ASCII Smuggling Tricks AI Code Reviewers by Hiding Payloads in Invisible Unicode

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Microsoft recently flagged a technique called ASCII smuggling, originally used to attack AI systems, now being repurposed by spammers to bypass email filters using invisible Unicode tag characters. These characters render as normal text to humans and thin filters, while concealing a hidden payload underneath. The same vulnerability applies to AI-powered code review tools, which analyze a diff as plain text and silently treat invisible Unicode characters as their ASCII equivalents, never inspecting the raw bytes. This means two code strings that differ only in hidden codepoints receive identical scores, allowing a malicious payload to pass review undetected. The recommended fix is to normalize all diffs before feeding them to any AI reviewer by stripping or flagging characters outside a strict allowlist of printable ASCII, collapsing the entire attack class at the intake stage.

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