Cybercriminals in the phishing business have adopted a new phishing method to obfuscate the source code for the forged page by using a custom web font to implement a substitution cipher that looks like plain text.
The threat actor used only two fonts, ‘woff’ and ‘woff2,’ both hidden via base64 encoding. Researchers were able to determine that the phishing page has a custom web font file that enables the web browser to render the ciphertext as plaintext.
As the Web Open Font Format (WOFF) expects the font to be in a standard alphabetical order, replacing the expected letters “abcdefghi…” with the letters to be substituted, the intended text will be shown in the browser, but will not exist on the page.
To further obfuscate the phishing attempt, the threat actor used branding imagery in SVG (scalable vector graphics) format, which can be rendered through code, eliminating the need to load them from a location that stores image resources, which would help with detection.
The new evasion approach has been spotted in a phishing kit with most of its resource files dated early June 2018, but malware researchers first observed it a month earlier.
Given the evasion method used, it is possible that the malicious framework was used in the wild even earlier than this point in time. While encoded source code and various obfuscation mechanisms have been well documented in phishing kits, this technique appears to be unique for the time being in its use of web fonts to implement the encoding.
Cybercriminals continue to introduce new techniques to evade detection and hide their activities from unsuspecting victims, security vendors, and even from savvy organizations proactively searching for brand abuse. While the substitution cypher itself is simple, the implementation via web font files appears to be unique, giving phishing method of cybercriminals yet another technique to hide their tracks and defraud consumers.