Trend Micro Uncovers Advanced Web Shell Attack Targeting IIS Servers

Trend Micro has uncovered a sophisticated web shell attack specifically targeting Internet Information Services (IIS) servers, revealing a complex intrusion methodology that demonstrates advanced persistent threat techniques. The cybersecurity firm's investigation disclosed a multi-stage attack that leveraged sophisticated mechanisms to establish persistent remote access and exfiltrate sensitive data.

The attackers exploited vulnerabilities in web server configurations, utilizing a meticulously crafted web shell to gain unauthorized access to the target infrastructure. By establishing initial access through compromised upload functionality, the threat actors systematically executed a series of reconnaissance and lateral movement activities designed to maximize their operational foothold.

Network forensics revealed that the attackers employed encoded PowerShell commands to create a reverse TCP shell, connecting to a command-and-control (C&C) server at 54.255.198.171. This infrastructure facilitated the download and execution of additional malicious tools, including remote access utilities like AnyDesk.exe and ngrok.exe, which enabled persistent remote system control.

To evade detection, the attackers implemented sophisticated obfuscation techniques. They renamed web shells to blend with legitimate files and used carefully crafted user-agent strings to mask their network communications. The threat actors strategically created new user accounts and modified existing credentials to maintain long-term access.

Incident diagram

The attack sequence included comprehensive system reconnaissance, utilizing built-in tools to gather critical information about the compromised environment. By leveraging utilities like 7z.exe, the attackers archived server directories and exfiltrated data through carefully constructed GET requests sent directly through the IIS server infrastructure.

Trend Micro researchers emphasized the critical importance of implementing robust security measures to mitigate such sophisticated web shell attacks. Recommendations include implementing stringent validation, enforcing strong authentication mechanisms, regularly patching systems, and ensuring comprehensive endpoint protection configurations.

The investigation underscores the evolving complexity of web-based threats and highlights the need for organizations to maintain vigilant, multi-layered security strategies. By understanding and anticipating such advanced attack methodologies, enterprises can significantly reduce their vulnerability to sophisticated cyber intrusions.

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