Spammers adopt ASCII smuggling to bypass email spam filters

ASCII smuggling, a tactic that encodes text using a special range of Unicode tags invisible to human readers, was initially highlighted as a way to make prompt-injection attacks on large language models harder to spot. The tags replicate the appearance of standard ASCII characters while remaining detectable only at the text-processing level. Spammers have repurposed this approach to conceal malicious instructions in email messages, effectively bypassing filters that rely on keyword detection.

Microsoft Defender for Office observed a dramatic increase in signatures matching ASCII smuggling, with daily detections climbing from roughly 21,000 to more than 1.3 million on a single day in early February, and reaching 2.5 million within four days. The influx persisted for months before dropping sharply around mid-May. According to Microsoft, the invisible nature of the tags makes them useful both for feeding hidden prompts to AI models and for obscuring spam content from automated defenses.