What Are Zero-Day Vulnerabilities?

We’ve discussed zero-day attacks, but understanding the vulnerabilities that give rise to these unforeseen intrusions is the key to shutting down attacks before they occur. Vulnerabilities are not inherently dangerous on their own, but are the flaws without which hacks and business disruptions could not happen. Their threat lies in their complete obscurity to vendors, as hackers easily evade detection and traverse networks unchecked. These vulnerabilities present an increasingly sophisticated challenge for security leaders — one that needs both anticipatory (prevention-bassed) and reactive defense strategies. This article focuses on zero-day vulnerabilities themselves.

What are Zero-Day Vulnerabilities?

A zero-day vulnerability refers to a security flaw in software, hardware, or firmware that is unknown to the vendor or developer, and therefore, unpatched. Because the vulnerability is not publicly disclosed, no fix or mitigation exists, exposing systems to exploitation.

Examples of vulnerabilities include, but are not limited to:

• Buffer Overflow: Overwriting adjacent memory through excess data, potentially leading to code execution.
• Cross-Site Scripting (XSS): Injecting malicious scripts into trusted websites to compromise user data.
• Default Credentials: Using factory-set usernames and passwords that are publicly known.
• Exposed API Keys: Storing sensitive keys in accessible locations, leading to unauthorized access.
• USB Exploits: Reprogramming USB devices to perform malicious actions.
• Weak Password Policies: Allowing easily guessed passwords or lacking multi-factor authentication.
• Open Ports: Exposing unnecessary network ports to external access.
• Software Dependency Flaws: Using insecure or compromised third-party libraries.
• Weak Encryption Protocols: Employing outdated or vulnerable cryptographic algorithms.

Zero-day vulnerabilities represent one of the most pressing threats in cybersecurity, as their unknown nature leaves no opportunity for preemptive defense. Attackers can exploit flaws in widely used software, firmware, hardware, libraries, or frameworks to breach even robust security defenses, while vendors are unaware those vulnerabilities exist.

The mitigation challenge here rests on building a culture of cyber resilience that pairs advanced security tools to detect the signs of these flaws with a robust incident response strategy. A broader challenge lies in addressing the systemic issues zero-days reveal — for instance, weaknesses in patch management, third-party integrations, and incident response processes exacerbate the risks. Managing zero-day threats demands more than technical solutions, it requires alignment across the organization to anticipate and withstand the unforeseen.

The LifeCycle of Zero-Day Vulnerabilities

A zero-day vulnerability generally stems from a flaw in the software or hardware code that can be triggered under certain conditions: whether through improper input handling, insufficient authentication protocols, or flawed data validation. Organizations are a key target.

In 2023, the 97 zero-day vulnerabilities exploited fell into 2 main categories: end-user platforms and enterprise-focused technologies.

Attackers who discover these flaws can craft exploits that directly target the vulnerability, allowing them to move undetected as they gain unauthorized access to vulnerable systems, execute code remotely, and perhaps move laterally to more sensitive assets. Once inside, they can:

• Exfiltrate data to sell
• Demand ransom for disabled systems
• Disable critical services outright
• Access intellectual property to aid corporate espionage
• Conduct surveillance for state-sponsored espionage
• Deploy malware to establish long-term system access
• Manipulate or destroy data to disrupt operations

Known security vulnerabilities are identified, cataloged, and typically addressed through vendor-issued security patches or software updates. Zero-days remain undiscovered by the vendor or security community until they are actively exploited. Because the threat emerges in real time with little to no advanced warning,  the security response demands a more immediate approach to mitigation.

Each phase of the vulnerability lifecycle, from discovery to remediation, presents unique challenges and opportunities to think about defense strategies. A clear grasp of how zero-days progress enables prioritization of proactive threat detection and timely mitigation.

