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A sophisticated malvertising effort is utilizing counterfeit Microsoft Teams installers to infiltrate corporate networks, exploiting tainted search engine results and misused code-signing certificates to distribute the Oyster backdoor malware.
The assault was neutralized by Microsoft Defender’s Attack Surface Reduction (ASR) protocols, which obstructed the malware from connecting with its command-and-control server.
This multi-stage assault emphasizes a rising trend of threat actors leveraging authentic services to seem credible and evade conventional security strategies.
By employing short-lived, valid code-signing certificates, the criminals managed to evade initial signature-based detection and deceive systems into trusting the harmful software.
Oyster Malware Via Microsoft Teams Installer
Conscia’s forensic analysis unveiled a quick and automated attack sequence that commenced with a basic web search.
On September 25, 2025, an employee’s search on Bing for Microsoft Teams resulted in a malicious redirect. Within a mere 11 seconds of the original search, the individual was routed from bing.com through a redirect domain (team.frywow.com) to a harmful website, teams-install.icu.
This swift redirection indicates an automated mechanism, probably driven by a malvertising initiative or a tainted search engine result that placed the harmful link prominently in the search rankings.
The domain teams-install.icu was crafted to mimic a legitimate Microsoft download webpage and was hosted on Cloudflare to further obscure its nefarious intent. Once the user arrived on the webpage, a file named MSTeamsSetup.exe was retrieved.
Approximately an hour later, the file was executed. Although it seemed to be a legitimate installer, it was actually the Oyster malware. The attack was halted only when Microsoft Defender’s ASR protocols recognized and obstructed the malware’s effort to link to its C2 server at nickbush24.com.
The essence of this campaign’s sophistication is rooted in its exploitation of code-signing certificates. The harmful executable was signed by an apparently legitimate entity referred to as “KUTTANADAN CREATIONS INC.” utilizing a certificate valid for only two days, from September 24 to 26, 2025.
This novel strategy enables threat actors to:
- Bypass Security: Signed files are frequently trusted by default, eluding antivirus and other signature-based inspections.
- Minimize Detection: The brief lifespan of the certificate diminishes the opportunity for security vendors to spot and revoke it.
- Automate Attacks: Attackers can systematize the process of acquiring and signing malware with new certificates for various campaigns.
Conscia research discovered other similar short-lived certificates utilized by signers like “Shanxi Yanghua HOME Furnishings Ltd,” hinting at a more extensive, well-coordinated operation.
This incident was neutralized prior to any data being exfiltrated or further payloads such as ransomware being deployed. The successful intervention illustrates that traditional security measures are no longer adequate. Trust in digital certificates cannot be absolute, and organizations must implement advanced endpoint protection.
Had the ASR protocols not been in action, the Oyster backdoor (also known as Broomstick or CleanUpLoader) would have established enduring access to the infiltrated system. This would have allowed the attackers to execute data theft, deploy additional malware, and move laterally across the network.
Key insights from this attack are evident: attackers are advancing their use of legitimate system tools (“living-off-the-land“), certificate trust is being actively weaponized, and the pace of automated attacks necessitates resilient, behavior-based security controls like ASR to avert a compromise that can transpire in seconds.
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