Phishing Attack Exploits Blob URLs to Evade Email Security and Detection Systems

Security researchers at Cofense Intelligence have observed a shift in phishing methodologies, with cybercriminals now harnessing blob Uniform Resource Identifiers (URIs) as a means to bypass traditional email security filters and forensic analysis tools.

This emerging tactic exploits the intrinsic characteristics of blob URIs-a mechanism generated by web browsers to temporarily store and access data locally-enabling attackers to deliver credential phishing pages directly to victims while evading detection.

Blob URIs: Legitimate Uses

Blob URIs, utilized extensively for legitimate purposes such as streaming multimedia content on platforms like YouTube, allow browsers to manipulate binary large objects (blobs), including images, audio files, and video streams.

These URIs begin with a “blob:http://” or “blob:https://” prefix and remain accessible exclusively to the browser session that generated them, effectively isolating the data from external network access.

In legitimate scenarios, this approach supports access control and optimizes network traffic by preventing direct user access to sensitive media files and caching temporary data locally.

However, adversaries have recognized that the same properties make blob URIs an effective vehicle for concealing and delivering malicious HTML content, as they frustrate conventional URL-based threat detection and analysis.

Advanced Phishing Campaign Flow

The observed phishing attack chain begins with a carefully crafted email that successfully slips past Secure Email Gateway (SEG) defenses, often leveraging allowlisted, trusted domains such as onedrive[.]live[.]com to host intermediary files.

Upon engaging with the phishing email, recipients are directed to these genuine platforms, where no immediate spoofing is evident, thereby eluding both automated defenses and user suspicion.

The attack then pivots when the user clicks a link purportedly related to encrypted messages or account actions; this interaction seamlessly redirects the victim to a threat actor-controlled page hosting a script that generates a blob URI.

The browser then decodes and renders a phishing credential capture page locally via this blob URI, effectively severing any direct network visibility or analysis by SEG solutions or security analysts.

Detection and mitigation are complicated by the ephemeral and localized nature of blob URIs.

Because the credential phishing page is never hosted on a conventional, reachable web server-instead existing transiently in the memory space of the victim’s browser-security teams cannot retrieve or directly analyze the final payload using standard automated tools.

Furthermore, current AI and threat intelligence models have limited training on blob URI patterns due to their relatively recent adoption for nefarious purposes, giving attackers an edge in evading both signature-based and behavioral defenses.

Credential submission on these phishing pages initiates the exfiltration of sensitive information-such as email addresses and passwords-back to adversary-controlled servers, often through covert network requests embedded within the locally rendered phishing page.

According to Cofense Report, this technique not only undermines the efficacy of traditional email protection and URL reputation filtering but also makes incident response and forensic investigation significantly more challenging.

The adoption of blob URIs by threat actors signifies an ongoing evolution in phishing tactics, as attackers seek new vectors to outpace rapidly advancing defensive technologies.

If successful campaigns continue to proliferate, the frequency of blob URI-based attacks is expected to escalate.

Security practitioners are being urged to update detection strategies, educate end-users to identify obfuscated blob URI links, and build analytical frameworks capable of inspecting browser memory-resident artifacts to counter this sharply sophisticated phishing threat.

Find this Story Interesting! Follow us on LinkedIn and X to Get More Instant updates