Phase	What Happens?
Discovery Phase	Attackers, researchers, or security professionals identify the vulnerability. Threat actors have a competitive edge through silent exploitation (if they find the flaw first). Hackers reverse-engineer code or use automated tools to find vulnerabilities before organizations do. Companies can find vulnerabilities by bug bounty programs or external audits.
Exploitation Window	Attackers actively exploit the vulnerability before a patch is available. Security strategies incorporate real-time monitoring, anomaly detection, and threat intelligence to help detect attacks quickly.
Disclosure Process	The vulnerability is disclosed to the vendor or the public. Responsible disclosure practices aim to minimize exploitation, but poorly coordinated disclosures can leave organizations scrambling to respond. Monitoring disclosures through trusted channels and communicating risks swiftly is key.
Patch Development	Vendors create and test a patch to address the vulnerability. Ensuring patch management processes are agile and collaborating with software vendors to prioritize updates for critical systems can help speed up the response.
Remediation Timeline	Vendors deploy patches and address secondary risks. This stage exposes systemic weaknesses in patch deployment, third-party integrations, and organizational readiness.

The Vulnerabilities Involved in High-Profile Zero-Day Attacks

When hackers weaponize zero-day vulnerabilities and use them to attack organizations, the fallout can be devastating. The unpredictability of zero-days is compounded by their high value within the cyber threat ecosystem. Advanced Persistent Threat (APT) groups and cybercriminal syndicates often invest heavily in discovering and weaponizing them in targeted attacks. 

The exploits often target foundational software or critical infrastructure in order to amplify their potential for widespread impact. In this context, the stakes are not merely technical but strategic, as zero-days have the capacity to undermine trust in enterprise systems and disrupt many businesses or entire industries, as the following real-world attacks show.

Fortra GoAnywhere MFT (2023)

A zero-day vulnerability (CVE-2023-0669) was discovered in Fortra’s GoAnywhere Managed File Transfer (MFT) system, specifically in the application’s web-based administrative console. The flaw involved an insecure direct object reference (IDOR) that allowed attackers to execute remote code without authentication. 

The Cl0p ransomware group exploited this vulnerability, targeting the file transfer system that many enterprises rely on, and successfully breached over 130 organizations. Among the victims was Community Health Systems, a U.S.-based healthcare operator, which confirmed that the attack exposed the personal data of approximately 1 million patients.

MOVEit Transfer Vulnerability (2023)

The MOVEit Transfer zero-day attack, also exploited by the Cl0p ransomware group, affected thousands of organizations and exposed the data of 83 million people. The vulnerability was a critical SQL injection flaw in the file transfer service, allowing attackers to send specially crafted SQL queries that bypassed authentication and accessed sensitive data within the databases.

Cl0p operators leveraged this flaw to infiltrate MOVEit servers, exfiltrate data, and compromise connected systems. The attack caused widespread impacts, with high-profile victims including Shell and the New York City Department of Education. Cascading supply chain disruptions also occurred; for example, the BBC’s payroll provider was affected, indirectly impacting the organization. The estimated total cost of the breach across affected companies reached $9 billion.

Lessons Learned and Industry Responses

Zero-day vulnerabilities have far-reaching implications, not only in terms of their exploitation but also in how they reshape industry practices to address them more effectively.

Following incidents like the MOVEit breach, supply chain vulnerability awareness skyrocketed. The breach underscored the importance of practices like robust asset inventory management and conducting due diligence on software providers and contractors. In fact, a survey published at the end of 2023 found 85% of companies increased supply chain/third-party security budgets over the previous 12 months, driven by the realization that vulnerabilities in external systems can cascade into broader organizational risks. 

There’s also been a shift toward “security by design,” encouraging software vendors to adopt more stringent validation and testing protocols during development to minimize flaws like input validation errors or weak authentication mechanisms. Other organizations prioritize proactive threat-hunting programs. Others turn to early detection security like runtime protection and behavioral analysis, hoping to catch attacks in progress. 

Collaboration across the cybersecurity community has also improved. Public-private partnerships, like the Cybersecurity and Infrastructure Security Agency’s (CISA) advisories, provide actionable intelligence on identifying and addressing vulnerabilities before they escalate. By sharing indicators of compromise (IOCs) and tactics used by advanced persistent threats (APTs), the industry is fostering a collective response to reduce the window of exploitation for zero-day vulnerabilities.

