An unusual phishing technique has been observed in the wild, hiding empty SVG files inside HTML attachments pretending to be DocuSign documents.
Security researchers at email security provider Avanan named it “Blank Image.” They explain that the attack allows phishing actors to evade detection of redirect URLs.
The phishing email sent to prospective victims purports to be a document from DocuSign, a widely abused brand as many recipients are familiar with it from their office jobs.
The victim is requested to review and sign the sent document that is named “Scanned Remittance Advice.htm.”
HTML files are popular among phishing actors because they are typically ignored by email security products and thus have higher chances of reaching the target’s inbox.
If a victim clicks on the “View Completed Document” button, they are taken to a genuine DocuSign webpage. However, if they attempt to open the HTML attachment, the ‘Blank Image’ attack is activated.
SVG smuggling code
The SVG image does not contain any graphics or shapes, so it renders nothing on the screen. Its role is that of merely a placeholder for the malicious script.
It’s worth noting that the use of SVG files inside HTML containing base64-obfuscated code isn’t new. The same technique was observed in malspam delivering Qbot malware in December 2022.
In the DocuSign-themed campaign that Avanan researcher spotted, the SVG is empty. The victim sees nothing on their screen but the URL redirect code still runs in the background.
“This is an innovative way to obfuscate the true intent of the message. It bypasses VirusTotal and doesn’t even get scanned by traditional “Click-Time Protection.” By layering obfuscation upon obfuscation, most security services are helpless against these attacks.” – Avanan
Users should treat emails with HTML code in them and .HTM attachments with caution. Avanan also suggests that administrators should consider blocking them them.