Attackers Use Unicode & HTML to Bypass Email Security Tools

Researchers spot cybercriminals using new techniques to help malicious phishing emails slip past detection tools.

Cybercriminals have been spotted using HTML/CSS and Unicode tricks to bypass tools meant to block malicious emails, marking a new twist in phishing techniques, security researchers report.
Attackers are continuously testing enterprise security systems and exploring new ways to get through. Some rely on hidden text and
zero-font attacks
, in which they put invisible characters between the letters of an email so it doesnt trigger email defenses with phrases like password expired or Office 365. These malicious emails appear legitimate to any unsuspecting user.
Security firm Inky noticed a new twist on this technique in which attackers use their knowledge of HTML/CSS and Unicode to disguise phishing emails. The company began to investigate when a customer reported a suspicious message disguised as a password expired email from Office 365. Researchers loaded the raw text into text editor Emacs and found a few interesting traits.
One of these is the Unicode soft hyphen, also known as syllable hyphen. In typesetting, this is used to tell the renderer where to safely break a line and insert a visible hyphen. The soft hyphen normally renders as invisible; however, it will appear as a Unicode character to security software scanning emails for malicious content. To a security tool, it may as well be an X.
When the Inky team scanned the malicious email for phrases like change your password, they didnt receive results because the attacker had written such phrases as c-h-a-n-g-e- -y-o-u-r- -p-a-s-s-w-o-r-d-. To a user they appear as normal; to a scanner they may not raise any flags because its pattern-matching settings arent configured to look for this type of content.
The fact that they render invisible is this weird quirk of Unicode, says Dave Baggett, founder and CEO at Inky. Clearly, the attacker knows a lot about Unicode and is being quite smart in crafting this. He notes there were about 10 Unicode characters included in this email alone.
This wasnt the only new technique seen in this Office 365
phishing email
, a type of malicious message Baggett describes as rampant. When the attacker typed Office 365, for example, they used the HTML to make it look like logotype. This big red text in the upper left corner is common in Office 365 phishing, he says, and people often register the text as a logo.
Attackers also used the display:none setting, an element of CSS that tells a browser to render text as invisible. The phisher made the error of putting text they wanted the user to see within a span element, even though the CSS was written to render spans as hidden. The attacker used the invisible span trick to hide repeating text 40008 between words of the phrase Password for user[@]example[.]com, a move Baggett says was intended to hide the malicious text from security tools.
If youre a developer, its useful to temporarily hide things to test, but here, theyre using it to make every span display:none, which is very weird, he notes. You would never do this on a webpage. He hypothesizes the idea was to fool security tools into thinking the text was visible.
The 40008 text could be another tactic to bypass the pattern-matching in security tools, Baggett adds. If there is a random number generated for every email, theres less of a chance tools will associate them with the same phishing kit.
This looks like someone took an existing template theyd been using and modified it to use this new trick, he says.
The
technique used here
is similar to
steganography
, or the practice of hiding surreptitious messages in text by using invisible or hard-to-see space, Baggett explains. Steganography is another common technique among cybercriminals who want to conceal malicious text. An attacker could also use zero-width Unicode characters to transmit messages in this manner.
A challenge in defending against this technique is there are different kinds of soft hyphens, he points out. Its in the attackers interest to use several unique Unicode characters to slip past security defenses; however, the more Unicode characters a company adds to its security tool, the slower it will be. Even if you could capture all the many ways an attacker can bypass defenses, it may not necessarily scale well.
Its in the attackers best interest to use more characters, Baggett says. Its in the SEG [Secure Email Gateway]s interest to have fewer characters in their matching patterns.

Last News
▸ ArcSight prepares for future at user conference post HP acquisition. ◂ Discovered: 07/01/2025 Category: security	▸ Samsung Epic 4G: First To Use Media Hub ◂ Discovered: 07/01/2025 Category: security	▸ Many third-party software fails security tests ◂ Discovered: 07/01/2025 Category: security

**Cyber Security Categories**
Google Dorks Database	Exploits Vulnerability	Exploit Shellcodes

CVE List

Tools/Apps

News/Aarticles

Phishing Database

Deepfake Detection

Trends/Statistics & Live Infos

Tags:
Attackers Use Unicode & HTML to Bypass Email Security Tools