Cybercriminals have compromised Microsoft email accounts belonging to organizations in the energy sector and used those trusted inboxes to distribute large volumes of phishing emails. In at least one confirmed incident, more than 600 malicious messages were sent from a single hijacked account.
Microsoft security researchers explained that the attackers did not rely on technical exploits or system vulnerabilities. Instead, they gained access by using legitimate login credentials that were likely stolen earlier through unknown means. This allowed them to sign in as real users, making the activity harder to detect.
The attack began with emails that appeared routine and business-related. These messages included Microsoft SharePoint links and subject lines suggesting formal documents, such as proposals or confidentiality agreements. To view the files, recipients were asked to authenticate their accounts.
When users clicked the SharePoint link, they were redirected to a fraudulent website designed to look legitimate. The site prompted them to enter their Microsoft login details. By doing so, victims unknowingly handed over valid usernames and passwords to the attackers.
After collecting credentials, the attackers accessed the compromised email accounts from different IP addresses. They then created inbox rules that automatically deleted incoming emails and marked messages as read. This step helped conceal the intrusion and prevented account owners from noticing unusual activity.
Using these compromised inboxes, the attackers launched a second wave of phishing emails. These messages were sent not only to external contacts but also to colleagues and internal distribution lists. Recipients were selected based on recent email conversations found in the victim’s inbox, increasing the likelihood that the messages would appear trustworthy.
In this campaign, the attackers actively monitored inbox responses. They removed automated replies such as out-of-office messages and undeliverable notices. They also read replies from recipients and responded to questions about the legitimacy of the emails. All such exchanges were later deleted to erase evidence.
Any employee within an energy organization who interacted with the malicious links was also targeted for credential theft, allowing the attackers to expand their access further.
Microsoft confirmed that the activity began in January and described it as a short-duration, multi-stage phishing operation that was quickly disrupted. The company did not disclose how many organizations were affected, identify the attackers, or confirm whether the campaign is still active.
Security experts warn that simply resetting passwords may not be enough in these attacks. Because attackers can interfere with multi-factor authentication settings, they may maintain access even after credentials are changed. For example, attackers can register their own device to receive one-time authentication codes.
Despite these risks, multi-factor authentication remains a critical defense against account compromise. Microsoft also recommends using conditional access controls that assess login attempts based on factors such as location, device health, and user role. Suspicious sign-ins can then be blocked automatically.
Additional protection can be achieved by deploying anti-phishing solutions that scan emails and websites for malicious activity. These measures, combined with user awareness, are essential as attackers increasingly rely on stolen identities rather than software flaws.
