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Active Signal

Microsoft warns spammers are using ASCII smuggling to bypass email filters

Source: Ars Technica
Time: 9:09 AM
Weight: 94/100High importance
How is it calculated?
A 0–100 editorial signal score. Higher scores carry more weight on the timeline.

Audio Brief

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What happened?

Microsoft researchers have observed a significant surge in spammers using a technique called "ASCII smuggling" to sneak malicious content past email filters.

Why it matters

The method uses a specific block of Unicode tags—specialized digital characters—that are invisible to human readers but legible to computer processors.

Original Source

Detailed explanation

Microsoft Defender for Office reported that detections of this activity spiked from approximately 21,000 to over 2.5 million daily instances during a major campaign earlier this year.

This shift matters because it targets the advanced machine learning and natural language processing models used in modern spam and phishing classification.

By embedding these invisible characters within sensitive keywords like "funding" or "credit," attackers can break words into unrecognizable fragments for automated scanners while the recipient still sees the full, deceptive word.

This allows malicious emails to bypass the infrastructure designed to flag suspicious mass messaging campaigns.

The technique was originally identified as a way to hide instructions in prompt injections, which are attacks that manipulate AI models into performing unauthorized actions.

Microsoft notes that spammers likely adopted the method because many traditional security filters were not yet programmed to recognize these specific Unicode tags.

To counter this, the company has provided guidance for developers to update their filtering systems to account for these hidden characters during text analysis.

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