Trusted Vendor Phishing: How Microsoft 365 Credentials Were Targeted

A trusted vendor email can lead to Microsoft 365 credential phishing when attackers compromise a legitimate business partner’s account and use it to distribute malicious content. Because the message comes from a known sender, recipients may be more likely to trust the email, open its attachment, and follow links that lead to a credential-harvesting page.

ProArch’s Security Operations Center identified and disrupted a coordinated phishing campaign that used this exact technique. A malicious PDF sent from a compromised vendor account redirected recipients through multiple domains before taking them to a page designed to capture login credentials.

The investigation connected suspicious sign-in activity across multiple users to the malicious email, uncovered attacker-created mailbox rules, and revealed that the campaign had reached more employees than the original security alerts indicated.

What Happened in the Microsoft 365 Vendor-Based Phishing Campaign?

ProArch’s Security Operations Center identified a coordinated credential-phishing campaign targeting multiple users in a client environment. The attack began with an email sent from an account belonging to a known and previously trusted vendor.

The email carried a malicious PDF attachment. When opened, the document redirected recipients through several intermediary domains before reaching a credential-harvesting page designed to capture login information.

Because the email appeared to come from a trusted business partner, several employees engaged with the attachment, and subsequent suspicious sign-in activity indicated that some accounts may have been compromised.

The investigation also uncovered attacker-created mailbox rules intended to hide the malicious activity from affected users, a classic sign of an attacker trying to maintain quiet access.

Rapid detection, correlation, and containment by the SOC stopped further spread and protected the client’s environment from deeper compromise.

Who Is Most at Risk from Vendor Email Compromise?

This incident is directly relevant to CISOs, IT administrators, and SOC teams managing Microsoft 365 environments — especially organizations that rely on regular email communication with external vendors and third parties. Any business where a compromised or abused vendor account could be used to deliver phishing content should take note of the tactics observed here.

How Did the Attackers Use a Malicious PDF to Steal Credentials?

The campaign began with unfamiliar sign-in alerts across multiple users. Correlating identity telemetry with email activity, the SOC traced the root cause to a single malicious email carrying a weaponized PDF.

How Was the Phishing Campaign Executed?

• Initial access: A malicious PDF attachment was delivered via an email account belonging to a known, previously trusted vendor.
• Multi-stage redirection: Sandbox analysis showed the PDF routed victims through several intermediary malicious domains before reaching a credential-harvesting page.
• Credential exposure: Users who interacted with the final page were positioned to have their Microsoft 365 credentials captured, correlating closely with subsequent suspicious authentication attempts from a common IP subnet.
• Broader distribution: A 30-day historical email review revealed the attachment had reached more users than the original alerts suggested, with some messages forwarded internally between employees — amplifying the campaign’s credibility and reach.
• Defense evasion: Malicious inbox rules were found in compromised mailboxes, apparently designed to hide security alerts and campaign-related emails from the account owner.
• Campaign persistence: After initial containment, continued IOC monitoring uncovered a second vendor-associated sender distributing the same attachment, showing the campaign was still active.

Why Does Vendor-Based Phishing Put Microsoft 365 Environments at Risk?

• Credential Theft — Harvested Microsoft 365 credentials could give an attacker direct access to email, files, and connected business applications.
• Business Email Compromise — Compromised mailboxes can be used to launch further attacks against internal staff, customers, or additional vendors.
• Third-Party / Supply-Chain Risk — Abusing a trusted vendor relationship bypasses the natural skepticism users apply to unknown senders, making this style of attack significantly more effective than generic phishing.
• Detection Evasion — Malicious mailbox rules can allow attackers to operate undetected for extended periods, increasing potential damage before discovery.
• Reputational and Trust Impact — Repeated phishing from a compromised partner can erode confidence in legitimate vendor communications going forward.

What Should Security Teams Do After a Vendor Phishing Attack?

What Immediate Actions Should You Take to Contain Credential Phishing?

• Block identified malicious domains, URLs, and sender addresses.
• Reset credentials for any accounts showing suspicious authentication activity.
• Review and remove unauthorized or unrecognized mailbox rules.
• Search mail flow logs for the identified attachment and sender to find additional exposed users.
• Tighten conditional access policies to flag or block sign-ins from atypical infrastructure.
• Deploy alerting for newly created mailbox rules, particularly those matching security-related keywords.

A Microsoft 365 Security Assessment can help identify gaps across identity and access controls, email security, monitoring, external sharing, and core tenant configuration.

How Can You Reduce Future Vendor Email Compromise Risk?

• Establish a clear process for notifying vendors when their accounts are suspected of compromise, and require remediation confirmation before restoring communication.
• Deliver targeted user-awareness training on vendor-impersonation and trusted-sender phishing techniques.
• Build automated correlation between identity, email, and mailbox-rule telemetry to shorten detection time for similar campaigns.
• Periodically reassess the risk posed by third-party vendors with access to sensitive communication channels.

What Indicators Should Organizations Monitor Next?

The SOC continues to monitor for additional senders and infrastructure tied to this campaign, along with similar vendor-impersonation attacks across our client base. Organizations that have communicated with vendors exhibiting unusual send behavior are encouraged to review their own environments against the indicators identified in this investigation.

Organizations needing the campaign’s published attachment, domain, IP, and mailbox-rule indicators should consult ProArch’s related trusted third-party phishing warning signs and response advisory.

Security teams can also review related Microsoft 365 phishing research from the ProArch SOC:

How Does ProArch SOC Detect and Stop Trust-Based Phishing Attacks?

This investigation highlights what sets a mature SOC apart from basic alert triage: the ability to connect a single sign-in alert to a broader, multi-stage attack chain spanning identity, email, mailbox, and network telemetry.

By correlating signals across systems, sandboxing suspicious attachments, and maintaining vigilance well after initial containment, ProArch’s SOC identified — and kept disrupting — a campaign that would have gone unnoticed by tools relying on sender reputation alone.

This proactive, connected approach is what keeps our clients protected against increasingly sophisticated, trust-based phishing techniques.