Email is the lifeblood of modern communication, but it remains a primary vector for cyberattacks. As organizations invest heavily in sophisticated email filtering solutions, threat actors continuously innovate to bypass these defenses. A recent campaign targeting Russia’s healthcare and technology sectors highlights an alarming new trend: the ingenious use of polyglot files to deliver malicious payloads, camouflaged within seemingly benign archives.

The Polyglot File Problem: A New Layer of Deception

In the final week of June 2025, security teams across Russia’s vital sectors observed an unusual surge of “routine” logistical and contractual emails. These messages, deceptively arriving from legitimate sender addresses and featuring familiar subject lines, harbored a dangerous secret. The attached archives, while appearing as ordinary ZIP files, exhibited a dual nature: they also functioned as executable libraries.

This hybrid format is the hallmark of a polyglot file. Derived from the Greek “polyglottos,” meaning “many tongues,” a polyglot file is a single file designed to be valid under multiple file formats. In this specific attack, the malicious archives were crafted to be simultaneously recognized as a ZIP file by email gateways and, critically, as an executable (like an EXE or DLL) by the victim’s operating system.

The inherent challenge for email filters lies in this duality. Traditional filters are designed to identify and block known malicious file types or patterns. However, if a file successfully masquerades as a harmless archive at the gateway level, it often bypasses initial scrutiny. Once opened by the unsuspecting recipient, the secondary, executable nature of the file can then be triggered, leading to compromise.

How Polyglot Attacks Bypass Email Filters

The efficacy of polyglot file attacks against email filters stems from several factors:

• Signature-Based Evasion: Many email security solutions rely on signature databases of known malicious files. Polyglot files, especially if newly crafted, may not have an existing signature, allowing them to slip through.
• Content-Type Ambiguity: Email gateways typically examine the file header and extension to determine its type (e.g., .zip, .pdf, .exe). A polyglot file is meticulously constructed to have valid headers for multiple formats, making it difficult for automated systems to definitively classify it as malicious.
• Sandbox Evasion: Advanced sandboxing techniques analyze files in an isolated environment. However, if the polyglot file’s executable component only triggers under specific user interaction (e.g., clicking an “extract” button or running a script after extraction), it might not exhibit malicious behavior during automated sandbox analysis.
• Trust Exploitation: By appearing as a common, non-executable archive type (like a ZIP), the attack leverages user trust. Users are generally more inclined to open a ZIP file than an unsolicited executable.

The Anatomy of the Attack Campaign

While specific details of the payload delivered in the Russia healthcare and technology sector attacks were not provided in the initial intelligence, the general modus operandi suggests a multi-stage approach:

1. Initial Compromise: The polyglot ZIP/executable file is delivered via email, often disguised as routine business correspondence.
2. User Interaction: The victim opens the “ZIP” file. Depending on the exact polyglot technique, this might directly execute the malicious code or present a seemingly innocuous prompt that, when acted upon, triggers the payload.
3. Payload Delivery: The executed code could then drop a variety of malware, including:
   • Information Stealers: To exfiltrate sensitive data.
   • Ransomware: To encrypt systems and demand ransom.
   • Remote Access Trojans (RATs): To establish persistent access for further exploitation.
   • Botnet Clients: To enlist the compromised machine into a malicious network.
4. Lateral Movement & Persistence: Once inside the network, the attackers would likely attempt to gain further access, elevate privileges, and establish persistence.

Remediation Actions and Proactive Defenses

Combating polyglot file attacks requires a multi-layered defense strategy that goes beyond traditional signature-based detection:

• Advanced Email Security Gateways (SEG): Implement SEGs with advanced threat protection capabilities, including behavioral analysis, content disarm and reconstruction (CDR), and deep file inspection that can identify anomalies in file structures.
• User Awareness Training: Conduct regular, up-to-date training for all employees on identifying phishing attempts, suspicious attachments, and the dangers of opening unsolicited files, even if they appear benign. Emphasize the principle of “assume breach.”
• Endpoint Detection and Response (EDR)/Extended Detection and Response (XDR): Deploy EDR/XDR solutions that monitor endpoint activity for suspicious processes, network connections, and file modifications. These tools can often detect the execution of a malicious payload even if the initial email filter failed.
• Application Whitelisting/Blacklisting: Restrict the execution of unauthorized applications. While challenging to implement broadly, it can prevent malicious executables from running.
• Least Privilege Principle: Ensure users operate with the minimum necessary privileges to perform their jobs. This limits the potential damage if an account is compromised.
• Regular Software Updates: Keep operating systems, email clients, and all software patched and up-to-date to mitigate known vulnerabilities.
• Network Segmentation: Segment your network to contain potential breaches and limit lateral movement by attackers.
• Robust Backup and Recovery Strategy: Implement frequent, immutable backups of critical data to minimize the impact of a successful ransomware attack.

Relevant Tools for Detection and Mitigation

Here’s a table of tools and categories that can aid in detecting and mitigating threats posed by polyglot files and similar advanced email attacks:

Tool Category/Name	Purpose	Link (Example)
Advanced Email Security Gateways (SEG)	Provide deep inspection, sandboxing, and CDR capabilities for incoming emails.	N/A (Vendor-specific solutions like Proofpoint, Mimecast, Microsoft Defender for Office 365)
Endpoint Detection and Response (EDR)	Monitors endpoints for suspicious activity, process execution, and file changes.	N/A (Vendor-specific solutions like CrowdStrike Falcon, SentinelOne, Microsoft Defender for Endpoint)
Threat Intelligence Platforms	Provide insights into new attack methods, IOCs, and threat actor TTPs.	Mandiant Advantage
Security Information and Event Management (SIEM)	Aggregates and analyzes security logs from various sources to detect anomalies.	N/A (Vendor-specific solutions like Splunk, IBM QRadar, Microsoft Sentinel)
File Analysis/Sandbox Tools	Automated or manual analysis of suspicious files in a controlled environment.	VirusTotal

Conclusion: The Evolving Landscape of Email-Borne Threats

The emergence of polyglot file attacks underscores a critical truth in cybersecurity: threat actors will always seek out the path of least resistance. As defenses improve, their methods become more sophisticated, leveraging inherent design characteristics of file formats and operating systems. Protecting against these advanced threats demands continuous vigilance, investment in multi-layered security solutions, and a strong emphasis on user education. By understanding the mechanics of these novel attacks, organizations can proactively strengthen their defenses and significantly reduce their attack surface.