In 2024, Unit 42 researchers observed a sharp increase in large-scale phishing campaigns using a novel technique involving the HTTP response header. Between May and July, they detected approximately 2,000 malicious URLs daily, which directed web browsers to refresh or reload pages automatically—without user interaction.
Unit 42 is a threat intelligence, incident response, and cyber risk expertise team backed by Palo Alto Networks technology.
Unlike traditional phishing tactics that rely on HTML content, this method manipulates the HTTP response header, allowing malicious links to execute before any visible content loads. “Since the original and landing URLs are often found under legitimate or compromised domains, it is difficult to spot malicious indicators within a URL string. Furthermore, attackers use personalized approaches that increase the likelihood that they will deceive their victim,” researchers said.
Phishing Campaign Targets Financial Sector and Government Entities
Malefactors have focused on high-profile targets in the global financial sector, government agencies, and popular internet portals. The use of compromised or legitimate domains adds to the challenge of identifying malicious links within URL strings. The campaigns are tailored to the recipient’s email domain, enhancing credibility and increasing the chances of victims being deceived.
Data shows that over 36% of attacks targeted the business sector, with 30% focused on government and educational entities. Companies using Microsoft Outlook for email were particularly at risk, as criminals often impersonate Outlook’s login pages to fool users.
How the Technique Works
Criminals behind phishing often use a range of readily available tools and mechanisms to hide their malicious intent and trick their victims. Unit 42 recently observed malefactors using header refresh techniques to embed phishing links and fashion convincing email subjects to fool customers.
“These malicious links, which have the targeted user’s email address embedded in the refresh field of the HTTP response header, direct the browser to automatically refresh or reload a webpage immediately. They do so without requiring user interaction,” the researchers explained.
“By carefully mimicking legitimate domains and redirecting victims to official sites, attackers can effectively mask their true objectives and increase the likelihood of successful credential theft. These tactics highlight the sophisticated strategies attackers use to avoid detection and exploit unsuspecting targets.”
Growing Threat Demands Enhanced Cybersecurity Measures
Palo Alto Networks customers benefit from advanced URL filtering technology, which identifies phishing URLs, extracts patterns, and blocks similar threats. However, the surge in phishing attacks using the HTTP refresh header highlights the need for increased awareness of this sophisticated technique.
Unit 42 researchers stressed that entities should be vigilant against the malicious use of HTTP refresh headers. While legitimate in some cases, this method is being exploited more and more by attackers to slip through security nets and carry out phishing attacks. If any firm suspects a compromise, they are advised to contact Unit 42’s Incident Response team for assistance.
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