unauthorized access). Poor planning or rushed design can amplify these risks.
• Supply chain dependencies with hidden faults (e.g., unpatched flaws in vendor-supplied databases or software components).

Why Are Zero-Day Vulnerabilities Dangerous?

Zero-days often exist in widely used systems, where one flaw can jeopardize millions of devices worldwide. Attackers exploit these weaknesses to evade traditional defenses that rely on known threat patterns. Organizations are left exposed as long as a zero-day remains unpatched, giving threat actors ample opportunity to carry out attacks.

How Attackers Find and Use Zero-Day Vulnerabilities

The most common techniques for rooting out zero-day vulnerabilities are fuzzing and reverse-engineering. Fuzzing attempts to crash a system by bombarding it with random data, and is particularly useful for finding code injection and denial-of-service vulnerabilities. Reverse-engineering, meanwhile, reveals the core structure and logic of code, which can help threat actors find ways to bypass authentication and escalate privileges. Undisclosed flaws may also be found for sale on the black market.

Whether threat actors keep the knowledge of a vulnerability to themselves or sell it, the next stage is exploitation.

What Is a Zero-Day Exploit?

A zero-day exploit is the means by which attackers capitalize on a zero-day vulnerability. In other words, the exploit is what transforms the vulnerability from a potential risk into an active threat. Because the vulnerabilities are unknown to defenders in most cases, basic defenses are largely ineffective against these exploits.

Types of Zero-Day Exploit

Most zero-day exploits directly target weaknesses in software, system architecture, or security protocols. These are some of the most common types:

• Remote code execution (RCE) enables attackers to run unauthorized commands on a system from a distance to steal data, spread malware, or even take control of applications or networks.
• Privilege escalation allows attackers to obtain high-level permissions, such as administrator rights, so they can access or manipulate sensitive systems and files.
• Authentication bypass exploits flaws in login protocols, firewalls, or other security measures to allow attackers to enter restricted systems without proper credentials.
• Flooding techniques can overwhelm a target system's bandwidth, memory, or processing resources, leading it to become unresponsive or crash (a denial of service attack).
• Malicious code injection feeds harmful queries or instructions into an application or database to disrupt operations; impersonate users; hijack sessions; or access, modify, or steal sensitive data.
• Memory corruption exploits errors in memory allocation, like buffer overflows, to overwrite code in critical areas of the system, enabling attackers to crash systems, escalate privileges, or execute malware.

Once one of these techniques succeeds, the zero-day exploit becomes a zero-day attack.

What Is a Zero-Day Attack?

A zero-day attack combines an unknown, unpatched flaw with one or more exploits to compromise a target system. From there, an attacker can install malware (such as spyware, ransomware, or remote access trojans) to help them steal data, conduct cyber espionage, or disrupt operations.

Modern IT environments are growing larger and more complex, giving zero-day attacks more opportunities than ever. In particular, cloud adoption, IoT devices, and hybrid infrastructures expand the attack surface. Competitive pressure can also lead companies to shorten development cycles and skip important security steps, opening the door to new vulnerabilities.

At the same time, cyberthreats are evolving rapidly. State-sponsored groups and well-funded independent threat actors are using advanced tools like fuzzers and AI-driven testing to quickly uncover and exploit flaws, putting organizations at greater risk.

Real-World Zero-Day Attacks and Vulnerabilities

Zero-days can affect any industry, from government to manufacturing, retail, finance, healthcare, and more. Over the years, they have been part of some of history's most impactful and damaging cyberattacks.

• Stuxnet (2010): This sophisticated worm exploited five Windows zero-days to target Iran’s nuclear program, causing critical damage to some 1,000 uranium centrifuges.
• Equifax Data Breach (2017): A zero-day vulnerability in the Apache Struts web application framework gave attackers access to more than 147 million people's sensitive personal data.
• Microsoft Exchange Server Attacks (2021): Chinese state-sponsored hackers exploited zero-day vulnerabilities in Microsoft Exchange Server to gain unauthorized access to email accounts and deploy malware.
• Legacy firewalls and VPNs: Google Threat Intelligence Group identified 20 zero-days in security and networking products in 2024 alone, accounting for 60% of enterprise technology zero-day exploits. Threat actors are increasing their focus on legacy solutions such as firewalls and VPNs, with Google citing Ivanti, Palo Alto Networks, and Cisco as notable targets.

Top 9 Best Practices for Zero-Day Defense

No hardware or software vendor can ever guarantee their solution is free of vulnerabilities. Thus, staying safe from zero-day attacks means reducing your exposure and enhancing overall security. The most effective way to do that is to adopt a comprehensive zero trust architecture, enabling your organization to:

• Minimize the attack surface by making applications and vulnerable assets like VPNs invisible to the internet, impossible for attackers to find.
• Prevent initial compromise by inspecting all traffic, including encrypted traffic, inline in real time to stop advanced threats like zero-day exploits and malware.
• Enforce least-privileged access by restricting permissions based on identity and context, ensuring only authorized entities can access authorized assets.
• Block unauthorized access with strong multifactor authentication (MFA) to validate user identities.
• Restrict lateral movement by connecting users directly to applications, not the network, reducing the potential blast radius of an attack.
• Shut down insider threats with inline inspection and monitoring to detect compromised users with access to your network and sensitive assets.
• Prevent data loss by inspecting data in motion and at rest to stop active data theft.
• Deploy proactive defense, such as deception technology, to trap and neutralize malicious actors in real time.
• Evaluate your security posture via third-party security risk assessments and purple team exercises to identify gaps in your security framework.

How Zscaler Helps Prevent Zero-Day Attacks

Stops zero-day threats in their tracks with the Zscaler Zero Trust Exchange platform, a comprehensive zero trust architecture built to minimize the attack surface, prevent compromise, eliminate lateral movement, and stop data loss.

Advanced Threat Protection monitors traffic in real time to detect and block malicious activity, including zero-day exploits. With AI-powered advanced analytics, the platform identifies suspicious behaviors and shuts them down before compromise can occur.

Unified Vulnerability Management continuously monitors your network and applications to identify and prioritize vulnerabilities. Rich insights offer an actionable path to remediation, empowering your organization to meaningfully reduce risk.