5 Biggest Cyberattacks in History

In our hyperconnected world, the threat of cyberattacks has become unavoidable reality. From data leaks affecting billions to silent breaches that span years, the world's biggest cyberattacks have reshaped how we think about digital security. These incidents aren't just cautionary tales — they're milestones in the evolution of cybersecurity risk.

In this blog, we'll break down five of the biggest cyberattacks in history — looking at how they happened, what damage they caused, and what we can learn from them today.

5 Biggest Cyberattacks in History:

1. Yahoo Data Breach (2013)

   The Yahoo breach stands as one of the world's biggest cyberattacks in terms of the number of affected users. In 2013, hackers compromised all 3 billion Yahoo accounts, stealing names, phone numbers, email addresses, security questions, and poorly encrypted passwords. Yahoo’s use of outdated encryption and its failure to detect or disclose the breach promptly turned a severe security failure into a historic catastrophe.

   
   

   What makes this incident even more critical is that Yahoo didn't fully disclose the extent of the breach until years later. The delayed response cost the company heavily — both in reputation and financially. Verizon reduced its acquisition offer by $350 million, and Yahoo faced dozens of lawsuits and regulatory penalties. This is a reminder that ignoring core security hygiene can be incredibly costly.

   
   

2. Aadhaar Data Leak (2018)

   India’s Aadhaar program is the world’s largest biometric ID system, covering over 1.1 billion citizens. In 2018, a significant vulnerability in a third-party API allowed unauthorized access to Aadhaar data — including names, Aadhaar numbers, phone numbers, and even biometric information.

   
   

   This breach showed how powerful systems — even those built for national security, can be undermined by insecure APIs. Selling access for a few dollars on the dark web highlighted the enormous privacy risks, making it clear that public data infrastructure must be protected with the same rigor as financial or military systems.

   
   

   

   
   

3. LinkedIn Data Scrape (2021)

   While not a classic breach, the LinkedIn data scrape affected over 700 million users, or roughly 92% of the platform's user base. Attackers used poorly restricted public APIs to collect user's full names, phone numbers, email addresses, employment history and more.

   
   

   Although the data was technically public, its sheer volume and ease of access exposed users to phishing and fraud. It emphasized that even open platforms must have safeguards — because scraping and aggregation at scale can transform harmless data into an exploitable asset for attackers.

   
   

4. Facebook Data Leak (2019/2021)

   Using Facebook's contact importer tool, hackers collected personal data tied to over 533 million users across more than 100 countries. The attacker exploited a now-patched vulnerability that let them input phone numbers and receive linked user profiles in return.

   
   

   The leak surfaced years later, showing that the life cycle of exposed data is often longer than we expect. Even though the vulnerability had been patched, the leaked data still put millions at risk. It underlined the need to treat all data with caution — even if it's several years old.

   
   

5. Marriott-Starwood Breach (2014-2018)

   In one of the biggest cyberattacks in the hospitality industry, attackers gained access to Marriott's Starwood reservation system and remained undetected for four years. They stole sensitive data from 500 million hotel guests, including passport numbers, travel histories, and encrypted credit card info.

   
   

   This breach exposed the risks of inherited vulnerabilities during mergers and acquisitions. Long-term, undetected access stemmed from weak legacy systems and lack of monitoring — reinforcing how crucial it is to audit and secure all IT assets, especially in traditional or acquired systems.

Conclusion from 5 of the Biggest Cyberattacks in History

These five events demonstrate how the world’s biggest cyberattacks don’t always start with sophisticated exploits. Often, they’re the result of overlooked vulnerabilities, insecure APIs, poor password practices, or simple human error.

The biggest cyberattacks in history have left billions of people exposed, entire sectors shaken, and governments scrambling to respond. If there’s one lesson they all share, it’s this: cybersecurity is no longer optional. Whether you’re a startup, a global enterprise, or a government agency, your security posture is only as strong as your weakest system.