Medium Severity

Evasion: Zero-width character stuffing in subject or sender name

Description

Flags inbound messages containing links or attachments where the subject line or sender display name contains 10 or more invisible unicode characters (such as zero-width spaces, soft hyphens, zero-width joiners, or byte order marks), a technique used to evade text-based detection engines. Messages from highly trusted sender domains that pass DMARC authentication are excluded.

References

No references.

Sublime Security
Created Oct 9th, 2026 • Last updated Oct 9th, 2026
Source
type.inbound
and (length(body.links) > 0 or length(attachments) > 0)
and any([subject.subject, sender.display_name],
        regex.icount(., '[\x{200B}-\x{200F}\x{00AD}\x{FEFF}\x{2060}]') >= 10
)

// negate highly trusted sender domains unless they fail DMARC authentication
and not (
  sender.email.domain.root_domain in $high_trust_sender_root_domains
  and coalesce(headers.auth_summary.dmarc.pass, false)
)
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