A zero-day vulnerability is a software flaw that is known to cybercriminals before the software developers are aware of it. The biggest problem is that this creates a critical window of exposure, where your systems are completely defenseless because there are no patches or fixes that exist to address the particular weakness.
The term “zero-day” refers to the fact that the software vendors have had zero days to work on creating a solution from the date they became aware of the problem.
The biggest threat for every company, no matter its size, is that these vulnerabilities can be exploited immediately, since being found by the hackers without any signs or warnings. Even worse is that cybercriminals keep zero-day discoveries in secret for as long as they can, in order to have more time to gain financial benefits from them, before the vendor releases a patch that fixes the particular vulnerability.
The truth is that the discovery of a zero-day vulnerability is a race against time: developers must quickly develop and deploy patches, hoping attackers haven’t compromised their client’s systems or at least minimizing this number.
Types of Zero-Day vulnerabilities
There are two fundamental types of zero-day vulnerabilities:
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Known zero-day vulnerability — The software developers know about the vulnerability and are in the process of developing a patch to fix this. Or, the software’s users have identified a vulnerability and are in the process of informing the developers.
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Unknown zero-day vulnerability — The software developers or users have not yet identified the vulnerability, so there is no patch currently in development. If malicious entities and cybercriminals do know about the vulnerability, this will still be classed as unknown because it is not known to the developers, even if these malicious entities are in the process of exploiting the flaw.
What Are Zero-Day Hacks or Exploits?
While zero-day vulnerabilities describe what happens when issues are left unpatched and systems are left exposed, this is not a problem for businesses all by itself. Instead, the danger begins to arise when malicious actors and cybercriminals exploit these vulnerabilities.
Exploiting a zero-day vulnerability basically means using the exposed area of the system to gain unauthorized access. While a patch should prevent this access, it cannot provide protection if it is not deployed or—in the worst case—if the patch has not been developed because the vulnerability has not yet been identified.
The concept of hacking a zero-day vulnerability may be synonymous with that of the zero-day exploit described above, or it may refer to a more sophisticated attack on the vulnerability. Hacking is generally considered to mean “manipulating computer systems to bypass access and authentication protocols” rather than simply gaining access to a system because those protocols are not in place. However, the end result is the same, whatever the definition—an unauthorized individual or device gains access to systems and data.
Zero-Day Examples
Below, we will talk about a few examples of zero-day security vulnerabilities that, unfortunately, were discovered and exploited because, as we all know, cybercriminals do not sleep in order to find weaknesses and use them to penetrate the security systems of companies with the main purpose of gaining financial benefits.
Log4Shell (2021)
This critical software vulnerability affected millions of corporations worldwide that used the Java logging library “Log4j,” which enabled hackers to compromise systems and execute arbitrary code remotely on the affected servers or leak sensitive information. It is safe to say that the impact was massive because it exposed sensitive data across major technology companies such as Apple, Amazon, and Microsoft.
Could you imagine such technological giants failing to prevent this attack? Log4Shell (CVE-2021-44228) can be considered one of the most severe target security vulnerabilities ever discovered due to flaws in the software code. The devastating nature of this emerging threat stems from the fact that its exploitation started long before security teams could develop a patch to fix it, leaving millions of vulnerable systems exposed and causing an unprecedented crisis in numerous security systems.
Microsoft Exchange Server ProxyLogon (2021)
Next in line is the “ProxyLogon” vulnerability (CVE-2021-26855) in Microsoft Exchange Server, which allowed the cybercriminals to bypass many security systems and gain administrative privileges in order to steal sensitive information without any user interaction. A Chinese state-sponsored group named HAFNIUM actively exploited this operating system flaw before Microsoft developed a fix. This resulted in a series of significant data breaches, impacting over 30,000 companies in the US and 250,000 servers globally.
On 2 March 2021, Microsoft released an emergency patch, breaking their regular update schedule in order to protect the sensitive information of its users. This vulnerability has definitely highlighted the dangers of email server security vulnerabilities, because they store tons of sensitive data and can serve as a gateway for hackers in their mission to compromise systems across the network and use the data for personal financial benefits.
LinkedIn Zero-Day Attack (2021)
The LinkedIn zero-day attack in 2021 was a major security breach where hackers exploited a previously unknown vulnerability in LinkedIn’s API, targeting the platform’s AutoFill feature. This cyberattack exposed the personal information of approximately 700 million users, representing nearly 92% of the platform’s user base at the time.
The fact that the cybercriminals stole all the information from those profiles, including users’ full names, personal email addresses, phone numbers, and physical addresses, is even more concerning. In June 2021, a cybercriminal revealed the breach by posting a sample of one million users’ data on a dark web forum and offering the entire dataset for sale. This brought concerns for potential identity theft, phishing attacks, social engineering, and business email compromise attacks, confirming the significant security implications of zero-day vulnerabilities even in major platforms.
Zerologon (2020)
A critical security update released by Microsoft on August 11, 2020, addressed a severe vulnerability in the NETLOGON protocol (CVE-2020-1472) identified by Secura’s research team. Despite receiving the highest possible CVSS severity score of 10.0, this vulnerability initially flew under the radar due to limited technical information in the initial disclosure. It is named “Zerologon” because of the fact that if a cybercriminal can successfully deploy this exploit, he could then authenticate by submitting a string of zeros.
