The cybersecurity landscape is constantly evolving, with threat actors continually unearthing novel methods to bypass defenses. While attention often focuses on zero-day exploits and sophisticated ransomware, a recent incident highlights a classic, often overlooked vector: the humble Microsoft Compiled HTML Help (CHM) file. This seemingly benign format, designed for offline documentation, is now being weaponized to deliver malicious payloads, proving that legacy technologies, when repurposed, can pose significant contemporary threats.

This post delves into a specific case where a CHM file masquerading as a financial document became a potent delivery mechanism for malware. Understanding this attack vector is crucial for cybersecurity professionals, IT teams, and developers alike to reinforce their defensive stances against increasingly resourceful adversaries.

The CHM File: A Deceptively Simple Attack Vector

Microsoft Compiled HTML Help (CHM) files, introduced as a replacement for WinHelp, are essentially compressed HTML documents, images, and other associated files compiled into a single binary. They are commonly used for software documentation, e-books, and help manuals. Their utility lies in their ability to package rich, interactive content for offline access.

However, the very features that make CHM files useful – their ability to execute JavaScript, display embedded content, and even launch external applications under certain conditions – also present a significant security risk when exploited maliciously. The inherent trust placed in help files, combined with default system behaviors, makes them an attractive target for social engineering and payload delivery.

“deklaracja.chm”: A Case Study in CHM Weaponization

A recent incident, observed on June 30, 2025, from Poland, involving a file named “deklaracja.chm” (Polish for “declaration”), serves as a prime example of CHM weaponization. This malicious CHM file was crafted to appear as a bank-transfer declaration.

The attack observed a typical social engineering pattern: the victim receives a seemingly legitimate document. Upon opening “deklaracja.chm,” the user is initially presented with a benign receipt image, designed to lull them into a false sense of security and perhaps distract them while malicious code executes in the background. This psychological manipulation is a cornerstone of effective phishing and social engineering campaigns.

While the initial report does not detail the specific payload delivered by “deklaracja.chm,” the method underscores a sophisticated approach: leveraging a trusted, ubiquitous file format to bypass traditional security controls that might focus on more common executables or document types like PDFs or Office files.

How CHM Files Can Be Exploited

The exploitation of CHM files typically revolves around several key mechanisms:

• Execution of Scripting Languages: CHM files can embed and execute various scripting languages, including JavaScript. Malicious actors can use this capability to download and execute further payloads, establish persistence, or perform reconnaissance on the victim’s system.
• ActiveX Controls and HTML Elements: While modern browsers have tightened security around ActiveX, CHM files can still leverage certain HTML elements and embedded controls that might permit file downloads or system interactions.
• Bypassing Security Prompts: In some scenarios, especially when delivered via email or downloaded from seemingly legitimate sources, the user might not receive typical security warnings associated with executable files, leading to a higher likelihood of execution.
• Zone Identifier Issues: Historically, issues related to the “Mark-of-the-Web” (MOTW) have allowed CHM files downloaded from the internet to execute with higher privileges than intended. While mitigations exist, misconfigurations or new bypasses can emerge.

Remediation Actions and Mitigations

Defending against CHM-based attacks requires a multi-layered approach, encompassing technical controls, user education, and continuous monitoring.

• Endpoint Detection and Response (EDR): Utilize EDR solutions with behavioral analysis capabilities to detect suspicious activity originating from CHM files, such as unexpected network connections, process injection, or file modifications.
• Email Security Gateways: Configure email security solutions to scan and, if possible, block CHM attachments that originate from external sources, especially if they are not anticipated or from untrusted senders. Implement sandboxing to analyze suspicious CHM attachments before delivery.
• Disable CHM Viewer (hh.exe) via GPO/Registry: For organizations with no legitimate need for the HTML Help Viewer, consider disabling its execution via Group Policy Objects (GPO) or registry modifications. This is a robust control but should be balanced against potential business impact.
• Application Whitelisting: Implement application whitelisting solutions (e.g., Windows Defender Application Control, AppLocker) to prevent the execution of unauthorized applications, including potentially malicious scripts launched from CHM files.
• User Awareness Training: Educate users about the risks associated with unexpected attachments, regardless of file type. Emphasize verification processes for financial documents or other sensitive information received via email. Teach them to recognize social engineering tactics.
• Patch Management: Ensure that operating systems and all software are kept up-to-date with the latest security patches. While not a direct CHM vulnerability (like CVE-2021-36942, which was related to HTML Help and RDP), general system hardening reduces the overall attack surface.
• Network Segmentation and Least Privilege: Limit the impact of a successful compromise by segmenting networks and enforcing the principle of least privilege, ensuring that even if a CHM file executes, its ability to spread or cause damage is contained.

Tools for Detection and Analysis

Several tools can aid in the detection, analysis, and mitigation of CHM-based threats:

Tool Name	Purpose	Link
Virustotal	Online service for analyzing suspicious files and URLs to detect malware using various antivirus engines and blacklisting services.	https://www.virustotal.com/
Cutter	A free and open-source reverse engineering platform that can be used to analyze compiled binaries, including CHM components if extracted.	https://cutter.re/
CHM Decompiler (e.g., KeyTools CHM Decompiler)	Tools to unpack and view the contents of CHM files, allowing security analysts to inspect embedded HTML, scripts, and other resources.	(Search for reputable CHM decompiler tools, e.g., KeyTools CHM Decompiler)
Sysinternals Process Monitor	Monitors and displays file system, Registry, and process/thread activity in real-time, useful for observing suspicious behavior upon CHM file execution.	https://learn.microsoft.com/en-us/sysinternals/downloads/procmon
Any.Run / Joe Sandbox	Interactive online malware analysis sandboxes where users can safely run and observe suspicious files, including CHM files, in a controlled environment.	https://any.run/ https://www.joesandbox.com/

Conclusion

The weaponization of Microsoft Compiled HTML Help files serves as a critical reminder that cybersecurity threats are not limited to the latest vulnerabilities. Attackers skillfully leverage legacy formats and social engineering to bypass modern defenses. The “deklaracja.chm” incident underscores the continuous need for vigilance, robust security measures, and proactive user education.

Organizations must treat all attachments with suspicion, regardless of their apparent file type or source. Implementing a defense-in-depth strategy that includes advanced endpoint protection, strong email security, application whitelisting, and regular security awareness training is paramount to mitigating the risks posed by resourceful adversaries who repurpose old tools for new attacks.