A previously undocumented malware family named SynkLoader has surfaced in targeted social engineering campaigns across corporate Microsoft Teams environments. Discovered by security researchers at Expel, the attack chain begins with direct messages from external Microsoft 365 tenants impersonating internal IT helpdesk staff and culminates in a fake Windows lock screen designed to capture domain passwords.
Anatomy of the Teams phishing campaign
The campaign exploits external communication settings in Microsoft Teams. Attackers create external Microsoft 365 accounts with display names resembling IT support or security teams. They reach out directly to corporate employees, claiming that the user needs an urgent security update, software patch, or VPN configuration fix. By moving the conversation from email to instant messaging, attackers capitalize on the higher level of trust users generally place in chat tools.
When the victim engages, the attacker sends a malicious archive or a link to a file hosting service containing an installer package. Once executed, the initial dropper unpacks SynkLoader, a modular malware framework combining Python, PowerShell, C#, and C++ components to bypass endpoint detection systems.
Capturing credentials with fake Windows lock screens
SynkLoader deploys a full-screen application that mimics the Windows 10 and Windows 11 logon screens, complete with the user’s account name, corporate branding, and password input fields. The fake lock screen hooks low-level keyboard shortcuts to prevent the user from switching windows, opening Task Manager, or accessing the desktop until they enter their domain password.
When the user types their credentials, the malware validates the format, stores the plaintext password, and transmits it via encrypted channels to attacker-controlled command and control servers. Once the password is captured, the fake screen dismisses itself, returning the user to their normal desktop session without raising immediate suspicion.
Multi-stage execution and persistence mechanisms
Beyond capturing credentials, SynkLoader functions as a versatile post-exploitation framework. The malware includes a traffic redirector module that establishes encrypted reverse proxies back to the attacker’s infrastructure. This allows the threat actors to tunnel arbitrary TCP traffic into the corporate network through the compromised workstation, effectively giving them an internal network vantage point.
The modular nature of the payload means attackers can dynamically push additional modules to the host. In observed incidents, operators downloaded reconnaissance scripts to enumerate Active Directory domain controllers, scan local subnets for vulnerable SMB shares, and extract cached browser credentials and session tokens from disk.
Evasion techniques and anti-analysis measures
SynkLoader incorporates several evasion mechanisms designed to thwart automated sandbox analysis and endpoint detection and response agents. The dropper checks for hypervisor artifacts, virtual machine drivers, and common debugger processes before executing the primary payload. If it detects an analysis environment, the binary terminates silently without performing malicious activity.
The malware also leverages living-off-the-land binaries, using native Windows utilities like certutil, bitsadmin, and PowerShell to stage components into temporary directories. By compiling critical modules into dynamically loaded libraries and injecting them into legitimate Windows processes (such as svchost.exe and explorer.exe), SynkLoader minimizes its footprint on disk and reduces the likelihood of triggering signature-based antivirus alerts.
Defensive countermeasures for security teams
To defend against SynkLoader and similar chat-based social engineering campaigns, organizations should review both technical controls and employee awareness programs:
- Restrict external Teams messaging: Audit your tenant’s external access settings in the Microsoft Teams admin center. If your business does not require open external federation, restrict domain communication to an approved allowlist or disable external 1:1 chat entirely.
- Implement Phish-Resistant MFA: Plaintext passwords captured by fake lock screens are significantly less damaging when authentication requires FIDO2 hardware keys, Windows Hello for Business, or certificate-based authentication.
- Monitor for unusual process trees: Set endpoint detection rules to flag unsigned binaries spawning full-screen windows, modifying keyboard hooks, or PowerShell scripts invoking credential-prompt APIs.
- Configure tenant boundary warnings: Ensure external chat banners in Microsoft Teams are prominent so employees are clearly alerted when interacting with users outside their organization.
- Audit collaboration app permissions: Review third-party application integrations and bot permissions across Microsoft Teams. Restrict automated messaging permissions to prevent compromised external accounts from sending unvetted attachments.
- Educate users on IT support workflows: Reiterate to staff that genuine IT support personnel will never distribute urgent software updates via unsolicited direct chat messages or ask users to re-enter passwords in non-standard prompts.
The growing threat of collaboration platform abuse
As email filtering solutions have improved at catching conventional phishing links, attackers are increasingly shifting focus to collaborative workspaces like Teams, Slack, and Discord. Protecting corporate networks requires extending visibility and access controls beyond the traditional email perimeter to all enterprise communication channels.
Incident response checklist for compromised endpoints
When security operations identify potential SynkLoader activity, rapid containment is necessary to prevent network-wide propagation. Immediately isolate the affected workstation from the local network and revoke active Entra ID and Active Directory session tokens for all users associated with the endpoint. Perform a forensic capture of running process memory to identify injected DLLs and active reverse proxy tunnels before initiating a clean operating system re-image.
Review Active Directory replication logs and Kerberos ticket requests originating from the host during the incident timeframe. Ensure that all service accounts and domain administrative passwords that may have been cached in LSASS memory on the workstation are reset immediately.