The flaw enabled attackers to compromise a domain controller without authentication, creating a vulnerable Netlogon session that could lead to full domain administrator access. What made this vulnerability particularly dangerous was its simplicity—an attacker only needed network connectivity to a domain controller to potentially exploit this weakness.
Apple iMessage Zero-Click Exploit (2021)
The 2021 Apple iMessage “FORCEDENTRY” exploit worked by sending a maliciously crafted PDF file through iMessage that exploited a vulnerability in Apple’s image rendering library (CoreGraphics). The attack required no user interaction—simply receiving the message was enough to trigger the exploit, which bypassed Apple’s BlastDoor security system. Once activated, the exploit deployed NSO Group’s Pegasus spyware, giving hackers complete access to the device, including messages, sensitive information, camera, microphone, location data, and stored credentials. The vulnerability was discovered on a Saudi activist’s iPhone and was patched by Apple in the iOS 14.8 update in September 2021.
The incident was particularly noteworthy because it showed how target security vulnerabilities could be used in sophisticated operations to steal data from high-value targets, requiring no mistakes or interaction from the victim to succeed. It served as a wake-up call to security teams about the sophisticated nature of such threats against modern mobile devices and the importance of rapid response to prevent data breaches.
RSA Security Breach (2011)
In 2011, cybercriminals used an unpatched vulnerability in Adobe Flash Player in order to gain access to the RSA security company network and particularly its core feature, SecurID. The attack was launched through sent Excel spreadsheet email attachments to several RSA employees. Hidden within the documents was an embedded Flash file exploiting the zero-day vulnerability in Adobe Flash Player.
The attack succeeded when one of the employees opened one of the corrupted attachments, which silently launched the installation of Poison Ivy, a remote administration tool that gave hackers complete control over the infected system. After infiltrating the RSA network, cybercriminals stole and transmitted the sensitive information they needed to external servers in their control.
The breach’s impact was identified when RSA admitted that the stolen data was actually sensitive information related to their SecurID two-factor authentication technology, particularly the “seed warehouse (a well-protected server that stored the secret keys known as “seeds,” providing access to every company worldwide using SecurID). Which was terrifying because of the fact that SecurID at that time was widely used by millions of organizations worldwide, including government and military agencies and banks, for protecting critical systems and sensitive data. This incident has shown the world that nobody is immune to cyberattacks; even security-conscious companies can fall victim to zero-day attacks, especially when combined with social engineering tactics.
What is the Difference between Zero-Day vs Critical Vulnerability
Zero-day vulnerabilities represent unknown security gaps in software that have never been discovered before—meaning vendors and security teams have literally “zero days” to prepare. The vulnerability itself represents a weakness in the software that leaves your systems completely exposed with no immediate fix available.
Critical vulnerabilities, on the other hand, are known threats that have been already identified, documented, and analyzed by the software’s vendor security researchers. While serious and being able to severely impact your organization’s system security and data confidentiality, there are already released software updates or mitigation strategies that can close them before being exploited by the hackers.
The key difference between zero-day vulnerabilities and critical vulnerabilities lies in awareness and preparation. In the first case, organizations face targeted attacks against unknown weaknesses, while critical vulnerabilities test your company’s ability to quickly implement existing security patches that are able to fix them.
The dangers of zero-days stem from the advantage that cybercriminals have, as no defenses exist and the impact remains unknown until after exploitation. The biggest risk for critical vulnerabilities stems from the widespread awareness of the weakness and the organizational challenges in promptly implementing patches. Many companies make the mistake of not installing the patches immediately after their release, leaving their systems exposed, whether it is due to operational constraints or legacy system limitations, which is one of the main reasons for successful cyberattacks.
How to detect a zero-day attack?
Detecting zero-day attacks presents unique challenges due to their unknown nature. Traditional security tools, including antivirus software and IDS/IPS systems, are not capable of identifying these threats since no existing signatures match these advanced threat patterns and approaches.
On the other hand, user behavior analytics proves to be the most effective detection method for zero-day threats. Most entities authorized to interact with your network typically follow consistent, predictable patterns in their interactions, but when activities deviate from these established norms, security teams should treat them as potential zero-day exploit indicators. Companies often first notice these attacks through anomalous network traffic patterns or unusual scanning activities from services or clients.
In such cases, security teams employ several complementary detection approaches. While real-time malware databases provide valuable reference points, they inherently lag behind zero-day threats that exploit newly discovered vulnerabilities. Machine learning is increasingly used nowadays because of its ability to analyze historical exploit information in order to establish baseline system behaviors, improving threat detection accuracy as they process more operational data.
Given the dynamic nature of vulnerability exploitation, organizations should implement a multi-layered detection strategy that combines these two approaches. This comprehensive security posture helps protect critical business assets and data from emerging zero-day threats. Continuous monitoring and adaptation of detection methods remain essential as attacks are ever-evolving, making it harder and harder to detect their patterns.
How to protect your business from zero-day attacks and exploits?
