A Standard for Safe and Reversible Sharing of Malicious URLs and Indicators
draft-grimminck-safe-ioc-sharing-02

Network Working Group                                  S. Grimminck, Ed.
Internet-Draft                                              9 April 2025
Intended status: Informational                                          
Expires: 11 October 2025

    A Standard for Safe and Reversible Sharing of Malicious URLs and
                               Indicators
                  draft-grimminck-safe-ioc-sharing-02

Abstract

   This document defines a consistent and reversible method for sharing
   potentially malicious indicators of compromise (IOCs), such as URLs,
   IP addresses, email addresses, and domain names.  It introduces a
   safe defanging format to prevent accidental execution or activation
   when IOCs are displayed or transmitted.  These techniques aim to
   standardize the safe dissemination of threat intelligence data.  This
   specification uses the URI syntax defined in RFC 3986 and follows the
   key word conventions from RFC 2119.

Status of This Memo

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   This Internet-Draft will expire on 11 October 2025.

Copyright Notice

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   extracted from this document must include Revised BSD License text as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Defanging Techniques  . . . . . . . . . . . . . . . . . . . .   3
   5.  Refanging Techniques  . . . . . . . . . . . . . . . . . . . .   4
   6.  Example Use Cases . . . . . . . . . . . . . . . . . . . . . .   4
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  Implementation Guidance . . . . . . . . . . . . . . . . . . .   5
   9.  Edge Cases and Special Handling . . . . . . . . . . . . . . .   5
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   11. Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   The secure sharing of malicious artifacts is vital to threat
   intelligence, open-source intelligence (OSINT), and incident response
   efforts.  However, sharing raw URLs, IP addresses, and email
   addresses associated with malware or threat actors poses a risk of
   accidental activation.

   The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”,
   “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and
   “OPTIONAL” in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document defines a clear and reversible method for defanging and
   refanging IOCs to support safe sharing across various platforms,
   formats, and use cases.  The requirements language (e.g., "MUST",
   "SHOULD") follows [RFC2119], and URI syntax adheres to [RFC3986].

2.  Terminology

   Defanging: The process of altering an indicator so that it cannot be
   accidentally activated or clicked.

   Refanging: The process of restoring a defanged indicator to its
   original, actionable form.

   IOC: Indicator of Compromise - data such as a URL, IP address, domain
   name, email address, or hash associated with malicious activity.

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3.  Problem Statement

   Inconsistent defanging practices hinder the reliable and automated
   exchange of threat intelligence.  For example:

   *  A URL defanged as "h**p://example[.]com" cannot be reliably parsed
      by tools expecting "hxxp://example[.]com".

   *  An IP address defanged with parentheses (e.g., "192.0.2(.)1") may
      fail to refang in systems expecting "[.]".

   Such inconsistencies reduce the effectiveness of threat detection and
   response.

4.  Defanging Techniques

   The following transformations MUST be consistently applied:

   *  Replace "http" and "https" schemes with "hxxp" and "hxxps"
      respectively.

   *  Replace every period (".") in domain names and IP addresses with
      "[.]".

   *  Replace the "@" character in email addresses or credentials with
      "[@]".

   Using encoded characters (such as %2e for ".") SHOULD be avoided to
   prevent ambiguity.

   Examples:

   *  Original: https://evil.example.com/path
      Defanged: hxxps://evil[.]example[.]com/path

   *  Original: http://username:password@attacker.com
      Defanged: hxxp://username:password[@]attacker[.]com

   *  Original: user@phishing.example.com
      Defanged: user[@]phishing[.]example[.]com

   *  Original: http://192.0.2.1
      Defanged: hxxp://192[.]0[.]2[.]1

   *  Original: http://[2001:db8::1]:8080
      Defanged: hxxp://[2001:db8::1]:8080

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   Note: Credentials in URIs (e.g., _username:password_) are included
   here for illustrative purposes only.  Sharing credentials, even in
   defanged form, is strongly discouraged in operational contexts.

   Note: IPv6 addresses enclosed in square brackets MUST retain their
   colon-based syntax (e.g., "::") and brackets.  These characters are
   essential to URI parsing and MUST NOT be altered.  Defanging should
   apply only to components such as the scheme ("http") or domains, not
   to the IPv6 address syntax.

5.  Refanging Techniques

   Tools designed to ingest defanged data SHOULD automatically reverse
   these transformations in a deterministic manner:

   *  Convert "hxxp" and "hxxps" back to "http" and "https"
      respectively.

   *  Convert "[.]" back to ".".

   *  Convert "[@]" back to "@".

   Refanging MUST maintain the original semantics of the data to avoid
   misinterpretation.  Examples:

   *  Defanged: hxxps://evil[.]example[.]com/path
      Refanged: https://evil.example.com/path

   *  Defanged: user[@]phishing[.]example[.]com
      Refanged: user@phishing.example.com

6.  Example Use Cases

   Common scenarios include:

   *  **OSINT Sharing**: A report lists defanged URLs (e.g.,
      "hxxp://malware[.]com/payload") to prevent accidental clicks.

   *  **Email Communication**: Security teams share defanged IOCs like
      "attacker[@]example[.]com" in email threads.

   *  **Threat Intelligence Platforms**: Automated ingestion of defanged
      IPs (e.g., "192[.]0[.]2[.]1") for blocklist updates.

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7.  Security Considerations

   While these defanging techniques reduce the risk of accidental
   activation of malicious indicators, defanged data SHOULD always be
   handled with caution.  For example:

   *  Defanged URLs in PDFs may still be rendered as hyperlinks; use
      plain-text formatting.

   *  Systems processing defanged indicators MUST treat them as
      potentially harmful data, applying sandboxing or isolated
      environments for analysis.

   *  Credentials (e.g., _username:password_) SHOULD NOT be shared, even
      in defanged form, due to inherent security risks.

8.  Implementation Guidance

   Software designed to parse threat intelligence feeds should
   explicitly support these defanging and refanging standards.
   Implementations SHOULD verify correct refanging through unit tests
   and validation scripts.  Example test case:

           Test Input: "hxxp://192[.]0[.]2[.]1"
           Expected Output: "http://192.0.2.1"

9.  Edge Cases and Special Handling

   **Internationalized Domain Names (IDNs)**: Defang punycode domains
   similarly (e.g., "xn--example[.]com").

   **Non-Standard URI Schemes**: For schemes like "ftp", apply analogous
   defanging (e.g., "fxp://example[.]com").

   **IPv6 Literals in URIs**: Do not alter colon characters (":") or
   brackets ("[", "]") in IPv6 addresses.  For example, "[2001:db8::1]"
   MUST remain unchanged.  Only scheme names or domain elements
   surrounding them should be defanged.

10.  IANA Considerations

   This document has no IANA actions.

11.  Normative References

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   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, May 2017,
              <https://www.rfc-editor.org/rfc/rfc8174>.

Author's Address

   Stefan Grimminck (editor)
   Email: ietf@stefangrimminck.nl

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