Why Your Microsoft 365 Login is at Risk: New Phishing Attack Hides in Azure Blob Storage

 

CYBERDUDEBIVASH • ThreatWire

Published: October 19, 2025

Why Your Microsoft 365 Login is at Risk: New Phishing Attack Hides in Azure Blob Storage

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ps://contoso.z13.web.core.windows.net/SignIn/ Email or phone Password Sign in → posts creds to C2 HTML smuggling / Redirect

Attackers host pixel-perfect Microsoft 365 sign-ins on Azure Blob Static Websites to borrow Microsoft-owned domains and evade filters.

TL;DR: Criminals are spinning up Azure Blob Storage static sites (e.g., *.z*.web.core.windows.net / *.blob.core.windows.net) that host look-alike Microsoft 365 pages. These pages steal credentials and MFA codes, then pivot into your tenant. Block fast: inspect URLs beyond the “Microsoft look,” restrict unknown Azure Storage hostnames, use Safe Links/Defender detonation, enforce phishing-resistant MFA, and monitor suspicious consent/OAuth token events.

Audience: US • EU • UK • AU • IN SOC/IR teams, M365 admins, IT helpdesks, and executive risk owners.

Why this works

• Trusted cloud domain halo: Many filters “allow” *.windows.net / *.azureedge.net traffic by default.
• Static Website + SAS: Attackers enable static hosting and share via short-lived SAS links or custom subpaths that look corporate.
• Pixel-perfect UX: Brand-accurate HTML/CSS mimics Microsoft’s login; some flows proxy live login.microsoftonline.com and steal session tokens (AiTM).
• HTML smuggling: Email carries a benign HTML file that reconstructs the payload in the browser to bypass attachment scanning.

What you’ll see in the wild

• Sender themes: “document share,” “payment remittance,” “DLP quarantine,” or “new Teams voicemail.”
• Links like: https://<random>.z13.web.core.windows.net/<brand>/index.html or https://<name>.blob.core.windows.net/<container>/signin/.
• On submit: credentials posted to attacker C2 or reverse-proxied to harvest MFA cookies (Adversary-in-the-Middle).

Immediate protections (do these now)

1. Turn on phishing-resistant MFA for admins and high-risk users (FIDO2 security keys / Windows Hello for Business). Avoid SMS/voice.
2. Microsoft Defender for Office Safe Links: rewrite & detonate links, block click-time to unknown *.web.core.windows.net/*.blob.core.windows.net when not owned by you.
3. Conditional Access: require compliant devices for risky sign-ins; block legacy protocols; enforce token binding / Continuous Access Evaluation.
4. Disable “user consent” to apps and enable admin consent workflow; verify Publisher Verification for OAuth apps.
5. Educate fast: teach users to expand the address bar fully—Microsoft login lives under login.microsoftonline.com or microsoft.com domains only.

Blocklist / Allowlist strategy for Azure Storage

• Allowlist only your storage hostnames (e.g., myorg.blob.core.windows.net, myorg.z13.web.core.windows.net); default-deny all others at proxy/SWG.
• Inspect referrers & destinations for web.core.windows.net and blob.core.windows.net accessed directly from email clicks.
• Use TLS inspection on egress where permitted to extract full host & path for policy decisions.

Detection ideas (SOC playbook)

KQL – unusual Azure Storage hosts clicked from email

EmailEvents
| where Timestamp > ago(14d)
| where ThreatTypes has_any ("phish","credentialPhish","url")
| join kind=leftouter UrlClickEvents on NetworkMessageId
| where Url has_any ("web.core.windows.net","blob.core.windows.net")
| summarize clicks=count(), users=dcount(AccountUpn) by UrlDomain, bin(Timestamp, 1d)
| order by clicks desc
  

KQL – consent grant anomalies

AuditLogs
| where OperationName == "Consent to application"
| where ResultDescription !~ "Admin consented"
| summarize grants=count(), users=dcount(InitiatedBy.user.userPrincipalName) by TargetResources[0].displayName, bin(TimeGenerated, 1d)
  

Hunt: bursts of sign-ins from fresh IPs / impossible travel right after a click to suspicious Azure Storage hosts; Sign-in logs with non-compliant device + unfamiliar token key IDs.

Helpdesk response (when a user reports a page)

1. Collect the full URL and browser HAR if possible; disable the user account temporarily and revoke refresh tokens.
2. Reset password and enforce step-up MFA re-registration; inspect mailbox rules and OAuth grants.
3. Search tenant for other clicks to the same hostname; purge the original email via Threat Explorer.

User coaching (60-second rule)

• Always check the domain in the address bar. If it’s not login.microsoftonline.com, do not type your password.
• Never approve MFA prompts you did not initiate. Prefer number matching or FIDO2 keys.
• Report suspicious pages using your company’s “Report Phish” add-in or forward to the SOC address.

Explore more on ThreatWire:

• Microsoft 365 security advisories
• OAuth consent & app governance
• Latest phishing tradecraft

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Why trust CyberDudeBivash? We publish practical, vendor-agnostic guidance for defenders across US/EU/UK/AU/IN with playbooks that SOCs can run today—no fluff, just actions.

Microsoft 365 phishing, Azure Blob Storage static website, web.core.windows.net, blob.core.windows.net, adversary-in-the-middle, MFA token theft, Safe Links policy, Conditional Access, OAuth consent phishing, app governance, M365 Defender, KQL hunting, enterprise email security, US EU UK AU India.

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Educational analysis. Implement changes in a test cohort before broad rollout; validate impact with staged phishing simulations and egress policy checks.