Cybersecurity experts have recently identified a sophisticated phishing technique that leverages blob Uniform Resource Identifiers (URIs) to bypass Secure Email Gateways (SEGs) and evade traditional security analysis tools. This method exploits the unique properties of blob URIs, which are designed to display temporary data accessible only by the browser that generated them.
Understanding Blob URIs
Blob URIs are a feature in web browsers that allow developers to create temporary URLs pointing to data stored in the browser’s memory. These URIs are typically used for handling local files or generating content dynamically without involving external servers. Due to their ephemeral nature and local scope, blob URIs are not accessible from outside the user’s browser session, making them an attractive tool for cybercriminals seeking to conceal malicious activities.
The Phishing Attack Mechanism
The attack begins with a seemingly innocuous email containing links to legitimate, allowlisted websites rather than directly to malicious domains. This initial misdirection helps the phishing attempt bypass email security filters that typically block messages with suspicious links. Upon reaching these intermediary pages, victims are then redirected through a series of steps that ultimately generate a local blob URI containing the actual phishing content.
Cofense researchers identified this technique starting in mid-2022 and have observed its growing adoption among threat actors. According to their analysis, this method is particularly effective because the final credential phishing page exists only in the victim’s browser, leaving no external URL for security tools to scan or block. This technical limitation creates a significant blind spot in conventional phishing detection systems.
Infection Chain Analysis
The infection chain follows a sophisticated multi-stage process. After the initial email bypasses the SEG, users are directed to legitimate services such as Microsoft OneDrive. What appears to be a standard login page or document access screen is actually a carefully crafted redirection mechanism. When victims click to “Sign in” or “View document,” they are seamlessly directed to a threat actor-controlled HTML page that generates a blob URI locally in the victim’s browser.
The resulting phishing page, rendered from the blob URI (typically appearing as “blob:https://domain.com/random-string” in the address bar), presents convincing login forms mimicking services like Microsoft 365 or OneDrive. Despite existing only in the local browser memory, these pages contain hidden functionality to exfiltrate captured credentials to remote servers controlled by the attackers.
Challenges in Detection
Traditional security measures, including SEGs and endpoint protection systems, are ill-equipped to detect and block blob URI-based phishing attacks. Since the malicious content is generated and rendered entirely within the user’s browser, it does not exist on any external server, making it invisible to conventional scanning tools. Additionally, the use of legitimate intermediary sites in the attack chain further complicates detection efforts, as these sites are often trusted and allowlisted by security systems.
AI-based security filters also struggle to catch these attacks, as blob URIs are rarely used maliciously and may not be well-represented in training data. Researchers warn that unless detection methods evolve, this technique is likely to gain traction among attackers.
Mitigation Strategies
To defend against such threats, organizations are urged … .
Additionally, user education remains a critical component in mitigating phishing risks. Training programs should emphasize the importance of verifying the authenticity of login prompts, especially when accessed through email links, and encourage users to report suspicious activities promptly.
Conclusion
The exploitation of blob URIs in phishing attacks represents a significant evolution in cybercriminal tactics, effectively circumventing both technological defenses and standard user awareness training that emphasizes checking URL validity before entering credentials. As this method gains traction among threat actors, it is imperative for organizations to enhance their security measures and stay vigilant against such sophisticated attacks.
