Hackers overcome email security filters by taking use of HTML functions

Email security remains one of the most critical defenses against cyber threats, protecting billions of messages exchanged daily. Yet, despite advancements in filtering technologies, an alarming percentage of phishing emails—over 15%—still successfully evade these defenses. This is not just a coincidence but a calculated effort by cybercriminals leveraging the intricate capabilities of HTML functions.

HTML, the backbone of web communication, offers numerous functionalities for styling, embedding links, and structuring content. Hackers manipulate these capabilities, turning what was designed to enhance usability into tools for bypassing detection. In 2023 alone, 38% of all phishing attempts incorporated advanced HTML techniques, signaling a growing trend among cybercriminals to exploit this vulnerability.

One key method involves obfuscating malicious URLs. By embedding links within innocuous-looking buttons or images, hackers craft emails that appear harmless at first glance. In recent studies, over 60% of phishing emails employed such tactics, capitalizing on the assumption that visually appealing elements are trustworthy.

Another technique is invisible text. Attackers ensure their payload evades human scrutiny and automated scanning tools by utilizing HTML to hide malicious code in seemingly blank spaces. Reports suggest nearly 42% of email-based attacks incorporate hidden text to circumvent detection.

Complex encoding strategies also play a role. Hackers leverage base64 encoding to disguise harmful content, embedding it directly into HTML elements. Such techniques have been identified in 28% of phishing campaigns analyzed during 12 months, further highlighting their effectiveness in eluding traditional email security protocols.

Dynamic content is another avenue of attack. By embedding JavaScript within HTML, attackers can alter email content after delivery, bypassing initial scans. Over 19% of spear-phishing attempts in the last quarter of 2024 were found to use such techniques, enabling a level of adaptability previously unseen.

These tactics are not just theoretical—they translate into measurable impacts. The financial toll of phishing emails reached an estimated $17 billion in 2023, with a significant portion attributed to campaigns that bypassed HTML-based filters. This figure represents a 21% increase from the previous year, underlining the urgency of addressing this issue.

Moreover, phishing campaigns are becoming more targeted. In 2024, nearly 25% of attacks were aimed at high-level executives, leveraging HTML-based tricks to exploit their busy schedules and high-stakes decision-making roles. This trend underscores the growing sophistication of email-based threats.

The effectiveness of these methods stems from their ability to exploit the very systems designed to protect users. Advanced email filters rely on pattern recognition, machine learning, and heuristic analysis, yet they struggle to detect the dynamic and obfuscated techniques employed through HTML functions.

Furthermore, the sheer scale of email communication amplifies the challenge. With over 347 billion emails sent daily, even a tiny success rate for phishing campaigns can result in many compromised systems. Hackers understand this dynamic and adapt their strategies accordingly.

Regulations and awareness campaigns, while essential, have needed to catch up with the rapid evolution of these tactics. In a survey this year, 64% of IT professionals admitted that their organizations lacked the tools or knowledge to counter HTML-based email threats effectively.

The rise of HTML-centric attacks also highlights the importance of proactive measures. Strengthening email security requires a multi-layered approach that goes beyond traditional filtering. Organizations must combine robust technical defenses with Employee training and regular threat assessments to stay ahead of evolving risks.

As cybercriminals continue to innovate, understanding their methods is the first step toward mitigation. The more we delve into their techniques, the better equipped we become to protect our digital communication channels.

HTML functions as a Cybercriminal’s Playground

Initially designed as a language to structure and present web content, HTML has become an unexpected tool for cybercriminals. Its versatility and dynamic nature allow hackers to manipulate emails to evade traditional security measures. With billions of emails sent daily, even a tiny percentage of successful phishing attempts can have devastating consequences, making HTML a favorite playground for attackers.

Hackers leverage HTML’s flexibility to mask their true intentions. By embedding malicious links, disguising harmful payloads, and manipulating visual elements, they create emails that look legitimate but are laden with threats. These methods exploit the complexity of email rendering and the inherent trust users place in visually appealing and well-structured messages. Studies show that over 38% of phishing emails rely heavily on HTML-based deception tactics.

The use of HTML has also enabled the proliferation of advanced obfuscation techniques. Attackers can encode content that bypasses automated filters and fool recipients into interacting with the email. This dual advantage—fooling machines and humans—has elevated HTML to the forefront of email-based cyberattacks, with a 25% increase in HTML-centric phishing campaigns reported in 2023.

