URL Defanger Tool

Several defanging conventions exist in the security community:

Protocol defanging:

• http → hxxp
• https → hxxps
• ftp → fxp

Dot replacement:

• . → [.] (most common)
• . → (.) (alternative)
• . → [DOT] (verbose)

At sign replacement:

• @ → [@]
• @ → [AT]

MITRE ATT&CK and STIX standards recommend the [.] and hxxp conventions. Most threat intel platforms recognize these patterns for automatic detection and refanging.

This tool supports multiple conventions based on your organization standards.

Each approach has different use cases:

Use defanging when:

• Sharing IOCs in threat reports
• Documenting malware analysis
• Posting in security forums or Slack channels
• Creating searchable threat intelligence
• Need to preserve the exact URL for analysis

Use screenshots when:

• Showing website appearance as evidence
• Demonstrating phishing page design
• Legal documentation requiring visual proof

Avoid URL shorteners for malicious URLs because:

• They create live, clickable links
• Risk of accidental clicks remains
• URL shortener services may block or scan the link
• You lose visibility into the actual destination

Best practice: Defang URLs in text, supplement with screenshots if visual context is needed. Never use URL shorteners for malicious URLs in security documentation.

Modern threat intelligence platforms have robust defanged URL handling:

Automatic detection:

• Platforms recognize common patterns (hxxp, [.], [@])
• URLs are stored in normalized format internally
• Search works with both defanged and regular formats

Popular platform behaviors:

MISP: Auto-detects and refangs for analysis, stores canonical form, displays defanged, correlation works across formats.

AlienVault OTX: Accepts defanged input, API returns normalized URLs, automatic indicator extraction.

VirusTotal: Search accepts defanged URLs, reports show both formats, API normalizes automatically.

Splunk/SIEM tools: Field extraction handles both formats, lookup tables can normalize, correlation rules match either.

Tip: When building custom tools, implement both defang detection and normalization to ensure interoperability with the broader threat intel ecosystem.

Defanging and obfuscation serve different purposes in cybersecurity:

Defanging:

• Purpose: Make URLs non-clickable while keeping them readable
• Method: Replace specific characters (http → hxxp, . → [.])
• Reversible: Easy to refang and restore
• Use case: Sharing threat intelligence safely
• Example: hxxps://malware[.]com/payload[.]exe

Obfuscation:

• Purpose: Hide actual URL destination from detection
• Method: Encoding, shorteners, redirects, homoglyphs
• Reversible: Sometimes requires decoding
• Use case: Evading security controls (attackers use this)
• Example: URL encoding like %68%74%74%70

Key differences:

• Intent: Defanging protects analysts; obfuscation deceives detection
• Transparency: Defanging is transparent; obfuscation is deceptive
• Users: Defenders defang; attackers obfuscate

Obfuscation techniques attackers use:

• URL encoding: %68%74%74%70
• Unicode/homoglyphs: аpple.com (Cyrillic a)
• Redirectors: bit.ly → evil site
• Open redirects: legitimate.com/redirect?url=evil.com

For security analysts: Defang URLs in reports, de-obfuscate attacker URLs for analysis, never click obfuscated links, use URL sandboxes.

Several automation options exist for defanging URLs:

Command-line tools:

• Python ioc-fanger library: pip install ioc-fanger
• Bash: echo URL | sed s/http/hxxp/g

SIEM/SOAR integration:

• Splunk: Use eval defanged=replace(url, ".", "[.]")
• Cortex XSOAR: Built-in defang transformer
• Phantom: URL defang action in playbooks

Browser extensions:

• Copy as Defanged (Chrome/Firefox)
• IOC Parser extensions

API integration:

• CyberChef API for batch processing
• Custom microservice with regex patterns

Email/Chat platforms:

• Slack workflows with defang bots
• Microsoft Teams Power Automate flows
• Email gateway rules to auto-defang outbound IOCs

Best practices:

1. Defang at the source (where IOCs are created)
2. Log original values separately for analysis
3. Test edge cases (IPv6, IDN, encoded URLs)
4. Document which convention your org uses
5. Train analysts to recognize defanged formats

Failing to defang URLs creates multiple security risks:

Risk 1: Accidental clicks - Analyst clicks malicious URL while reviewing, leading to browser exploitation, malware download, or credential harvesting.

Risk 2: Automated systems - URL previews fetch malicious content (link unfurling in Slack/Teams), email clients render tracking pixels, security scanners trigger malware delivery.

Risk 3: Information leakage - Referer headers reveal investigation, IP addresses exposed to attacker, timing information leaked.

Risk 4: Active content - Email HTML renders malicious sites, Markdown renderers create clickable links, documentation systems auto-link.

Risk 5: False positives - Security tools block legitimate reports, firewall blocks security team documentation.

Mitigation: Always defang in shared spaces, use URL sandboxes (VirusTotal, URLScan.io), implement browser isolation, train staff on defanged formats.

International Domain Names (IDNs) require special consideration when defanging:

What are IDNs? Domain names containing non-ASCII characters (e.g., münchen.de, 中国.cn). They use Punycode encoding for DNS compatibility.

IDN defanging challenges:

Visual similarity attacks (homoglyphs):

• аpple.com (Cyrillic a) vs apple.com (Latin a)
• Standard defanging does not reveal these attacks
• Need to show both Unicode and Punycode versions

Punycode representation:

• münchen.de → xn--mnchen-3ya.de
• Defang both versions for completeness

Best practices for IDN defanging:

1. Show both representations (Unicode and Punycode)
2. Flag potential homoglyphs when characters look similar to ASCII
3. Use Punycode for machine processing
4. Preserve original encoding during defanging

Example output:

• Display: münchen[.]de

• Punycode: xn--mnchen-3ya[.]de

• Warning: None (legitimate IDN)

• Display: аpple[.]com

• Punycode: xn--pple-43d[.]com

• Warning: Contains Cyrillic characters resembling Latin

This tool handles IDNs by preserving original encoding while applying standard defanging rules.