What Is a Zero-Day Exploit and Why Is It So Dangerous?

What Makes a Zero-Day Exploit So Critical?

A zero-day exploit is one of the most dangerous tools in a threat actor’s arsenal. It allows attackers to exploit a software vulnerability before the affected vendor discovers it—and before any fix exists.

Because defenders have “zero days” to prepare for or patch the issue, these attacks give cybercriminals a tactical edge. They bypass defenses, escalate privileges, and compromise sensitive systems with little resistance, sometimes before security teams even know they have an.

As more organizations shift to cloud infrastructure and rely on third-party software, the opportunity for zero-day exploitation and blind spots continues to grow.

What Is a Zero-Day Exploit?

A zero-day vulnerability is a flaw in software or hardware that the vendor doesn’t yet know about. Once an attacker discovers and weaponizes it, the vulnerability becomes a zero-day exploit.

This timeline usually plays out in five stages:

1. A software flaw exists.
2. An attacker discovers and develops an exploit.
3. The exploit is used in the wild before the vendor is aware of the issue.
4. The vendor eventually becomes aware and releases a patch.
5. The vulnerability is cataloged as a Common Vulnerabilities and Exposures (CVE) entry.

The time between exploitation and patching is called the zero-day window—and it’s a period of maximum risk. It is also referred to as a zero-day because there are zero days between when it is discovered and when hackers are exploiting it and when organizations must patch.

Why Are Zero-Days So Dangerous?

Unlike other exploits, zero-day attacks don’t rely on outdated systems or stolen passwords. They use unknown flaws, giving attackers several distinct advantages. They can:

• Bypass traditional defenses like firewalls and antivirus tools
• Remain undetected, even during incident response
• Spread rapidly when integrated into malware or worms
• Target high-value assets such as databases or authentication systems

Ransomware groups have increasingly relied on zero-day vulnerabilities to target cloud and file transfer systems in recent months. These exploits allow them to breach hundreds of organizations at once, often without triggering alarms.

Real-World Impact: C10p and Cleo Software

In 2024, the ransomware group C10p exploited two zero-day vulnerabilities in Cleo file transfer software: CVE-2024-55956 and CVE-2024-50623.

Using these flaws, the group:

• Breached dozens of supply chain vendors
• Targeted transportation and logistics providers
• Disrupted customer operations across multiple sectors

This campaign demonstrated how attackers can leverage a single vendor’s software flaw to trigger cascading impacts across industries and across several targets.

Common Targets of Zero-Day Exploits

Zero-day exploits can target virtually any system, from those used at large corporations to those used at small businesses. Top targets can include:

• Operating systems (Windows, macOS, Linux)
• Web browsers (Chrome, Firefox, Edge)
• File transfer and collaboration software
• Virtual private network (VPN) platforms
• Cloud service providers
• Email clients

Many of these platforms form the backbone of modern operations. Their compromise can often deliver disproportionate impact.

Nation-state actors often use zero-days to infiltrate hardened networks, while ransomware groups use them to automate mass extortion campaigns. Many nation-state actors or advanced persistent threat (APT) actors harbor zero-days to use at a later date of their choosing.

Who Uses Zero-Days?

Zero-days are not limited to elite threat actors. A growing market for vulnerabilities exists in dark web forums, exploit exchanges, and private broker communities.

The main buyers include:

• Ransomware groups seeking stealthy entry points
• Nation-state teams focused on espionage and surveillance
• Cybercriminal gangs targeting valuable data
• Hacktivists aiming to disrupt high-profile targets

SecurityScorecard tracks numerous threat groups globally, including several known to leverage zero-day vulnerabilities

How Zero-Days Spread

Threat actors typically deliver zero-day payloads through several methods, including:

• Phishing emails with malicious attachments or links
• Compromised websites that exploit browser flaws
• Trojanized software updates from trusted vendors
• Third-party platforms with embedded vulnerabilities

Once inside, attackers may escalate privileges, install backdoors, or pivot laterally to other systems.

Detection and Defense Strategies

Detecting a zero-day exploit before damage occurs is difficult. However, organizations can minimize impact through layered defenses and rapid response protocols.

1. Behavioral Monitoring

Deploy Endpoint Detection and Response (EDR) to flag:

• Unusual file execution
• Suspicious network connections
• Unauthorized access attempts

2. Threat Intelligence

Integrate feeds that identify emerging zero-days and Indicators of Compromise (IOCs). SecurityScorecard’s STRIKE Team has identified threat actor behavior tied to zero-day exploitation in the wild, and SecurityScorecard offers timely analysis and reporting of high-risk threats like these.

3. Network Segmentation

Segment internal systems to prevent attackers from moving laterally after compromise.

4. Application Approval Lists

Restrict which software is allowed to run. Prevent execution of unknown or unapproved binaries.

5. Patch Management Hygiene

While you can’t patch what’s unknown, maintaining a program that’s designed to close known vulnerabilities limits escalation paths once attackers gain initial access.

Third-Party Software and Zero-Day Risk

Bad actors frequently enter enterprise environments through vulnerabilities in third-party platforms. In the past year, for example, 46.75% of third-party breaches involved software vulnerabilities, according to SecurityScorecard’s 2025 Global Third Party Breach Report. File transfer software alone has become the most common attack vector for third-party breaches, the report found. Just two vulnerabilities in file transfer software accounted for 63.5% of all vulnerability-based attacks.

To put that into context, C10p was behind 41.5% of attributed third-party compromises due to its exploitation of zero-day vulnerabilities, according to SecurityScorecard research.

Many organizations mistakenly believe vendor risk assessments are enough. But attackers move faster than annual reviews. Zero-day detection must extend into your supply chain in real time.

SecurityScorecard’s Supply Chain Detection and Response (SCDR) solution provides early alerts for:

• Vendor environments showing exploitation indicators
• Breach chatter on ransomware sites
• IPs and infrastructure associated with threat actors

These capabilities help close the visibility gap between your systems and those of your vendors.

Closing Thoughts

Zero-day exploits represent a cybersecurity risk that demands urgency when uncovered. Their effectiveness stems from one fact: Defenders can’t stop what they don’t yet understand.

Attackers, however, continue to innovate. As organizations adopt more third-party and cloud-based tools, their exposure to unpatched and unrecognized vulnerabilities grows. That’s why resilience depends on proactive defense, including behavioral detection, segmented networks, threat intelligence, and visibility across the software supply chain.

Security teams that watch for weak links, anticipate blind spots, and take context-informed action before the damage is done are taking a step in the right direction to avoid zero-day vulnerabilities and their fallout.

Protect Your Supply Chain with Real-Time Threat Detection

SecurityScorecard’s SCDR solution offers continuous monitoring of your third-party ecosystem, enabling swift identification and mitigation of cyber threats. Enhance your organization’s resilience by proactively managing supply chain risks.

🔗 Understand SCDR