In 2025, another wave of large-scale data breaches reminded the world that cybersecurity risk is a constant operational challenge. Cyberattacks now occur with staggering frequency: small businesses are targeted roughly every 11 seconds, while larger enterprises face more than 1,900 attempted attacks each week.

This year’s most significant breaches shared familiar patterns, with attackers infiltrating through third-party systems, stealing credentials through malware or social engineering, and taking advantage of delayed detection to maximize damage. Across every industry, cybercriminals are finding new ways to exploit old weaknesses, proving that even well-resourced organizations can be undone by a single misconfiguration, unmonitored access point, or outdated system.

Below we recap the most notable breaches of 2025, how they unfolded, and what every business can learn from them to build stronger defenses in the coming year.

1. The 16 billion credential mega-leak

In June 2025, researchers discovered what may be the largest data exposure in history: roughly 16 billion login credentials compiled from infostealer malware logs, phishing kits, and prior data breaches. The aggregated database included credentials tied to major platforms including Google, Apple, and Meta, putting billions of users at risk of credential stuffing and identity theft.

While no single company was targeted directly, the leak revealed a systemic vulnerability: password reuse. Attackers with one valid credential could access dozens of services automatically, exploiting the fact that many users and employees reuse passwords across multiple systems.

Security firms issued urgent alerts to businesses and consumers to reset passwords, enable MFA, and deploy anomaly detection tools to monitor login patterns. Even users who weren’t directly affected faced a new wave of phishing and social-engineering attempts leveraging exposed emails and partial credentials.

Lesson learned: Prioritize credential hygiene and phishing resilience

The 16 billion-credential leak underscores how vulnerable organizations are to credential-reuse attacks. Passwords remain the weakest link in cybersecurity, even in 2025.

To strengthen identity protection:

2. AT&T Customer Data Leak

In May 2025, AT&T made headlines again when a dataset containing the personal details of more than 86 million customers appeared for sale on the dark web. The exposed data included full names, addresses, dates of birth, email addresses, and at least 44 million Social Security numbers.

AT&T said the data may have originated from older breaches, but customers were understandably frustrated to see their data circulating online again. Multiple class-action lawsuits followed, and regulators opened new investigations into the company’s security practices.

Even if the exposure was not the result of a fresh intrusion, it showed how damaging it can be when compromised data reemerges. Once information escapes an organization’s control, it can remain in circulation indefinitely and become more dangerous when enriched or recombined with other stolen data.

Lesson learned: Build stronger access management

Even as threat actors become more advanced, stolen or reused credentials remain the single most common attack vector. Verizon’s 2025 Data Breach Investigations Report found that over 74% of breaches involve the human element, including credential misuse and social engineering.

To reduce your risk exposure:

4. Yale New Haven Health System

In March 2025, Yale New Haven Health System, one of Connecticut’s largest healthcare networks, discovered that an unauthorized party had gained access to internal systems containing sensitive patient information affecting approximately 5.5 million individuals.

According to investigators, the actor exfiltrated files that may have included names combined with an address, telephone number, email address, date of birth, race/ethnicity, patient type, medical record number, and/or Social Security number. Interestingly, the threat actor was able to access this information even without accessing the YNNHS electronic health record (EHR) systems or financial payment systems. 

The organization quickly hired Mandiant to conduct a forensic investigation, notified affected individuals, and offered credit monitoring. Regulatory scrutiny from both HIPAA and state authorities followed, along with several class-action lawsuits.

Lesson learned: Integrate monitoring and detection for rapid response

When visibility is limited, detection delays multiply the damage. The faster a breach is detected, the lower the cost and impact. IBM’s 2025 Cost of a Data Breach Report found that companies with fully deployed security AI and automation saved an average of $1.7 million in breach costs and reduced containment time by 44%.

To strengthen incident detection and response:

5. Allianz Life Insurance Company

In July 2025, Allianz Life Insurance Company of North America disclosed a breach that compromised personal information belonging to most of its 1.4 million U.S. customers. Attackers gained access through a third-party cloud-based customer relationship management (CRM) platform, using social engineering to infiltrate the vendor’s environment.

Although Allianz emphasized that its internal systems remained secure, the compromised data included customer names, addresses, dates of birth, email addresses, phone numbers, and even Social Security numbers. The insurer quickly engaged law enforcement, began notifying affected customers, and offered free identity-theft protection.

For an insurance company built on trust and privacy, the reputational impact was immediate. Even though the breach originated outside its network, customers saw it as a failure of stewardship — a reminder that even third-party systems can create severe first-party consequences.

Lesson learned: Strengthen third-party risk management

Modern organizations rely on an ever-expanding ecosystem of vendors, from cloud providers to CRM platforms and payment processors. But each connection widens the attack surface. According to SecurityScorecard’s Global Third-Party Breach Report, nearly 60% of all breaches now originate from a third-party vendor.

To strengthen vendor risk management:

Key takeaways: Essential cybersecurity hygiene for 2026

Across industries and continents, the biggest breaches of 2025 stemmed not from one-in-a-million zero-day exploits or cutting-edge AI deepfakes, but from common, preventable weaknesses. 

The lesson for 2026 is clear: focus on the fundamentals. 

Continuous monitoring, disciplined vendor management, credential hygiene, and strong incident response are business-critical initiatives, not nice-to-haves. Organizations that treat security as an ongoing business process, not a compliance checkbox, will be the ones that stay out of next year’s headlines.

To build stronger resilience in 2026, organizations should double down on the following best practices that consistently separate prepared companies from those caught off guard:

1. Vendor and third-party risk cannot be an afterthought

Nearly every high-impact breach in 2025 involved a third-party, supply-chain-or vendor-access vector. Whether it was a cloud-based CRM, vendor portal or SaaS platform, the root was often external to the core system. Organizations must govern vendor access, continuously monitor vendor security posture, segment vendor systems, and assume vendors may become the weakest link.

2. Credentials and access controls remain foundational

The 16 billion-credential event reminds us that no matter how sophisticated the threat actors become, simple things like credential hygiene, password reuse, lack of MFA and unused or un-reviewed privileged accounts still dominate. Make MFA mandatory, enforce strong password policies, ensure least-privilege access, and monitor for credential misuse or abnormal access patterns.

3. Continuous monitoring, detection, and incident response are essential

It’s not enough to try to build a perfect fortress. Attackers will find weak spots. What separates organizations is how quickly they detect, respond, and contain a breach. Real-time monitoring, anomaly detection, and well-rehearsed incident response plans are critical for minimizing damage. 

4. Non-financial data is still valuable

Exposing names, emails, phone numbers, dates of birth or purchase history may seem less severe than credit-card numbers or bank info, but attackers know how to monetize this data. Organizations should treat all sensitive PII with the same seriousness, classify data accordingly, and protect it.

5. Data minimization and segmentation reduce exposure

Collecting and retaining more data than necessary increases risk. As Secureframe Founder and CEO Shrav Mehta notes, organizations that clearly identify their most critical assets and collect only the data they need can focus resources where they matter most. “If you don’t need to store certain data,” he explains, “you shouldn’t collect it in the first place.” Prioritizing data protection at the source helps reduce exposure and simplifies compliance.

6. Culture, training, and proactive governance must be prioritized

Security is not just an IT issue. When social engineering is the root cause (as it often was in 2025) human-factor controls matter. Build a culture where employees and external partners are aware, enabled to report suspicious activity, and trained on evolving tactics. Regular phishing tests, tabletop exercises, vendor and employee training all pay dividends.