New Phishing Attack Using Invisible Characters Hidden in Subject Line Using MIME Encoding

By CyberDudeBivash · 30 Oct 2025 · cyberbivash.blogspot.com · cyberdudebivash.com

LinkedIn: ThreatWire cryptobivash.code.blog

Heads-up: Attackers are inserting invisible Unicode (e.g., zero-width/soft-hyphen) into MIME-encoded email subjects to break keyword-based filters and slip phishing mail into inboxes. Recent samples split the subject across multiple =?UTF-8?B?...?= chunks to hide terms like “password”/“invoice”. 

This guide explains how the trick works, why your secure email gateway can miss it, and the exact detections, header rules, and user-awareness updates you should ship today.

TL;DR — Subject lines get obfuscated with invisible Unicode inside MIME encoded-words (RFC 2047). Some filters see “Invo…ce”; your client renders “Invoice”. Block mixed CL/TE? Great — but also normalize/strip zero-width & soft-hyphen from headers, and alert on multi-segment Subject: headers. Contents

1. How the Attack Works
2. Detections & Header Rules
3. SIEM/XDR Hunts
4. User Awareness: What to Teach
5. Recommended Tools (Partner Links)
6. CyberDudeBivash Services & Apps
7. Sources
8. FAQ

How the Attack Works

1. MIME encoded-word split: Subject is encoded as several =?UTF-8?B?...?= chunks (limit ≈75 chars each) to hide terms when filters don’t reconstruct correctly. 
2. Invisible Unicode insertion: Zero-width space (U+200B) or soft-hyphen (U+00AD) get Base64-encoded inside those chunks. Mail clients render a clean word; some filters miss the keyword match. 
3. Bypass impact: Inbox delivery rises; users see “Invoice overdue” while back-end scanners saw “Invo​ice over​due”. Recent reports show live campaigns using this technique. 

Detections & Header Rules

• Normalize headers: Strip/replace U+200B, U+00AD, U+200C, U+200D, U+2060 in Subject:, From:, Reply-To: before policy checks. 
• Alert on multi-segment subjects: Flag >1 encoded-word in Subject: especially mixed encodings or inconsistent charsets. 
• Disallow control-like code points: Block emails where header decoded text contains bidirectional or invisible characters.
• Header length heuristics: Unusual ratio of encoded length to decoded length, or subjects with many hyphenation points (soft-hyphen patterns). 
• SPF/DKIM/DMARC: Enforce p=quarantine/reject; mismatch + header anomalies = elevate to high risk.

SIEM/XDR Hunts (generic)

• Mail gateway: search for Subject: containing =?UTF-8?B? repeated 2+ times; decode and diff decoded vs rendered.
• User clicks: spikes on messages where decoded subject contains terms like “invoice”, “password”, “DocuSign” but original header had ZWSP/soft-hyphen.
• Web telemetry: referers from mail clients to newly registered domains; blocklisted TLDs + first-seen domains.

User Awareness: What to Teach

• Be suspicious of urgent subjects that look slightly “off” (odd spacing, hyphenation, or gibberish when copied).
• Verify sender domain carefully; watch for homoglyph look-alikes (Latin vs Cyrillic “a/o”).
• Never enter credentials from an email link; open the site directly or use a known bookmark.

Recommended by CyberDudeBivash (Partner Links)

Boost filtering, detection, and training fast:

Kaspersky EDR/XDR
Correlate mail events with endpoint/browser signalsEdureka — Email Security & DFIR
Hands-on training for your SecOps teamTurboVPN
Secure admin access while tuning mail gateways

Alibaba Cloud (Global)
Spin up safe sandboxes for mail pipeline testsAliExpress (Global)
Security keys & lab gear for phishing drillsRewardful
Run your customer referral program securely

CyberDudeBivash Services & Apps

We can harden your mail stack now: header normalization, SEG tuning, DMARC enforcement, brand-spoof hunts, and staff training.

• PhishRadar AI — detects obfuscated subjects/URLs & prompt-injection
• SessionShield — protects admin sessions & mail admin consoles
• Threat Analyser GUI — dashboards & correlation for mail events

Explore Apps & ProductsBook Email Security SprintSubscribe to ThreatWire

Sources

• SANS ISC: recent phishing with invisible characters in subject (soft-hyphen + multi-segment MIME). 
• CybersecurityNews: attacker implementation via RFC 2047 encoded-words with Base64 UTF-8 examples. 
• Microsoft Security Blog: trend of inserting invisible Unicode to break keyword detection.
• RFC 2047 guidance (encoded-word limits & splitting). 
• Background: zero-width/soft-hyphen characters & prior email obfuscation techniques. 

FAQ

Q: Is this a client bug?
A: Not exactly. Clients decode/compose per spec; the gap is when filters fail to normalize/fully decode before policy checks.

Q: Will URL rewriting alone stop it?
A: No. The lure is the subject; you must sanitize headers and hunt for encoded-word abuse. Prior ZWSP tricks also bypassed some URL protections. 

Next Reads

• Daily CVEs & Threat Intel — CyberBivash
• CyberDudeBivash Apps & Services Hub

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