GTIG: UNC6671 "BlackFile" vishing → AiTM → rogue-MFA → programmatic SharePoint exfiltration of 1M+ files per victim; DLS shutdown signals probable rebrand [SINGLE-SOURCE]

Google Threat Intelligence Group published on 2026-05-15 an analysis of UNC6671 — a financially-motivated extortion cluster operating under the "BlackFile" brand since February 2026 — documenting a real-time vishing + adversary-in-the-middle chain that bypasses traditional MFA and pivots to mass SharePoint exfiltration (Google Threat Intelligence Group, 2026-05-15). The chain starts with a phone call placed to a victim's personal mobile number in which an operator impersonates internal IT helpdesk and directs the target to an attacker-registered lookalike single sign-on portal (Tucows-registered hostnames in the <org>.enrollms[.]com and <org>.passkeyms[.]com namespaces); the operator captures credentials and TOTP / push approvals live and immediately registers a new attacker-controlled MFA device for persistent post-vishing access, mapping to T1556 Modify Authentication Process. Post-compromise, BlackFile uses Python requests and PowerShell scripts against the Microsoft Graph API and direct SharePoint file-stream URLs to exfiltrate, with single-victim file counts exceeding one million; the API requests surface Microsoft Office's ClientAppId (d3590ed6-52b3-4102-aeff-aad2292ab01c) in the M365 audit log AppAccessContext field — the same value legitimate Office clients carry — to blend in with normal Office activity. The detection break is the underlying user-agent: legitimate Office clients do not present python-requests/2.28.1 or WindowsPowerShell/5.1 as the user-agent header against Graph or SharePoint endpoints. GTIG also notes that the FileAccessed audit event distinguishes the bulk-API extraction pattern from interactive FileDownloaded events. Geographic focus is North America, Australia, and the UK — but the playbook is language-agnostic; any European helpdesk-fronted M365 / Okta environment is one successful call away from the same outcome. The BlackFile data-leak site went offline in late April 2026 and relaunched on 2026-05-11 with a shutdown announcement, which GTIG assesses as probable rebrand rather than cessation. GTIG explicitly distinguishes UNC6671 from ShinyHunters (UNC6240). MITRE ATT&CK additionally: T1566.004 Spearphishing Voice, T1557 Adversary-in-the-Middle, T1528 Steal Application Access Token. Detection priorities: alert on Okta system.multifactor.factor.setup events not preceded by a user-initiated session; flag M365 audit FileAccessed events with AppAccessContext.ClientAppId == d3590ed6-52b3-4102-aeff-aad2292ab01c AND a user-agent containing python-requests or PowerShell; require Conditional Access compliant-device for Graph API access from administrative accounts; and move helpdesk-privileged accounts to FIDO2 phishing-resistant MFA.