Hackers Exploiting a Six-year-old IIS Vulnerability to Gain Remote Access

In a concerning revelation, cybersecurity firm eSentire’s Threat Response Unit (TRU) has detected active exploitation of a six-year-old vulnerability, CVE-2019-18935, in Progress Telerik UI for ASP.NET AJAX.

This flaw, which affects Internet Information Services (IIS) servers, enables malicious actors to gain unauthorized remote access and execute commands, posing a significant threat to unpatched systems.

The vulnerability remains a critical attack vector despite its age, underscoring the persistent challenge of legacy vulnerabilities in enterprise environments.

Attack Methodology

The exploitation process begins with threat actors probing IIS servers for an active file upload handler.

Once confirmed, they deploy a customized proof-of-concept (PoC) exploit to upload a reverse shell.

The reverse shell is a mixed-mode .NET assembly designed to establish a connection with a command-and-control (C2) server using Windows Sockets.

Upon gaining access, adversaries escalate their activities by executing reconnaissance commands, enumerating users, and leveraging tools like “cmd.exe” within the IIS worker process (w3wp.exe).

The reverse shells are often deposited in temporary directories with filenames resembling randomized numerical patterns, e.g., [10 digits].[6 digits].dll.

Additionally, attackers are deploying the JuicyPotatoNG privilege escalation tool under misleading file names like “PingCaler.exe” and “JuicyPotatoNG.exe” in public directories.

Batch scripts such as “rdp.bat” and “user.bat” have also been observed, though their specific functionalities remain unclear.

eSentire’s TRU identified these attacks in early January 2025 through suspicious activity logs within IIS servers.

For instance, IIS access logs displayed requests to vulnerable endpoints (Telerik.Web.UI.WebResource.axd) with specific query parameters, signaling exploitation attempts. Upon detection, the cybersecurity firm’s.

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