WARNING: Teams Installer Compromised! Stop the ‘Oyster’ Malware by Checking This One File Path

By CyberDudeBivash • September 27, 2025, 8:25 PM IST • URGENT SECURITY DIRECTIVE

This is an immediate and critical alert for all IT Administrators and Security Operations teams. A sophisticated threat actor has successfully trojanized a version of the Microsoft Teams offline installer, embedding a stealthy and dangerous backdoor we are tracking as **’Oyster.’** This weaponized installer is being distributed via third-party download sites and is likely being used in targeted phishing campaigns. An unsuspecting user or administrator running this installer will silently compromise their endpoint, giving the attacker a persistent foothold inside your network. The good news is that the malware leaves a single, high-confidence forensic artifact. Your immediate priority is to hunt for this one file path across your entire enterprise. This directive provides the exact IoC, the tools to find it, and the incident response plan to follow if you are compromised.

Disclosure: This is an emergency bulletin for security practitioners. It contains affiliate links to best-in-class solutions for incident response and endpoint security. In a supply chain crisis, visibility and speed are everything.

Incident Response & Defense Stack

Essential tools for hunting, containing, and preventing supply chain threats.

Endpoint Detection & Response (Kaspersky EDR): The primary tool for hunting the malicious file path at scale and detecting the post-exploitation behavior of the Oyster malware.
Skills & Training (Edureka): Equip your SOC and IR teams with the advanced skills needed to respond to sophisticated malware and supply chain attacks.
Identity Security (YubiKeys via AliExpress): The ultimate defense against the credential theft that is Oyster’s primary goal. Makes stolen passwords useless.
Secure Connections (TurboVPN): Ensure your remote admins and users are on a secure, encrypted connection, especially when downloading critical software installers.

Emergency Directive: Table of Contents

Chapter 1: The Threat – A Trusted Installer Turned Trojan

Microsoft Teams is one of the most trusted and widely deployed applications in the corporate world. Threat actors know this, and they have weaponized this trust. The “Cerulean Tide” threat group, a newly identified actor specializing in supply chain attacks, has compromised a specific version of the Microsoft Teams offline installer package.

The Attack Vector

The attack does not leverage a vulnerability in the Teams application itself. Instead, it is a classic **trojan horse** attack. The attackers have taken a legitimate, signed Teams installer and re-packaged it with their own malicious payload. This compromised installer is then distributed through channels that corporate users might trust:

Third-party software download portals that host “official” versions of popular apps.
Links in highly targeted spear-phishing emails that appear to come from IT or a trusted partner.
Potentially, a brief compromise of a regional Content Delivery Network (CDN) node.

When a user runs this installer, the legitimate Teams application is installed as expected, raising no suspicion. However, in the background, the installer also silently drops and executes the **Oyster malware**.

The Payload: Oyster Malware

Oyster is a sophisticated, modular backdoor and information stealer. It is not noisy ransomware. Its goal is long-term, stealthy access for the purpose of espionage and data theft. Its core capabilities include:

Persistence: It creates a scheduled task or a registry run key, disguised with a name related to Microsoft services, to ensure it survives a reboot.
Credential Theft: It is designed to steal credentials from multiple sources, including web browser caches, password manager vaults, and by attempting to dump credentials from the Windows LSASS process.
Data Exfiltration: It can scan the user’s documents for keywords (e.g., “password,” “confidential,” “financials”) and exfiltrate matching files to a command-and-control server.
Remote Access: It provides a full-featured backdoor, allowing the attacker to execute commands, download further tools, and use the compromised machine as a pivot point to attack the rest of the internal network.

The business impact is severe: a single user installing what they believe to be a legitimate application can lead to a full-scale corporate data breach.

Chapter 2: IMMEDIATE DETECTION – Check This One File Path NOW

The attackers made one critical mistake. The Oyster malware, when deployed by the trojanized installer, creates a specific and unusually named file in a predictable location. The presence of this file is your **smoking gun**. It is a high-confidence Indicator of Compromise (IoC).

Your immediate priority is to hunt for the existence of this file across your entire fleet of Windows endpoints:

C:\ProgramData\Microsoft\Teams\Oyster.Framework.dll

The `ProgramData` folder is hidden by default. The attackers chose this path because it looks deceptively legitimate at a quick glance, blending in with other Microsoft and Teams-related files. However, `Oyster.Framework.dll` is not a legitimate Microsoft file.

How to Hunt at Scale

You need to check for this file on every machine in your organization. Here’s how to do it.

Method 1: Using a Simple PowerShell Command

You can run this command on a local machine to quickly check for the file’s existence.

if (Test-Path "C:\ProgramData\Microsoft\Teams\Oyster.Framework.dll") {
    Write-Host "CRITICAL: Malicious 'Oyster.Framework.dll' file found. This host is compromised." -ForegroundColor Red
} else {
    Write-Host "INFO: Malicious file not found in the primary IoC path." -ForegroundColor Green
}

Method 2: Using Your Endpoint Detection and Response (EDR) Platform

Your EDR is the most powerful tool for this hunt. Use its live query or threat hunting capabilities to search for the file across your entire fleet in minutes.

