A0Backdoor Campaign Abuses Microsoft Teams and Quick Assist for Initial Access
BlueVoyant researchers identified a social-engineering campaign in which operators linked to Blitz Brigantine / Storm-1811 / STAC5777, a cluster associated with the Black Basta ecosystem, impersonate IT support over Microsoft Teams after overwhelming targets with spam email. Victims are persuaded to launch Quick Assist, giving the attackers remote access that is then used to deploy A0Backdoor through digitally signed MSI packages masquerading as legitimate Microsoft software such as Microsoft Teams and CrossDeviceService. The activity has targeted organizations in sectors including finance and healthcare, and the use of multiple code-signing certificates suggests the operators prepared the toolchain well before the observed intrusions.
Once installed, A0Backdoor fingerprints the host and communicates with operators using DNS tunneling over public resolvers, helping the malware blend into normal traffic while maintaining persistence and command-and-control. The reporting is substantive threat intelligence rather than promotional or advisory content, and the only other mention of the same event is a newsletter item that cites the A0Backdoor research as one entry among many malware stories. Other references in the set cover unrelated vulnerabilities, policy developments, AI security issues, phishing, ransomware trends, and separate malware campaigns, and do not describe this specific Teams-and-Quick-Assist intrusion chain.
Get ahead of threats like this
Mallory correlates global threat intelligence with your attack surface — know if you’re exposed before adversaries strike.
How this story unfolded
3 events from the most recent confirmed update back to the earliest known activity.
BlueVoyant discloses A0Backdoor campaign details and mitigations
BlueVoyant publicly reported the A0Backdoor social-engineering campaign, attributing it to Blitz Brigantine and describing its abuse of Microsoft Teams and Quick Assist. The disclosure included technical details on the malware and recommendations such as restricting Quick Assist, limiting external Teams access, and monitoring for suspicious MSI and DNS activity.
Attackers deploy A0Backdoor via signed MSI and DLL sideloading
During the campaign, the operators used digitally signed MSI installers disguised as Microsoft software and a malicious hostfxr.dll for DLL sideloading to install the newly identified A0Backdoor. The malware fingerprints infected systems and uses DNS tunneling and DNS MX queries over public resolvers to communicate while evading detection.
Blitz Brigantine begins Teams and Quick Assist intrusion campaign
A threat group tracked as Blitz Brigantine, Storm-1811, and STAC5777 began targeting finance and healthcare organizations using spam-email flooding, fake IT support contact over Microsoft Teams, and Windows Quick Assist to gain access. The activity is linked to the Black Basta ransomware ecosystem and was active from at least August 2025.
Related entities
Vulnerabilities, threat actors, malware, products, organizations, and breaches Mallory has linked to this story.
Sources
2 references tracked. Mallory keeps watching after this page renders.
See the full picture, correlated to your attack surface.
Map indicators from this story to your assets and identify affected systems in minutes.
Every observed campaign, victim, and pivot linked to actors named in this story.
Malware, exploits, and IOCs connected to the activity described here.
YARA, Sigma, and Snort rules deployed to your SIEM as soon as they’re published.
Get matching new stories delivered to your team as they break — not the next morning.
Ask questions about this story and take action on the answers.
Related stories
Other stories Mallory is tracking that overlap on entities, sources, or topic labels.
Microsoft Teams Social Engineering Abuses Quick Assist to Deploy A0Backdoor
A social-engineering campaign targeting employees at **financial and healthcare** organizations is abusing **Microsoft Teams** chats/calls to trick users into granting remote access via *Windows Quick Assist*, enabling deployment of a newly identified malware family dubbed **A0Backdoor**. The activity begins with **email bombing** (flooding inboxes with spam) followed by an attacker impersonating internal IT over Teams, offering help and then persuading the victim to start a Quick Assist session; the tradecraft overlaps with tactics previously attributed to **Blitz Brigantine / Storm-1811**, which Microsoft has linked to **Black Basta**-associated operations. Post-access, the actor deploys **digitally signed MSI installers** masquerading as Teams-related components and *CrossDeviceService* (associated with Windows *Phone Link*), sometimes delivered via **tokenized links** from Microsoft personal cloud storage to appear trustworthy and hinder collection. BlueVoyant reported the installers drop files into user `AppData` paths that mimic legitimate Microsoft locations and use **DLL sideloading** (e.g., a malicious `hostfxr.dll`) to execute an in-memory loader that decrypts shellcode, performs sandbox checks, and ultimately extracts and runs **A0Backdoor** (using **AES**-encrypted payloads and a **SHA-256**-derived key), with anti-analysis behavior including excessive thread creation intended to disrupt debugging.
Apr 30, 2026
Microsoft Teams Vishing Used to Gain Remote Access and Deploy Malware
Attackers used **Microsoft Teams** to impersonate IT or helpdesk staff and socially engineer employees into granting access or executing malicious actions, turning collaboration tooling and trusted support workflows into the initial access vector. One report describes a compromise at an Italy-based consumer services company where a Teams meeting invite and screen sharing session led the victim to run a staged **PowerShell** chain that deployed **PhantomBackdoor**, a multi-stage **WebSocket-based** backdoor associated with earlier spear-phishing activity. The observed sequence included post-call PowerShell execution, device reconnaissance, and establishment of WebSocket command-and-control. A second report describes a similar **vishing** intrusion in which a threat actor posing as support staff called employees through Teams and, after multiple attempts, convinced one user to grant remote access through **Quick Assist**. The attacker then directed the victim to a spoofed credential-harvesting site, used a malicious **MSI** and sideloaded **DLL** to launch follow-on payloads, and established outbound C2. While the malware families and exact post-compromise chains differ, both accounts document the same operational pattern: **Teams-based social engineering**, abuse of legitimate remote assistance or user-guided execution, credential theft or payload staging, and transition to hands-on intrusion activity inside a corporate environment.
Mar 21, 2026
Microsoft Teams Vishing Campaign Deploys Nimbus RAT via Quick Assist and Cloud Services
Attackers used **email bombing** and fake IT support messages over **Microsoft Teams** to trick employees into launching **Windows Quick Assist**, giving remote access that led to the deployment of **Nimbus RAT**. eSentire said the campaign targeted corporate organizations globally, with notable impact on the legal sector, and observed a full compromise path from Teams contact to RAT execution in under 20 minutes. In one documented intrusion, the victim received more than 280 legitimate subscription emails before being contacted by an actor-controlled Teams account, then was directed to download malware from a compromised **Microsoft 365** tenant hosted on **SharePoint**. Nimbus RAT is a Java-based backdoor bundled with **OpenJDK** to run on Windows systems and uses **Google Drive** and **Google Sheets** for command-and-control, helping malicious traffic blend into normal SaaS activity. Researchers said the malware supports command execution, file and registry manipulation, screenshot capture, in-memory payload execution, and credential theft, while a secondary tool, **InboxSetupPro**, uses **OneDrive** for exfiltration and steals communications data including **Signal** attachments and large offline email archives. eSentire reported **1,540** similar Teams-related events across **172** customer environments, with activity rising sharply from December 2025 through March 2026, prompting recommendations to restrict external Teams communications, disable Quick Assist where possible, monitor transfers to public cloud storage, and train users to spot email-flooding and fake helpdesk tactics.
Jun 4, 2026