The Marriott Starwood breach 2018 was a watershed moment in data security, affecting millions worldwide. The incident revealed deep-rooted security challenges in the integration of large corporate networks, especially following mergers and acquisitions.

Marriott International is one of the world’s largest hotel chains, operating thousands of properties across more than 130 countries. In 2016, Marriott acquired Starwood Hotels & Resorts Worldwide, expanding its portfolio with brands like Westin, Sheraton, and St. Regis. The acquisition also meant inheriting Starwood’s legacy IT infrastructure, which played a pivotal role in the breach.

The integration of Starwood’s systems into Marriott’s environment was a complex process, involving the consolidation of disparate networks, databases, and security protocols. This complexity, combined with legacy vulnerabilities, created an environment ripe for exploitation by cybercriminals.

• 2014: Attackers first gained unauthorized access to Starwood’s network.
• September 2018: Marriott discovered an attempt to access the Starwood guest reservation database.
• November 2018: Marriott publicly disclosed the breach, revealing the scale and scope of the incident.

The breach persisted undetected for nearly four years, making it one of the longest-running cyber intrusions in the hospitality sector.

Understanding the mechanics of the Marriott Starwood breach 2018 provides valuable insights into the tactics, techniques, and procedures (TTPs) employed by advanced threat actors targeting large organizations.

The initial compromise of Starwood’s network is believed to have occurred in 2014, prior to the Marriott acquisition. According to CrowdStrike, attackers exploited vulnerabilities in Starwood’s IT environment, possibly through phishing emails or exploitation of unpatched systems. Once inside, they established a persistent foothold, enabling long-term access.

The attackers utilized a combination of advanced techniques, including:

• Credential theft: Stealing legitimate user credentials to move laterally within the network.
• Remote Access Trojans (RATs): Deploying malware to maintain persistent access and exfiltrate data.
• Encryption and data exfiltration: Compressing and encrypting stolen data before transferring it out of the network to evade detection.

Forensic analysis indicated the use of tools commonly associated with nation-state actors, though attribution remains complex. The attackers’ ability to remain undetected for years points to a high level of sophistication and operational security.

For more on attacker TTPs, see MITRE ATT&CK.

The breach lasted from 2014 until its discovery in 2018, making it a multi-year intrusion. During this period, attackers had continuous access to the Starwood guest reservation database, systematically harvesting sensitive guest information.

Such prolonged access is indicative of insufficient network segmentation, inadequate monitoring, and delayed detection—common issues in legacy environments.

The Marriott Starwood breach 2018 resulted in the exposure of a vast array of sensitive data, affecting millions of individuals worldwide.

According to Marriott’s official statements and independent analysis by BleepingComputer, the following types of data were compromised:

• Names
• Mailing addresses
• Phone numbers
• Email addresses
• Passport numbers
• Date of birth
• Gender
• Arrival and departure information
• Reservation dates
• Loyalty program account information
• Encrypted payment card numbers and expiration dates

While Marriott stated that payment card data was encrypted using Advanced Encryption Standard (AES-128), the possibility that attackers also obtained the necessary decryption keys could not be ruled out. For a deeper understanding of encryption algorithms like AES, see Understanding AES: The Cornerstone of Modern Cryptographic Defense.

The breach impacted up to 500 million guests who made reservations at Starwood properties. Of these:

• Approximately 327 million guests had a combination of personal information exposed.
• The remaining guests had less sensitive data, such as name and email address, compromised.

This scale makes the Marriott Starwood breach 2018 one of the largest data breaches in history, rivaling incidents like the Yahoo and Equifax breaches.

Timely detection and transparent disclosure are critical in mitigating the impact of data breaches. The Marriott Starwood breach 2018 provides a case study in both the challenges and obligations organizations face in this regard.

Marriott discovered the breach in September 2018, when an internal security tool flagged an attempt to access the Starwood guest reservation database. Subsequent investigation revealed that unauthorized access had been ongoing since 2014.

Forensic experts from Mandiant were engaged to assist in the investigation, confirming the scope and duration of the intrusion.

• September 8, 2018: Marriott detected suspicious activity in the Starwood database.
• November 19, 2018: Marriott completed initial investigation and began notifying regulatory authorities.
• November 30, 2018: Marriott publicly disclosed the breach, informing affected guests and the media.

The delay between detection and public disclosure was due to the need for thorough investigation and regulatory compliance, but it also drew criticism from some quarters for not informing the public sooner.

The Marriott Starwood breach 2018 had far-reaching consequences for customers, the company, and the broader cybersecurity landscape.

For affected guests, the breach resulted in:

• Increased risk of identity theft and fraud due to exposure of personal and financial information.
• Potential for phishing attacks leveraging stolen data to craft convincing scams.
• Loss of trust in Marriott’s ability to protect customer data.

