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Recently, a slew of activity by the advanced persistent threat group Lazarus has focused on finding vulnerable Microsoft IIS servers and infecting them with malware or using them to distribute malicious code. This article describes the details of the malware attacks and offers actionable suggestions for protecting Microsoft IIS servers against them.

The North Korean state-sponsored Lazarus hacking group is breaching Windows Internet Information Service web servers to hijack them for malware distribution. South Korean security analysts at ASEC previously reported that Lazarus was targeting IIS servers for initial access to corporate networks.

The notorious North Korean state-backed hackers, known as the Lazarus Group, are now targeting vulnerable Windows Internet Information Services web servers to gain initial access to corporate networks. The latest tactic of targeting Windows IIS servers was discovered by South Korean researchers at the AhnLab Security Emergency Response Center.

The infamous Lazarus Group actor has been targeting vulnerable versions of Microsoft Internet Information Services servers as an initial breach route to deploy malware on targeted systems. The findings come from the AhnLab Security Emergency response Center, which detailed the advanced persistent threat's continued abuse of DLL side-loading techniques to deploy malware.

Hackers are deploying a new malware named 'Frebniss' on Microsoft's Internet Information Services that stealthily executes commands sent via web requests. Microsoft IIS is a web server software that acts as a web server and a web app hosting platform for services like Outlook on the Web for Microsoft Exchange.

The Cranefly hacking group, aka UNC3524, uses a previously unseen technique of controlling malware on infected devices via Microsoft Internet Information Services web server logs. Like any web server, when a remote user accesses a webpage, IIS will log the request to log files that contain the timestamp, source IP addresses, the requested URL, HTTP status codes, and more.

Threat actors are increasingly abusing Internet Information Services extensions to backdoor servers as a means of establishing a "Durable persistence mechanism." Attack chains taking this approach commence with weaponizing a critical vulnerability in the hosted application for initial access, using this foothold to drop a script web shell as the first stage payload. This web shell then becomes the conduit for installing a rogue IIS module to provide highly covert and persistent access to the server, in addition to monitoring incoming and outgoing requests as well as running remote commands.

Microsoft says attackers increasingly use malicious Internet Information Services web server extensions to backdoor unpatched Exchange servers as they have lower detection rates compared to web shells. Microsoft previously saw custom IIS backdoors installed after threat actors exploited ZOHO ManageEngine ADSelfService Plus and SolarWinds Orion vulnerabilities.

Microsoft says attackers increasingly use malicious Internet Information Services web server extensions to backdoor unpatched Exchange servers as they have lower detection rates compared to web shells. "In most cases, the actual backdoor logic is minimal and cannot be considered malicious without a broader understanding of how legitimate IIS extensions work, which also makes it difficult to determine the source of infection," the Microsoft 365 Defender Research Team said Tuesday.