Using Blob URLs to Bypass SEGs and Evade Analysis
Summary:
Blob URIs (Uniform Resource Identifiers) are browser-generated links used to handle temporary, local data such as images, videos, or HTML content, allowing web applications like YouTube to display content securely without exposing direct file locations. While blob URIs serve legitimate purposes such as limiting access to media files and reducing network load, they have recently been exploited by cybercriminals for credential phishing. Beginning in mid-2022, attackers started embedding blob URIs within phishing campaigns to evade detection by Secure Email Gateways. These campaigns typically involve a phishing email that links to a trusted, allowlisted page (e.g., Microsoft OneDrive), which then redirects the user to a malicious HTML file that generates a blob URI containing a fake login page. Because blob URIs reference data stored in local browser memory, they can't be directly accessed or analyzed over the internet, making them difficult for automated security tools to detect or inspect. This evasive technique not only bypasses standard email defenses but also hinders AI-based threat detection, allowing attackers to exfiltrate sensitive credentials under the guise of legitimate-looking login portals.
Security Officer Comments:
Blob URIs can be recognized in a browser's address bar by their unique prefix, typically starting with “blob:http://” or “blob:https://”, indicating that the content is being rendered from local browser memory rather than a traditional web server. This visibility provides a small window for users and security professionals to identify potential misuse, but most users are unlikely to notice or understand the significance of the prefix. As attackers continue to find success bypassing Secure Email Gateways and other conventional defenses with blob URI-based phishing, the adoption of this tactic is expected to rise. Its ability to hide malicious content from automated scanners and present convincing phishing pages makes it an increasingly attractive tool for cybercriminals looking to steal credentials while avoiding detection.
Suggested Corrections:
Users should always be cautious of individuals or organizations that ask for personal information. Most companies will not ask for sensitive data from their customers. If in doubt, users should verify with the company itself to avoid any potential issues.
Users should always take a close look at the sender’s display name when checking the legitimacy of an email. Most companies use a single domain for their URLs and emails, so a message that originates from a different domain is a red flag.
As a general rule, users should not click links or download files even if they come from seemingly “trustworthy” sources.
Check for mismatched URLs. While an embedded URL might seem perfectly valid, hovering above it might show a different web address. In fact, users should avoid clicking links in emails unless they are certain that it is a legitimate link.
Users should always be on the lookout for any grammatical errors and spelling mistakes. Legitimate companies will often employ proofreaders and editors who ensure that the materials they send out are error-free.
Users should not be frightened or intimidated by messages that have an alarmist tone. They should double check with the company if they are uncertain about the status of their accounts.
Phishing emails are designed to be sent to a large number of people, so they need to be as impersonal as possible. Users should check whether the message contains a generic subject and greeting, as this can be a sign of a phishing attempt.
Although not every end user has access to advanced anti-phishing software, they can still use the built-in protection of their email clients to filter messages. One example is setting the email client to block all images unless approved.
Legitimate companies will never send confirmation emails unless there are specific reasons for doing so. In fact, most companies will avoid sending unsolicited messages unless it’s for company updates, newsletters, or advertising purposes.
Users should always take the context of an email or message into account. For example, most online accounts do away with viewable member numbers, so users should be wary if they receive emails containing a “member number” for services that generally don’t use them.
It is important to take note of unusual information in the text of the message. Any mentions of operating systems and software that are not typically used by consumers can often be indicators of a phishing attempt.
If it seems suspicious, it probably is. Users should always err on the side of caution when it comes to sending out personally identifiable information through messages and emails.
Link(s):
https://cofense.com/blog/using-blob-urls-to-bypass-segs-and-evade-analysis
Blob URIs (Uniform Resource Identifiers) are browser-generated links used to handle temporary, local data such as images, videos, or HTML content, allowing web applications like YouTube to display content securely without exposing direct file locations. While blob URIs serve legitimate purposes such as limiting access to media files and reducing network load, they have recently been exploited by cybercriminals for credential phishing. Beginning in mid-2022, attackers started embedding blob URIs within phishing campaigns to evade detection by Secure Email Gateways. These campaigns typically involve a phishing email that links to a trusted, allowlisted page (e.g., Microsoft OneDrive), which then redirects the user to a malicious HTML file that generates a blob URI containing a fake login page. Because blob URIs reference data stored in local browser memory, they can't be directly accessed or analyzed over the internet, making them difficult for automated security tools to detect or inspect. This evasive technique not only bypasses standard email defenses but also hinders AI-based threat detection, allowing attackers to exfiltrate sensitive credentials under the guise of legitimate-looking login portals.
Security Officer Comments:
Blob URIs can be recognized in a browser's address bar by their unique prefix, typically starting with “blob:http://” or “blob:https://”, indicating that the content is being rendered from local browser memory rather than a traditional web server. This visibility provides a small window for users and security professionals to identify potential misuse, but most users are unlikely to notice or understand the significance of the prefix. As attackers continue to find success bypassing Secure Email Gateways and other conventional defenses with blob URI-based phishing, the adoption of this tactic is expected to rise. Its ability to hide malicious content from automated scanners and present convincing phishing pages makes it an increasingly attractive tool for cybercriminals looking to steal credentials while avoiding detection.
Suggested Corrections:
Users should always be cautious of individuals or organizations that ask for personal information. Most companies will not ask for sensitive data from their customers. If in doubt, users should verify with the company itself to avoid any potential issues.
Users should always take a close look at the sender’s display name when checking the legitimacy of an email. Most companies use a single domain for their URLs and emails, so a message that originates from a different domain is a red flag.
As a general rule, users should not click links or download files even if they come from seemingly “trustworthy” sources.
Check for mismatched URLs. While an embedded URL might seem perfectly valid, hovering above it might show a different web address. In fact, users should avoid clicking links in emails unless they are certain that it is a legitimate link.
Users should always be on the lookout for any grammatical errors and spelling mistakes. Legitimate companies will often employ proofreaders and editors who ensure that the materials they send out are error-free.
Users should not be frightened or intimidated by messages that have an alarmist tone. They should double check with the company if they are uncertain about the status of their accounts.
Phishing emails are designed to be sent to a large number of people, so they need to be as impersonal as possible. Users should check whether the message contains a generic subject and greeting, as this can be a sign of a phishing attempt.
Although not every end user has access to advanced anti-phishing software, they can still use the built-in protection of their email clients to filter messages. One example is setting the email client to block all images unless approved.
Legitimate companies will never send confirmation emails unless there are specific reasons for doing so. In fact, most companies will avoid sending unsolicited messages unless it’s for company updates, newsletters, or advertising purposes.
Users should always take the context of an email or message into account. For example, most online accounts do away with viewable member numbers, so users should be wary if they receive emails containing a “member number” for services that generally don’t use them.
It is important to take note of unusual information in the text of the message. Any mentions of operating systems and software that are not typically used by consumers can often be indicators of a phishing attempt.
If it seems suspicious, it probably is. Users should always err on the side of caution when it comes to sending out personally identifiable information through messages and emails.
Link(s):
https://cofense.com/blog/using-blob-urls-to-bypass-segs-and-evade-analysis