Cybersecurity attackers have found a new way to evade email security filters, using invisible Unicode characters to conceal phishing lures in their messages. This technique, known as ASCII smuggling, allows malicious emails to slip through defenses and reach unsuspecting victims’ inboxes.
The attack relies on encoding malicious instructions with Unicode characters from the Tags block (U+E0000–U+E007F), making them invisible to human eyes but not to AI-powered email security filters. Microsoft threat researchers discovered a large-scale phishing campaign using this technique, which peaked at an astonishing 2.37 million daily messages in late February. Although the volume has dropped significantly since then, the operation is still active.
In this campaign, attackers insert an invisible Unicode character inside finance-related lure words to split them. For example, a keyword like “funding” becomes something like “fun[invisible character]ding”, which evades email filters that rely on word lists to detect suspicious or malicious messages. Microsoft says the method has been used in millions of finance-themed phishing messages and works as intended, although its own Defender still caught over 99% of the messages based on other signals.
The domains used by the attackers are finance-themed and promote business funding, loans, and credit services. The messages were delivered through infrastructure associated with the legitimate ActiveCampaign email-marketing platform. After receiving Microsoft’s report of service abuse, ActiveCampaign said its moderation systems detect invisible Unicode characters the same way they detect unobfuscated text and treat heavy use as suspicious.
Microsoft recommends that defenders strip or normalize Unicode tag characters and other invisible code points before applying keyword, regex, or signature-based detection. This can help mitigate the risk of prompt-injection attacks. Additionally, applying normalization to email content passed to AI assistants can also help prevent these types of attacks. It’s worth noting that even with robust defenses in place, attackers who have gained valid credentials can still bypass prevention measures, as evidenced by the Blue Report 2026, which measured defense technique effectiveness across 338 million simulations.
As a user, it’s essential to be aware of this new tactic and take necessary precautions. When receiving emails with finance-related lures, be cautious and verify the sender before taking any action. Additionally, make sure your email security filters are up-to-date and configured correctly to detect invisible Unicode characters. By staying informed and vigilant, you can reduce the risk of falling victim to these types of attacks.
Source: Bleeping Computer — 2026-09-06