Cyberattack Bulletin: Defending Against Codefinger Ransomware in AWS S3 >

Cyberattack Bulletin:  Defending Against Codefinger Ransomware in AWS S3 >

Vectra AI Japan、ITR発行の最新レポート内「NDR市場」にて国内トップシェアを獲得
詳細を見る
Hamburger icon top line
Hamburger icon middle line
Hamburger icon bottom line
Back to all detections

M365 Ransomware

M365 Ransomware
Overview
Possible Root Causes
Example Scenarios
Business Impact
Steps to Investigate
MITRE ATT&CK Techniques
Related Detections
FAQ

Detection overview

The Microsoft 365 (M365) Ransomware detection identifies behaviors indicative of ransomware activity within an M365 environment. Ransomware is a type of malware that encrypts files or data and demands payment for their release. While traditional ransomware attacks target endpoint devices and servers, modern attackers increasingly exploit cloud services like M365 to disrupt operations, encrypt cloud-hosted files, and extort organizations. This detection helps security teams identify and mitigate potential ransomware threats before they cause significant damage.

Triggers

  • A series of file modifications typically associated with ransomware.

Possible Root Causes

  • An account is being used to access an organization’s cloud storage and encrypt and rewrite files.
  • In some cases, automated jobs or services that perform widespread file renaming may trigger this detection.

Business Impact

  • Ransomware attacks directly impact access to the organization’s data and are popular among attackers due to the possibility of a quick transition from attack to monetization.
  • After files have been encrypted, the attacker will ask the organization to pay a ransom in return for a promise to provide the encryption key which would allow the files to be decrypted.
  • Even if an organization is willing to pay the ransom, there is no guarantee that the encryption key will be provided by the attacker or that the decryption process will work.
  • Absent the encryption key, an organization must rely on restoration of files from backups.

Steps to Verify

  • Review the integrity of the affected files and determine whether they appear encrypted.
M365 Ransomware

Possible root causes

Malicious Detection

Attackers target M365 environments with ransomware to disrupt business operations, exfiltrate sensitive data, and demand ransom payments. Cloud-based ransomware attacks often originate from compromised user accounts, exploited OAuth applications, or adversaries leveraging automation tools to encrypt files stored in SharePoint and OneDrive. Since M365 is widely used for collaboration, a successful ransomware attack can impact multiple users, leading to significant data loss and operational downtime.

Benign Detection

Legitimate users may trigger similar behavior during bulk file migrations, large-scale document updates, or scripted data management tasks. IT administrators, for example, might move or modify large numbers of files when restructuring a SharePoint site or performing backups. Understanding the context of these changes—such as whether they were planned, who initiated them, and if they align with normal business activities—is essential for distinguishing between benign and malicious actions.

M365 Ransomware

Example scenarios

Scenario 1: Phishing attack leading to ransomware encryption

An employee receives an email that appears to be from Microsoft, prompting them to approve an OAuth request. Unknowingly, they grant permissions to a malicious application, which immediately begins encrypting files across their OneDrive. The M365 Ransomware detection triggers an alert due to rapid file modifications, prompting an investigation that identifies the compromised account and stops further damage.

Scenario 2: IT-admin initiated bulk file changes

An IT administrator is migrating large amounts of SharePoint data to a new site, causing a surge in file modifications. The detection is triggered, but upon review, the security team confirms it as a planned migration, avoiding unnecessary incident response actions.

‍

M365 Ransomware

Business impact

If this detection indicates a genuine threat, the organization faces significant risks:

Data loss and operational disruption

Ransomware can encrypt critical business files, making them inaccessible and severely impacting daily operations.

Financial and reputational damage

Paying a ransom does not guarantee data recovery, and public disclosure of a ransomware incident can harm an organization's reputation.

Regulatory non-compliance risks

Data breaches and loss of access to sensitive information may lead to violations of compliance requirements, resulting in legal and financial penalties.

M365 Ransomware

Steps to investigate

Review affected user activity

Identify the user or account associated with the suspicious file modifications and check for any unauthorized access or anomalies in their login behavior.

Analyze file modification patterns

Determine if the changes involve encryption, renaming, or mass deletions that resemble ransomware activity, and compare against normal business operations.

Check for related security alerts

Investigate if other security tools (such as Microsoft Defender or SIEM alerts) report concurrent suspicious activities, such as brute-force login attempts or abnormal OAuth permissions.

Contain and remediate the threat

If ransomware activity is confirmed, isolate the compromised account, revoke access tokens, and restore affected files from backups. Notify incident response teams to take further action.

M365 Ransomware

MITRE ATT&CK techniques covered

Data Encrypted for Impact

T1486
M365 Ransomware

Related detections

Ransomware File Activity

AWS Ransomware S3 Activity

See our detections in action

Our interactive demo provides a deep dive into the advanced capabilities of our cybersecurity platform, showcasing real-time detection, comprehensive analysis, and proactive threat mitigation.

Don't just read about the possibilities – experience them.

Launch the free tour
Ask for a custom demo

FAQs

How does Vectra AI detect ransomware in M365?

Vectra AI uses behavioral analytics to identify unusual file modifications that match ransomware activity, distinguishing them from normal user actions.

What should I do if this detection triggers a false positive?

Validate the user’s intent, check for known business activities that may cause similar behavior, and fine-tune detection policies if necessary.

How can attackers exploit M365 for ransomware attacks?

Attackers may gain access through credential theft, phishing, or malicious OAuth applications to encrypt or delete cloud-stored files.

What is the best way to respond to a confirmed M365 ransomware attack?

Immediately isolate affected accounts, revoke access tokens, restore files from backups, and conduct a forensic investigation to identify the root cause.

How does this differ from endpoint-based ransomware detection?

Unlike endpoint solutions, this detection focuses on cloud-based ransomware behaviors, such as abnormal file changes in M365 rather than malware running on a device.

Can this detection identify all types of ransomware?

While it effectively detects ransomware behaviors, novel ransomware variants with unique attack methods may require additional investigation.

Does this detection work in real-time?

Yes, Vectra AI provides real-time monitoring and alerting, enabling rapid response to potential ransomware threats.

Can Vectra AI prevent ransomware attacks?

While Vectra AI focuses on detection, it helps security teams respond quickly to minimize damage and prevent further spread.

Does this detection work for all M365 environments?

Yes, it applies to all organizations using M365 services, including OneDrive and SharePoint.

Can attackers bypass this detection?

While advanced attackers may attempt to evade detection, Vectra AI's Attack Signal Intelligence continuously monitors and correlates with other security events to identify and mitigate threats effectively.