What is Blob-URI Phishing?

What is Blob-URI Phishing?

Blob-URI Browser Phishing is a new and stealthy phishing method that abuses the way modern browsers handle blob URIs (Uniform Resource Identifiers). Here’s a breakdown of how it works, why it’s dangerous, and how to defend against it:

What Is a blob: URI?

A blob: URI refers to a Binary Large Object that is created in a browser using JavaScript. For example:

blob:https://example.com/1d2f3g4h5i

It’s not a real web address—it’s a browser-generated reference to data stored in memory (like HTML, images, or even a full webpage). Crucially:

It does not show up as a traditional domain.

The browser treats it as content from a trusted origin (e.g., example.com).

How Blob-URI Phishing Works

Victim clicks a link in an email or message.

Instead of going to a malicious website, JavaScript on a trusted page (like a Dropbox preview) generates and launches a blob-based login screen.

The page looks like a legit login portal—Microsoft 365, Google, etc.—but it’s rendered entirely from JavaScript in the browser.

No external network request is made to a suspicious domain, so email filters and web security tools don’t flag it.

When the victim enters credentials, the JavaScript captures them and sends the data silently to the attacker.

Why It’s Effective

Bypasses URL-based filters – No phishing domain is needed.

No obvious download or redirect – Users stay “on site.”

Trusted origin deception – It appears to be from a legit domain (e.g., blob:https://onedrive.live.com).

Looks exactly like real login pages – even with branding and animations.

How to Defend Against Blob URI Phishing

User Awareness

Teach users to avoid entering credentials into login forms opened from suspicious emails—even if the URL looks clean.

Warn that “blob:” links are not safe just because they’re tied to Microsoft or Google domains.

Security Controls

Use browser isolation or sandboxing solutions.

Disable blob URIs in email environments where possible (e.g., secure email gateways).

Configure endpoint security to flag blob URLs in suspicious contexts.

Zero Trust Login Behavior

Use SSO (Single Sign-On) and device-based authentication, so even if a password is stolen, the attacker can’t use it.

Favor passkeys or certificate-based logins that cannot be phished.

Log and Monitor

Look for browser processes spawning blob:-based URIs, especially when tied to user input or credential submission.

Summary

Blob-URI Phishing is dangerous because it:

Stays within the browser, hiding from traditional defenses.

Mimics trusted environments, tricking even savvy users.

Exfiltrates data silently, without network red flags.

You need a combination of user training, endpoint protection, and identity-aware defenses to effectively combat it.

Please reach our if you’d like to learn more about how Cloud Security Solutions can help.

Cloud Security Solutions

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