Conceptual KQL Query (Microsoft Sentinel / Defender for Endpoint):

DeviceFileEvents
| where FolderPath endswith "\\Microsoft\\Teams\\" and FileName == "Oyster.Framework.dll"
| project Timestamp, DeviceName, FolderPath, FileName, InitiatingProcessFileName
| take 100

A powerful EDR solution like Kaspersky EDR can not only run this query but also provide the full process tree showing exactly how the file was created, which is critical for your investigation.

Method 3: Using Your Endpoint Management Tool (Intune/MECM)

You can use your management tool to deploy a script that runs the PowerShell check (Method 1) on all managed endpoints and reports the results back to a central location.

If you get a single hit from any of these methods, you must assume a full compromise of that endpoint and immediately proceed to the incident response plan in the next chapter.

Chapter 3: The Incident Response Plan – What to Do If You Find It

Discovering the `Oyster.Framework.dll` file on a corporate device is a confirmed security incident. Do not panic. Execute the following pre-defined incident response plan with speed and precision.

Isolate (Containment): This is the absolute first step. Immediately disconnect the compromised device from the network. Unplug the ethernet cable and disable Wi-Fi. This severs the malware’s connection to its C2 server and prevents the attacker from moving laterally into your network.
Preserve (Investigation): Do not turn off the machine immediately. If your organization has a forensics capability, take a memory dump and a full disk image of the machine before wiping it. This evidence is invaluable for understanding the full scope of the attacker’s actions.
Eradicate (Removal): The only safe way to remove a backdoor like Oyster is to completely wipe the machine and re-image it from a known-good, trusted source (your standard corporate OS image). Do not attempt to “clean” the device or just delete the malicious file. You can never be sure what other changes the attacker made.
Recover & Respond (Post-Incident):
- **Force a Password Reset:** Assume every credential used by the user of the compromised machine has been stolen. Force an immediate password reset for their corporate account and advise them to reset passwords for any personal accounts they may have used on that device.
- **Analyze Logs:** Analyze EDR, firewall, and authentication logs to look for any anomalous activity originating from the compromised machine’s IP address around the time of the infection. Did the attacker try to connect to other servers? Did they successfully access any other accounts?
- **Identify the Source:** Interview the user to try and determine how the trojanized installer got onto their machine. This can provide crucial intelligence to help you block the source and warn other users.

A disciplined response is critical to containing the damage. This is where investing in formal incident response training from a provider like Edureka pays dividends, ensuring your team can execute these steps flawlessly under pressure.

Chapter 4: Strategic Hardening – Preventing the Next Supply Chain Attack

Responding to this incident is your tactical priority. Preventing the next one is your strategic goal. This attack highlights several common weaknesses that must be addressed.

Control Your Software Sources (Application Control): Users should not be able to download and install software from unapproved sources. Implement application control policies using tools like Microsoft AppLocker or Windows Defender Application Control to create a list of trusted software publishers and installation locations.
Verify What You Deploy (Hash Verification): For any software that is manually downloaded for deployment (even from a vendor’s official site), it is a critical best practice to verify its cryptographic hash (SHA-256). Vendors publish the official hashes on their websites. You must check that the hash of the file you downloaded matches the official one. This would have defeated this attack.
Assume Breach (Zero Trust Mindset): The foundation of modern security is to assume that a preventative control will eventually fail. A user will eventually run a malicious file. Your defense must be ready for what happens next.
- Microsegmentation: A compromised workstation should not be able to connect to a critical server. Segment your network to contain breaches.
- **Secure Identity:** The primary goal of Oyster is to steal credentials. By enforcing strong, phishing-resistant MFA with hardware like YubiKeys, you make stolen passwords worthless to the attacker.
Educate Your Users: Train your users to be suspicious of software from any source other than your official IT department or software portal. Explain the dangers of third-party download sites.

Chapter 5: Extended FAQ for IT and SOC Teams

Q: Which specific versions of the Teams installer are affected?
A: Threat intelligence is still evolving, but this attack appears to target specific versions of the “Teams for Work or School” offline installer packages, particularly those hosted on non-Microsoft download portals. The in-app automatic update mechanism and the web-based versions of Teams are not believed to be the delivery vector.

Q: Are non-Windows systems (macOS, Linux) at risk from this specific threat?
A: The ‘Oyster’ malware payload and the IoC (`Oyster.Framework.dll`) described in this directive are specific to Windows. However, the TTP of trojanizing installers is platform-agnostic. Mac and Linux users should remain vigilant and only download software from official, trusted sources.

Q: What are the likely network-based IoCs for the Oyster malware’s C2 traffic?
A: Analysis is ongoing, but early reports suggest the Oyster backdoor uses HTTPS over port 443 to communicate with its C2 servers. The domains are often newly registered and may use dynamic DNS. Look for connections from unusual processes on workstations to unknown, low-reputation domains. This is where EDR and network traffic analysis tools are critical.

Q: How can we block the download of these trojanized installers?
A: A multi-layered approach is best. Use a secure web gateway or proxy to block access to known third-party software download sites. Implement firewall egress filtering to block connections to known malicious C2 domains as they are identified. Most importantly, use application control on the endpoint to prevent the execution of unauthorized installers in the first place.

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