What can we learn from the Largest Data Breaches of the Decade

Between 2015 between 2015 and 2025 between 2015 and 2025, some of the most significant data breaches of the past have changed how companies view cybersecurity risks. These were not simply technical glitches, they exposed problems with patching and identification management, vendor security and governance of data.

This guide explains the reasons for what happened, what security teams immediately changed and practical advice you can implement now to lessen the impact of breaches.

A Quick Look at the Biggest Data Breaches (2015-2025)

Yahoo (2013-2014 and disclosed in 2016-2017)

• Effect: 3 billion accounts compromised. three billion of the accounts affected

• Instruction: Poor encryption and delayed disclosure exacerbated the harm

• Status: Still the largest breach recorded to date

Equifax (2017)

• Effect: 147.9 million Americans affected

• Reason: Unpatched Apache Struts vulnerability and expired TLS certificate

• Lektion: Patch management failures make minor vulnerabilities

Marriott / Starwood (2014-2018)

• Impact: 339 million guest records, including passport numbers

• Outcome: Fined by UK ICO

• Instruction: Long-term undetected breaches add financial and legal risk

Capital One (2019)

• Effect: The impact of HTML0 is over 100 millions customers

• The reason: Cloud WAF and IAM incorrect configuration (SSRF)

• Lektion: Cloud security failures tend to be due to related to configuration issues not flaws in the platform.

SolarWinds (2020)

• Effect: Global supply chain compromise through Orion updates

• The lesson: Third-party software risk is risk for the enterprise.

Colonial Pipeline (2021)

• Effects: Fuel shortages across the U.S. East Coast

• The reason: Ransomware + weak security of credential

• The lesson: Cyber resilience is an issue of business continuity

MGM Resorts & Caesars (2023)

• Effect: More than 100 million dollars in losses

• Cause: Help desk social engineering

• Lektion: People and process problems can thwart technology

23andMe (2023)

• Impact: 6.9 million users affected

• Cause: Credential stuffing

• Lektion: Password reuse is an inherent risk to the system and not an error made by a user

MOVEit Transfer (2023)

• Impact: 2,700+ organizations, ~93 million individuals

• Lektion: Single supply-chain flaws can spread across the globe

AT&T (2024)

• Effect: 73 million customers, both former and current.

• The reason: Long-retained legacy data released

• The lesson: Old data is an ongoing risk

Change Healthcare (2024)

• Effect: 190 million Americans likely to be affected

• Reason: Ransomware + no MFA on the server that is critical.

• The lesson: Identity security is today the most important security measure

“Stolen Snowflakes” Attacks (2024-2025)

• Goals: Ticketsmaster Santander, others

• Reason: Credential theft, not exploits for platforms

• Leçon: Identity compromise is usually external, indirect and it is scalable

The Common Thread Behind Massive Breaches

Some of these incidents were typical “hacks.” Some involved fraud or misuse of identity. But, they all have the same theme:

Insufficiency of data collection and poor controls = catastrophic results

10 Security Lessons Teams Implemented Immediately

1. Patch Like Your Brand Depends on It

• Monitor the time to patch for critical security holes (<=7 days)

• Automate vulnerability-to-ticket workflows

• Monitor exploit activity, not only software versions

2. Delete Data You No Longer Need

• Automated PII retention and deletion. PII storage and removal

• Verify and log data destruction

• Legacy data = legacy risk

3. Treat Vendors as Attack Entry Points

• Maintain a Software Bill of Materials (SBOM)

• Patch and disclosure of breach SLAs

• Track and segment third-party tools

4. Secure Cloud Configurations by Default

• Inforce least privilege IAM

• Access to metadata of a Block instance

• Always check for drift in the configuration.

5. Identity Is the New Perimeter

• Mandatory MFA is required for every user including service accounts.

• Make sure you use phishing-resistant MFA (FIDO2) for administrators

• Monitoring tokens misused and session hijacking

6. Protect Identity Providers and Email Systems

• Conduct independent cloud identity audits

• Signing keys rotate and monitor any anomalies

• Externalize key custody whenever you can.

7. Strengthen Help Desks Against Social Engineering

• Password resets cannot be made via chat or phone without solid re-verification

• Utilize Just-in-Time access to admin

• Run “assume breach” tabletop exercises

8. Build Ransomware Resilience

• Backups that are offline and immutable

• Tested restoration runbooks

• Segmentation to ensure critical operations are protected

9. Prepare for Regulatory Scrutiny

• Communications to respond to an incident that are pre-approved

• Escalation pathways that are clear for regulators

• The security and privacy programs

10. Assume Credential Reuse Will Happen

• Breached-password detection

• MFA enforcement, with the possibility of friction when opting out

• Rate-limiting and detection of abuse at the feature level

A Realistic 90-Day Cybersecurity Action Plan

Weeks 1-2: Stabilize

• Find out about crown-jewel systems and the data

• Inforce MFA across the entire organization

• Patch vulnerabilities that are exploited (KEV list)

Weeks 3-6: Reduce Blast Radius

• Implement least privileges and JIT access to admin

• Critical systems for segmentation

• Automated retention of data and deletion

Weeks 7-10: Detect and Respond Faster

• Check for the abuse of tokens and identity anomalies

• Harden help desk identity verification

• Do exercises to recover ransomware

Weeks 11-13: Secure the Supply Chain

• Inspect all third-party software

• Enforce vendor security SLAs

• Monitor vendor access behavior

Metrics That Actually Change Outcomes

• Mean time to identify (MTTD) identities that are not as expected

• The percentage of critical vulnerabilities fixed within 7 to 15 days

• The percentage of admin accounts that use MFAs that are phishing-resistant

• The hygiene of key/token rotation as well as the presence of alarms for anomalies

• Vendor SBOM, incident-response compliance

• Automated PII deletion coverage

Final Takeaway

The biggest security breaches in the last past decade were not caused by impossible zero-day attacks. They were the result of fundamental security flaws:

• Systems that are not patched

• Data that is stored over

• Insecure identity controls

• Vulnerable help desks

• Third-party risk not managed

The best part?
These problems are easily identifiable, measurable and can be fixed.

Companies that take action based on these lessons could dramatically reduce the consequences of a breach and in some cases even prevent it from happening altogether.