RTL / LTR Scripts & Browser Gaps — How Attackers Hide Malicious URLs By CyberDudeBivash (Bivash Kumar Nayak)

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TL;DR

Attackers abuse Unicode bidirectional controls (e.g., RIGHT-TO-LEFT OVERRIDE U+202E), mixed-script homoglyphs, and browser rendering quirks to make malicious URLs look benign in addresses, filenames, emails and UIs. This allows silent phishing, file-name spoofing, and evasion of basic URL filtering. Defenders must normalize and inspect for invisible bidi characters, enforce IDN/punycode display rules, and add logging & detection for mixed-script URLs.

How the trick works — short & precise

Bidi override characters (U+202E, U+202A, etc.) change the visual order of text. Example: evilexe\u202Egnp.exe may render as exe.png to a user while the real filename is evilexegnp.exe.
Mixed-script homoglyphs replace characters (e.g., Latin a with Cyrillic а) so apple.com looks identical but the Unicode code points differ.
Punycode / IDN tricks let attackers register domain names that visually match popular domains but are different under the hood (e.g., xn--pple-43d.com).
Browser & app display differences: some browsers/panels render bidi markers or decode IDNs differently (address bar vs. tab title vs. link text), creating user confusion.
Result: users click “what looks like” safe links; attackers get clicks into credential-harvesting pages, drive-by exploits, or spoofed download filenames.

Real-world attack patterns

Phishing email with anchor text https://bank.example.com but the actual href uses mixed scripts or RTL overrides to point to hxxp://evil.example.
Malicious attachment named invoice\u202Egnp.pdf.exe that appears as invoice.pdf in some file managers.
Fake login pages hosted on IDN domains that display as g00gle.com visually.
Adtech / redirected URLs that use URL shorteners containing bidi or homoglyphs so analysts misread the landing domain in logs.

Detection — SOC & devnotes (practical, deployable)

1) Quick detection regexes & checks

Detect bidi control characters in URLs/filenames (U+202A..U+202E, U+200E, U+200F):
- Regex (PCRE): [\x{202A}-\x{202E}\x{200E}\x{200F}]
Detect high ratio of mixed scripts (Latin + Cyrillic + Greek) in a single domain/label:
- Heuristic: if more than 1 script class present in same label → flag.
Detect Punycode (IDN) domains:
- Regex: (^|\.)xn--[0-9a-z\-]+

2) Sigma-style hunt (pseudo)


title: Suspicious URL with Unicode Bidi Controls
id: cdb-url-bidi-2025
description: Detects URLs or filenames containing Unicode bidirectional override characters
logsource:
  product: webproxy
detection:
  selection:
    Url|contains_regex: '[\x{202A}-\x{202E}\x{200E}\x{200F}]'
  condition: selection
level: high

3) Endpoint/EDR checks

Alert on downloads whose filename contains bidi chars or that contain more than one script class.
Monitor browser navigation events where destination host contains xn-- (Punycode) or suspicious mixed-script labels.

4) SIEM enrichment

Normalize logged URLs to code point sequences and store both “visual” (rendered) and “raw” forms. Flag differences between link text and href. Correlate with user-click events.

Mitigation & hardening (short → mid → long)

Immediate (hours → days)

Canonicalize & normalize incoming URLs in mail gateways and web proxies: remove or encode bidi control characters, and compare normalized hostnames to blocklists.
Force display of raw IDN/punycode in admin/privileged UIs (show xn--), or show an unmistakable icon/tooltip when IDN is used.
Disable auto-execution of downloaded files and show full file name including hidden characters in download dialogs.
Email gateway rules: if anchor text ≠ href (domain mismatch) — treat as suspicious and quarantine.
User education: show examples of RLO tricks and instruct to always hover and inspect full URL.

Mid-term (weeks)

Policy: block or warn on IDNs in critical systems and require allowlisting of domains for admin users.
Browser hardening: apply enterprise policies that force punycode display for IDNs and disable permissive rendering of bidi markers (many browsers have enterprise flags).
Dev/CI controls: sanitize filenames from uploads and downloads (strip bidi + invisible controls).

Long-term (months)

Platform fixes: work with vendors (browser, mail client, file explorer vendors) to ensure consistent display of Unicode controls and to show raw machine-readable names on hover.
Domain & trademark monitoring: proactively monitor IDN registrations for target brand look-alikes.

Defensive coding checklist (for dev teams)

When validating URLs: check href != visible text; if mismatch, require user confirmation.
Strip control characters from filenames and URL path segments before saving or executing.
Convert IDN domains to punycode and validate against allowlists for sensitive flows.
Log both rendered and raw forms of user-supplied URLs for incident triage.

For phishing analysts — quick triage workflow

Hover link → copy href and paste into a text editor that shows invisible chars (e.g., hex view).
If URL contains xn-- or bidi chars, fetch WHOIS/punycode and use a controlled sandbox to screenshot landing page.
Check certificate subject for mismatch (IDN abuse often lacks valid cert for brand).
Check web proxy logs for repeated short-lived IDN or mixed-script domains.

IoCs & triage rules (examples)

Filenames containing \u202E or other bidi code points.
Domains with xn-- labels that resolve to uncommon hosts.
URL anchor text that visually equals a popular domain but href points elsewhere.

Incident response (if users clicked / infection suspected)

Contain: isolate affected host and capture browser process memory & network connections.
Collect: browser history, download folder (show raw filenames), clipboard contents, and email source.
Hunt: search fleet for other users who received identical emails or who visited the same IDN domain.
Remediate: rotate credentials, revoke sessions, remove any dropped payloads. Reimage if arbitrary code execution found.

User awareness messaging (short, pasteable)

“If a link looks like a trusted site but came from email or ad, hover it — check the actual href. If the address contains odd characters, or you see xn-- in the domain, don’t click and report it to security.”

Why browsers & apps differ (why this remains an issue)

Unicode is complex and the Unicode Bidi algorithm was designed for correct rendering of mixed-direction text (Hebrew/Arabic + Latin). Browsers and apps historically prioritized user-friendly rendering over security; subtle differences in how address bars, tab titles, and link text are rendered cause spoofing opportunities. Vendors have made improvements, but new variants (homoglyphs + mixed-script) keep appearing.

Quick policy templates (for CISO/Security Ops)

Blocklist policy: block all inbound emails with hrefs where anchor_text != href domain or where href contains bidi controls or xn-- unless pre-approved.
Privileged user rule: admin consoles must be accessible only from devices with IDN display enforcement and no third-party ads.

#CyberDudeBivash #Bidi #RTL #Phishing #URLSpoofing #IDN #Punycode #DotNet #ThreatIntel #Cybersecurity

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