Disguising Malicious Links through HTML Tags

Hackers use HTML tags like <a> to embed malicious URLs behind seemingly harmless text or buttons. For instance, users might see a “Click Here” button redirecting them to a phishing site. According to a report, 62% of phishing emails 2023 utilized hidden links to trick users into providing sensitive information. Attackers also use link-shortening services and custom domain redirection to obscure the proper destination further, making detection even harder.

Once clicked, these links can lead to websites that steal credentials or download malware onto the victim’s device. Research indicates that nearly 34% of users who interact with these links unknowingly compromise their security, showcasing the devastating effectiveness of this tactic.

Embedding Invisible Content in Email

Invisible text and images are another way hackers use HTML to bypass detection. By setting text color to match the background or using minuscule font sizes, attackers hide malicious code within the email. Automated filters often ignore these elements but remain functional when the email is rendered in the recipient’s inbox. Approximately 42% of phishing emails 2024 included hidden elements designed to bypass email security filters.

These invisible components often execute malicious scripts or trigger the download of harmful payloads. In some cases, invisible text serves as a decoy to fool advanced filters, leading to an 18% higher success rate for phishing campaigns using this technique.

Dynamic Content Manipulation Using HTML and JavaScript

Some attackers embed scripts within HTML emails to dynamically modify content after delivery. This tactic allows bypassing initial scans, as the malicious payload is only activated once the email reaches its target. In Q3 2024, over 19% of spear-phishing campaigns used such dynamic content, allowing attackers to adapt their message or payload based on the recipient’s environment.

Dynamic manipulation also enables attackers to insert targeted content, such as personalized messages or industry-specific bait, increasing the likelihood of user interaction. The adaptability of this method resulted in a 27% rise in phishing success rates compared to static approaches, making it a potent weapon in the hands of cybercriminals.

Beyond the Basics: Advanced HTML Strategies in Cyber Attacks

Advanced HTML Strategies in Cyber Attacks refer to sophisticated techniques employed by cybercriminals to exploit the features of HTML, the backbone of web communication, to bypass email security systems and deceive users. Unlike basic methods, these strategies involve complex manipulations of HTML elements, attributes, and functionalities to hide malicious payloads, evade detection, and increase the success rate of attacks. Hackers use these advanced techniques to disguise harmful content, create dynamic and interactive elements, and manipulate email rendering in ways that traditional filters struggle to identify.

These strategies go beyond simple phishing attempts by leveraging encoding, hidden elements, dynamic content, and embedded forms. For instance, attackers may encode malicious links to appear harmless or use invisible text and iFrames to deliver threats without triggering security alerts. The growing sophistication of these methods is evident in the rising percentage of phishing campaigns using advanced HTML tactics, making them a significant challenge for cybersecurity defenses. Understanding these strategies is critical for developing stronger protections against evolving cyber threats.

Cloaking Malicious Code with Base64 Encoding

Attackers use Base64 encoding to embed harmful scripts or links directly into HTML elements. This encoding converts malicious payloads into harmless-looking strings that evade email filters. In 2024, 28% of phishing emails were identified as using Base64 encoding, demonstrating its effectiveness in avoiding detection and delivering attacks covertly.

Exploiting HTML Attributes for Payload Delivery

Hackers manipulate HTML attributes like SRC or data to deliver malicious content. Attracting harmful scripts in these attributes means attackers avoid scrutiny from essential filtering tools. Research shows that 33% of HTML-based phishing emails include manipulated attributes to bypass defenses.

Implementing CSS for Phishing Deception

CSS (Cascading Style Sheets) is often used alongside HTML to create visually convincing phishing emails. Attackers use CSS to disguise forms or overlay fake login prompts. A recent study found that 17% of phishing emails employed CSS-based visual tricks to deceive recipients into sharing sensitive credentials.

Leveraging HTML Forms for Credential Harvesting

HTML forms allow attackers to create fake login portals directly within emails. These forms collect credentials and transmit them to the attacker’s server. Reports indicate that phishing emails containing embedded forms increased by 22% in 2024, highlighting their growing popularity among cybercriminals.

Creating Dynamic Email Content with iFrames

iFrames embedded in emails are used to load external malicious content while keeping the email itself clean. This technique bypasses static content analysis and dynamically serves threats. Over 15% of phishing emails 2023 leveraged iFrames to maintain stealth and flexibility in delivering payloads.

For more:

https://www.inky.com/news/attackers-use-unicode-html-to-bypass-email-security-tools

https://www.trellix.com/blogs/research/the-anatomy-of-html-attachment-phishing