How do you protect yourself and your business against zero-day vulnerabilities? How do you make sure that your business systems are not exposed? Here are a few things to bear in mind as you shore up your defenses:
Advanced Endpoint Protection
Nowadays every organization can benefit from using endpoint security solutions that have evolved beyond traditional antivirus software. These systems employ behavior-based detection methods, moving away from signature-based detection that can’t identify the sophisticated new threats.
Machine learning algorithms analyze patterns to identify suspicious activity, while runtime application self-protection provides additional security layers. These solutions offer automated response capabilities to contain potential threats quickly, combined with real-time system monitoring that can spot subtle signs of compromise.
Implementing Vulnerability and Patch Management
It is a well-known fact that a robust vulnerability management program is capable of reducing your company’s attack surface. Leveraging real-time monitoring allows for immediate and effective identification of potential weaknesses across your infrastructure, while systematic patch management ensures that every endpoint connected to the network receives the latest security updates.
Furthermore, vulnerability management solutions have the ability to simulate attacks on software code or review it for errors, thus finding new vulnerabilities and warning you about them. Additionally, prioritizing critical software patches, testing them in a controlled environment, and deploying them automatically across the organization takes your security posture to a whole new level, minimizing the time between identifying a vulnerability and closing it.
Use Network Security Controls
Strong network-level protections form a crucial barrier against zero-day attacks. Modern next-generation firewalls are known for performing deep packet inspection in order to identify malicious traffic, while intrusion detection and prevention systems are blocking every suspicious activity, intercepting the attack at its earliest phase. Network traffic analysis plays a key role in identifying unusual patterns that might indicate an attack. Secure email gateways are equally important because they prevent phishing attacks, which serve as the initial entry point for zero-day exploits.
Furthermore, implementing a web application firewall in your security posture is vital, as it safeguards web applications by analyzing and filtering internet traffic. It works by examining both HTTP and HTTPS communications, serving as a borderline between external users and your web applications. One of the most effective ways to prevent zero-day attacks is by implementing a web application on the network edge.
WAF has the ability to intercept incoming requests and screen them for potential threats before they reach their destination. A web application firewall is known to be closely monitoring all incoming traffic, having the capability to filter out malicious inputs that might target a specific vulnerability.
Equipping your company with WAF will protect it from various sophisticated attacks, not only zero-day exploits but also from Cross Site Scripting (XSS) attempts, where hackers are trying to inject malicious scripts into trusted websites. It is also capable of blocking SQL injection attacks that are trying to manipulate your database queries and prevents both DoS and DDoS attacks.
Use VLANs.
Virtual Local Area Networks (VLANs) create secure boundaries within your network by separating traffic into isolated segments, similar to having separate secure rooms in a building. By implementing either physical or virtual dividers, organizations can keep critical server communications protected from general network traffic.
This security approach means that even if attackers successfully breach the network’s outer defenses, they remain trapped within specific segments, unable to access or steal data from critical systems protected in other VLANs.
Monitor and manage any abnormalities.
Abnormal user behaviors or analytics data could provide an indication that a vulnerability exists in the system.
Communicate the danger to all team members.
Make sure all of your team members are engaged in the need to protect systems and promote widespread vigilance.
Utilize deployment software tools.
This set of tools assists your business as you implement and assess software deployments across multiple endpoints from one centralized location.
Put remote assistance software in place.
Ensure that all users have support and assistance across remotely deployed devices and desktops, utilizing software solutions to make remote support effective and immediate.
Incident Response Preparation
Even if you implement all the solutions in your security strategy, it doesn’t make you immune to cyberattacks; yes, it significantly reduces that risk, but there is still a chance to face such a scenario. For that reason it is vital to prepare for potential breaches; maintaining documented incident response procedures that all of your team members understand and can execute quickly is a cornerstone in bringing your business back to its feet. Regular training and simulation exercises are mandatory in order to keep your team ready to respond in case the disaster happens.
Establishing clear communication channels, escalation paths, and procedures will improve the response effectiveness when tough times come. Last but not least, your company must have a robust backup and recovery plan to minimize downtime and data loss, while business continuity planning ensures critical operations can continue during an incident. The best practice is to follow the 3-2-1 backup strategy, keeping three copies of your data, stored on two different types of media, and one copy kept off-site. This ensures that no matter what happens, you won’t lose a single file and can quickly recover your database.
Protect Your Business from Zero-Day Bugs and Vulnerabilities — Put Action1’s Services and Solutions on the Case
Action1 reinvents patching with an infinitely scalable, highly secure, cloud-native platform configurable in 5 minutes — it just works and is always free for the first 200 endpoints, with no functional limits. Featuring unified OS and third-party patching with peer-to-peer patch distribution and real-time vulnerability assessment with no VPN needed, it enables autonomous endpoint management that preempts ransomware and security risks, all while eliminating costly routine labor. Trusted by thousands of enterprises managing millions of endpoints globally, Action1 is certified for SOC 2 and ISO 27001.
The company is founder-led by industry veterans Alex Vovk and Mike Walters, who founded Netwrix, which has grown into a multi-billion-dollar industry-leading cybersecurity company.