The Current State of Zero-Day Vulnerability Exploits

The UK and its allies warned in 2024 that malicious actors are increasingly targeting zero-days in their cyber attack campaigns. According to the data, more than half of the top 15 exploited vulnerabilities in 2023 were zero-days. On dark web marketplaces, demand for these security flaws is skyrocketing, and even nation-state actors are increasingly targeting them. As zero-day vulnerabilities evolve, their potential to create cascading effects and evade traditional defenses has grown significantly, posing ever-greater challenges for security teams.

Here is a look at how zero-day attacks are becoming increasingly sophisticated and detrimental:

• Increasing supply chain impacts: the MOVEit data breach showed that when zero-days hit a service or software used by many businesses, the impacts can rapidly escalate. Hackers know this and increasingly look for the most valuable zero-days in the most widely used tools to maximize the blast radius of their activities. 
• Zero-day vulnerabilities in artificial intelligence (AI) and machine learning (ML) models: As more companies integrate AI and ML into all sorts of business processes, these tools increasingly become the target of zero-days. Because AI tools are often treated as “black boxes,” attackers might exploit flaws in the training data or algorithms and uncover new vulnerabilities in systems that rely on AI-driven decision-making.
• Fileless attacks: Fileless malware, which operates entirely in system memory without leaving traces on disk, presents a growing threat. These attacks are harder to detect with traditional file-based security tools, making zero-day vulnerabilities within this space a significant target and emerging challenge. The rise of fileless exploits will push cybersecurity defenses to focus more on runtime and behavioral analysis to identify malicious behavior in real time.

Detection and Prevention Strategies

Subtle signs that a system may harbor a zero-day vulnerability include unusual traffic patterns, memory anomalies, or unexpected privilege escalations. These anomalies often emerge when a vulnerability is being probed or partially exploited. By analyzing application behavior and error logs, organizations can detect signs of latent vulnerabilities. Leveraging shared vulnerability intel from sources like Microsoft, Consortia, or independent security researchers enables organizations to identify systemic flaws across industries and adjust proactively.

Monitoring Techniques for Vulnerabilities

Behavioral analytics and anomaly detection tools, powered by machine learning, identify patterns consistent with exploitable vulnerabilities. For instance, tools that monitor for repeated input anomalies can flag flawed data validation mechanisms. Detailed network traffic analysis further uncovers deviations that might suggest exploitation attempts targeting unpatched vulnerabilities.

For cloud-native environments, runtime observability offers insights into vulnerabilities specific to Kubernetes, serverless, and containerized workloads. By analyzing abnormal resource usage, unexpected API calls, or deviations in container behavior, such tools can pinpoint vulnerabilities before attackers fully exploit them.

Mitigating Risks from Vulnerabilities

When vulnerabilities cannot be immediately patched or eliminated, limiting their impact is critical:

• Network isolation: Segmenting networks to reduce the potential blast radius of vulnerability exploitation.
• Adaptive authentication: Monitoring for abnormal access attempts that may exploit authentication-related vulnerabilities and requiring additional factors to verify identity.
• Dynamic behavioral analysis: Continuously monitoring user and application behavior to uncover patterns suggestive of underlying flaws.

Improving Recovery Processes

While the focus is on vulnerability detection and prevention, some exploits may still succeed. A detailed recovery strategy, integrated with automated backup solutions, can help organizations restore systems affected. Regular simulations that model scenarios based on vulnerability exploitation can also refine responses and ensure alignment between technical and business priorities.

Cutting-edge Zero-Day Vulnerability Defense

Moving beyond foundational detection and prevention frameworks, advanced techniques can help identify and address underlying vulnerabilities before or as they are exploited.

Deception Technology

Deploying deception systems such as honeypots or honeynets within critical environments offers a novel way to uncover hidden vulnerabilities. These systems mimic real assets and lure attackers into engaging with decoys. Insights from these interactions provide actionable intelligence, revealing the tactics, techniques, and procedures (TTPs) attackers use to exploit vulnerabilities. This can inform the discovery of new zero-day vulnerabilities and strengthen defenses against future exploits.