The exposure of passport numbers and other sensitive identifiers raised concerns about long-term privacy and security risks for travelers.

Marriott faced significant repercussions, including:

• Reputational damage and loss of customer confidence.
• Financial costs associated with investigation, remediation, legal fees, and regulatory fines.
• Operational disruption as resources were diverted to address the breach and implement new security measures.

The breach also prompted scrutiny of Marriott’s due diligence during the Starwood acquisition and its post-merger integration processes. To avoid similar pitfalls, organizations should consider conducting a professional password audit, testing & recovery to identify vulnerabilities inherited during mergers and acquisitions.

The Marriott Starwood breach 2018 triggered investigations by regulatory bodies worldwide, including the UK Information Commissioner’s Office (ICO) and the U.S. Federal Trade Commission (FTC).

• In July 2019, the ICO announced its intention to fine Marriott £99 million (approximately $124 million USD) for violations of the General Data Protection Regulation (GDPR).
• Class-action lawsuits were filed by affected customers in multiple jurisdictions.

These consequences underscored the legal and financial risks associated with inadequate cybersecurity and data protection practices.

Marriott’s response to the breach involved immediate containment, support for affected guests, and long-term security improvements.

Upon discovering the breach, Marriott took several steps to contain the incident and prevent further unauthorized access:

• Engaged leading cybersecurity firms for forensic investigation and remediation.
• Disabled compromised accounts and removed malware from affected systems.
• Implemented enhanced monitoring and detection capabilities across its networks.
• Notified law enforcement and regulatory authorities as required by law.

For more on incident response best practices, see CISA’s Incident Response Resources.

Marriott provided a range of support services to affected individuals, including:

• Dedicated website and call center for breach-related inquiries.
• One year of free identity monitoring services for U.S. guests.
• Guidance on steps to protect against identity theft and fraud.

While these measures were appreciated by some, others criticized Marriott for not offering more comprehensive compensation or support, particularly for non-U.S. guests.

In the aftermath of the breach, Marriott undertook a comprehensive overhaul of its cybersecurity posture, including:

• Accelerated integration of Starwood’s systems into Marriott’s secure environment.
• Enhanced network segmentation and access controls to limit lateral movement by attackers.
• Deployment of advanced threat detection and response tools, including Security Information and Event Management (SIEM) solutions.
• Regular security assessments and penetration testing by third-party experts.
• Employee training and awareness programs to reduce the risk of social engineering attacks.

These efforts aimed to address the root causes of the breach and prevent similar incidents in the future. For more on configuring effective brute-force protection and password recovery strategies, see how to configure a bruteforce attack.

The Marriott Starwood breach 2018 offers valuable lessons for organizations of all sizes, particularly those involved in mergers and acquisitions or managing complex, legacy IT environments.

• Due diligence in M&A: Thorough security assessments are critical before, during, and after mergers and acquisitions to identify and remediate inherited vulnerabilities.
• Continuous monitoring: Implementing robust monitoring and detection capabilities is essential for early identification of suspicious activity.
• Network segmentation: Limiting access between systems can contain breaches and prevent attackers from moving laterally.
• Patch management: Regularly updating and patching systems reduces the attack surface for known vulnerabilities.
• Incident response planning: Having a tested incident response plan enables swift and effective action when breaches occur.

For more on best practices, consult the NIST Cybersecurity Framework.

• Conduct regular security audits of all IT assets, especially following organizational changes.
• Invest in advanced threat detection technologies, such as endpoint detection and response (EDR) and SIEM.
• Enforce strong authentication and least privilege access controls across all systems.
• Educate employees on recognizing and reporting phishing and other social engineering attacks.
• Establish clear breach notification procedures to ensure timely and transparent communication with stakeholders.

Organizations can also refer to guidance from the CIS Controls and ISO/IEC 27001 for comprehensive security frameworks. Additionally, understanding the password cracking myths busted: what works today can help organizations separate effective security measures from outdated practices.

The Marriott Starwood breach 2018 serves as a stark reminder of the persistent and evolving threats facing organizations today. With 500 million guests affected, the breach underscored the importance of proactive cybersecurity measures, especially during periods of organizational change. By learning from this incident and implementing best practices, organizations can better protect their data, their customers, and their reputations in an increasingly hostile digital landscape. For insights into the latest tools and methods for staying secure, see the Password Recovery Tools 2025: Top Picks Ranked.

• Marriott International: Privacy and Cybersecurity
• CrowdStrike: Marriott Starwood Breach Lessons Learned
• BleepingComputer: Marriott Reveals Data Breach Impacting 500 Million Starwood Guests
• MITRE ATT&CK Framework
• Mandiant: Marriott Starwood Breach Analysis
• ICO: Statement of Intent to Fine Marriott International
• CISA: Incident Response Resources
• NIST Cybersecurity Framework
• CIS Controls
• ISO/IEC 27001 Information Security Management