PDFs Emerge as Top Threat Vector in Email-Based Cyberattacks

Twenty two percent of malicious attachments distributed through email are PDF documents, reveals Check Point Research (CPR). This trend capitalizes on the widespread corporate adoption of PDFs, as 87% of organizations use them as a business communication standard, according to CPR.

The effectiveness of PDFs as an attack vehicle, according to CPR, lies in their functional duality. To the end-user, they appear as static and secure documents. For automated security systems, however, they present a significant analysis challenge. Elad Paz, Research Team Leader, and Sharon ben Moshe, Malware Analyst, CPR, explain that PDFs operate analogously to a CAPTCHA test. They are designed for correct human interpretation while evading detection by automated platforms, a feature that threat actors systematically exploit.

The prominence of the PDF as a cyberattack vector is not new, but its methodology has evolved considerably. Historically, campaigns relied on exploiting known vulnerabilities (CVE) in PDF reader software. Continuous security improvements in these readers, especially in web browsers that now natively integrate viewers, and the frequency of updates have reduced the reliability of this method for mass attacks.

Similarly, attacks that depend on executing dynamic content, such as embedded JavaScript, have become less frequent since 2022, when these attacks showed the highest deployment. Security professionals consider this attack type "noisy" because its anomalous behavior increases the probability of detection. CPR indicates that the effectiveness of JavaScript-based exploits is inconsistent across different PDF readers, and many security vendors already have signatures to identify them.

This hardening of technical defenses has forced a strategic shift by threat actors. The predominant tactic has now moved from software exploitation to social engineering, reports CPR findings. Cybercriminals use the PDF format because it is perceived as a legitimate and reliable information container. By leveraging user familiarity with this file type, they achieve a higher success rate, prompting victims to perform actions that initiate the attack chain, such as clicking a malicious link.

Anatomy of a Link-Based Attack Campaign

An analysis of recent campaigns shows a clear pattern in attack execution, along with a sophisticated set of evasion techniques designed to bypass modern security countermeasures.

The most observed technique by CPR is the link-based campaign. Its execution is structurally simple but effective. The attack begins with the delivery of a PDF file that contains a link to a phishing site or a malicious file download. To increase credibility, the link is often associated with an image or text that impersonates trusted brands like Amazon, DocuSign, or Adobe Acrobat.

Detecting these campaigns is difficult because the attacker has total control over the elements of the deception. The link, text, and image can be modified easily. This flexibility negates the effectiveness of security tools that rely on reputation or static signatures. Although the attack requires human interaction, this represents an advantage for the attacker, as automated analysis environments, or sandboxes, often fail when processing tasks that depend on human decision making.

Advanced Evasion Techniques

Threat actors demonstrate a deep understanding of how detection systems work and apply specific techniques to neutralize them.

1. URL Evasion: Threat actors employ several tactics to hide the final destination of a malicious link. They use legitimate URL redirection services from providers like Bing, LinkedIn, or Google AMP that are often on security whitelists. This technique makes it difficult for URL reputation systems to identify the threat. Another method involves embedding QR codes in the document, prompting the victim to scan them with a mobile device. 

2. Static Analysis Evasion: The PDF format specification (ISO 32000) is a document of nearly 1,000 pages, and its complexity allows for content obfuscation. Attackers exploit this by encoding the annotations that define clickable areas in non-standard ways. These are interpreted correctly by common PDF readers but not by static analysis tools.

3. File Obfuscation: Methods such as encryption, filters, and indirect objects within the PDF structure are used to hide malicious content. While these techniques might make a file appear corrupt to a strict analyzer, most PDF readers are designed to prioritize robustness and user experience. 

4. Machine Learning (ML) Evasion: As security systems increasingly adopt ML, attackers develop countermeasures. A common technique is embedding malicious text within images. This forces systems to use Optical Character Recognition (OCR), a process that can be prone to errors, especially if the attacker manipulates the image quality or subtly alters characters. Additionally, they may add invisible or minuscule text to confuse Natural Language Processing (NLP) models, making correct semantic interpretation of the document more difficult.

Projections and Strategic Mitigation

Given this landscape, CPR urges organizations to adopt a multilayered security approach. Threat actors are expected to continue to refine these evasion techniques, sp defenses must go beyond signature-based detection and static analysis. It is critical to implement threat emulation solutions that can dynamically analyze file behavior in a controlled environment and endpoint protection solutions that can block the attack chain if an initial threat penetrates.

Furthermore, continuous staff training is critical. Security policies must include education to identify signs of social engineering, rigorous sender verification for unsolicited emails, and a protocol to not interact with links or QR codes in documents whose origin is not fully verifiable. Instructing users to hover over links to preview the real URL before clicking remains a basic and effective practice that must be reinforced at a corporate level.