Proactive Threat Hunting

Advanced security teams engage in proactive vulnerability identification by analyzing telemetry data from endpoints, network activity, and cloud services. Threat hunting focuses on identifying subtle indicators of compromise tied to potential software weaknesses that could evolve into vulnerabilities. By uncovering systemic flaws or patterns in exploitation attempts, teams can prioritize patching or other mitigation efforts.

AI-Powered Predictive Analytics

Predictive analytics leverages artificial intelligence to analyze historical data on vulnerabilities and attack vectors, forecasting areas of weakness in systems or applications. By pinpointing configurations or behaviors associated with high-risk vulnerabilities, predictive models enable organizations to preemptively harden systems and applications against likely exploitation paths.

Post-Exploitation Safeguards

While the focus is on preventing vulnerabilities from being exploited, some safeguards are essential to limit damage when an exploitation occurs:

• Immutable backups: Ensure data backups cannot be altered or encrypted by attackers, even if vulnerabilities are exploited.
• Dynamic application controls: Automatically revert compromised applications to a known secure state to neutralize impacts.
• Enhanced privilege restrictions: Contain the spread of attacks by closely monitoring and restricting privilege escalation linked to vulnerability exploitation.

Strengthen Zero-Day Protection with Upwind

Upwind’s runtime insights and cloud baselines can play a crucial role in identifying zero-day attacks in progress. By monitoring deviations from normal activity within cloud workloads, APIs, and resources, Upwind can detect anomalies that serve as early indicators of a zero-day exploitation so that companies can limit damage.

For those zero-day vulnerabilities that go undetected initially, Upwind’s runtime software bill of materials (SBOM) proves invaluable for swift remediation. With detailed visibility into all the packages, libraries, and frameworks in use across a multi-cloud or hybrid cloud environment, security teams can effectively track which components have recently been disclosed as vulnerable. Want to see it in action? Get a demo.

Frequently Asked Questions

What makes zero-day attacks different from other cyber threats?

What differentiates zero-day attacks is their inherent stealth and unpredictability. Traditional attacks might rely on publicly disclosed weaknesses with known fixes, making them more “routine” in terms of detection and response. In contrast, zero-day exploits tend to catch organizations off guard, often bypassing even sophisticated defenses. 

After all, who can anticipate and fix a vulnerability without knowing it’s there? Ultimately, thwarting zero-days hinges on improving organizational processes to minimize all types of vulnerabilities, but also recognizing that complete elimination of vulnerabilities is not realistic and finding attacks quickly at runtime can help secure systems and apps inadvertently built with weaknesses.

Can zero-day vulnerabilities be completely prevented?

Complete prevention of zero-days is an unrealistic goal in the current threat landscape. Why? That’s because: 

• Human error is inevitable in human-made code 
• Modern software is complex, with many third-party dependencies
• Attacks are evolving, so anticipating future developments and heading them off with code is unrealistic
• Supply chain risks introduce vulnerabilities from outside developers, making organizational efforts just part of an overall strategy

Threat hunting and real-time monitoring are some strategies that can reduce risks and help prevent and minimize attacks when they do happen.

What organizations are at the most risk from zero-day vulnerability attacks?

Organizations with critical infrastructure (defense, technology) and sensitive data (healthcare, financial) are especially attractive to zero-day vulnerability exploits. The MOVEit Transfer breach is one example, targeting healthcare systems where patient data presented an appealing target. Health records, financial data, intellectual property, and government files are typical targets.

As for infrastructure, energy, utilities, water, transportation, and defense, attackers more often seek to disrupt services rather than obtain sensitive data. They’re more interested in financial gain or political leverage.

However, organizations outside these industries are also targets. In fact, all organizations are at some risk from zero-day attacks.

For other types of organizations, attackers may seek data and competitive secrets, or simply focus on undermining their consumers’ trust. Companies with complicated supply chains can find they’re also caught up in attacks. And companies like SaaS providers and eCommerce platforms are also common victims. Their architectures can rely on cloud platforms and third-party and open-source software. Cloud-native environments appeal to attackers because compromising one tenant can potentially impact others, rippling across organizations.