@bernierllc/validators-mime-structure

Primitive validator for MIME message structure validation - multipart boundaries, content-type correctness, transfer encoding, and MIME part structure.

Installation

npm install @bernierllc/validators-mime-structure

Usage

Basic Validation

import { validateMimeStructure, isValidMimeStructure } from '@bernierllc/validators-mime-structure';

const mimeMessage = `Content-Type: multipart/mixed; boundary="boundary123"

--boundary123
Content-Type: text/plain

Hello World
--boundary123--`;

// Get detailed validation problems
const problems = await validateMimeStructure(mimeMessage);

if (problems.length === 0) {
  console.log('Valid MIME structure');
} else {
  problems.forEach(problem => {
    console.log(`${problem.severity}: ${problem.message}`);
  });
}

// Quick validation check
const isValid = await isValidMimeStructure(mimeMessage);
console.log(isValid ? 'Valid' : 'Invalid');

Configuration Options

import { validateMimeStructure } from '@bernierllc/validators-mime-structure';

const problems = await validateMimeStructure(content, {
  // Enable/disable specific checks
  checkBoundaries: true,            // Validate multipart boundaries
  checkContentType: true,           // Validate Content-Type headers
  checkTransferEncoding: true,      // Validate Content-Transfer-Encoding
  checkPartStructure: true,         // Validate MIME part structure
  checkBoundaryMismatch: true,      // Detect boundary mismatches

  // Configuration
  maxNestingDepth: 10              // Maximum multipart nesting depth
});

Helper Functions

import {
  extractBoundary,
  isMultipart,
  parseMimeParts
} from '@bernierllc/validators-mime-structure';

// Extract boundary from Content-Type header
const boundary = extractBoundary('multipart/mixed; boundary="test"');
console.log(boundary); // "test"

// Check if message is multipart
const isMultipartMsg = isMultipart(mimeMessage);
console.log(isMultipartMsg); // true

// Parse MIME message into parts
const parts = parseMimeParts(mimeMessage);
parts.forEach(part => {
  console.log('Headers:', part.headers);
  console.log('Body:', part.body);
});

API Reference

validateMimeStructure(content, options?, utils?)

Validates MIME message structure and returns an array of validation problems.

Parameters:

• content: string - MIME message content to validate
• options?: Partial<MimeStructureOptions> - Validation options
• utils?: SharedUtils - Shared utilities from validators-core

Returns: Promise<Problem[]>

Example:

const problems = await validateMimeStructure(mimeMessage, {
  checkBoundaries: true,
  maxNestingDepth: 5
});

isValidMimeStructure(content, options?)

Checks if MIME message structure is valid (no errors).

Parameters:

• content: string - MIME message content to validate
• options?: Partial<MimeStructureOptions> - Validation options

Returns: Promise<boolean>

Example:

if (await isValidMimeStructure(mimeMessage)) {
  console.log('Valid MIME structure');
}

extractBoundary(contentType)

Extracts boundary string from Content-Type header value.

Parameters:

• contentType: string - Content-Type header value

Returns: string | null

Example:

const boundary = extractBoundary('multipart/mixed; boundary="abc123"');
// Returns: "abc123"

isMultipart(content)

Checks if content is a multipart MIME message.

Parameters:

• content: string - Content to check

Returns: boolean

Example:

if (isMultipart(content)) {
  console.log('This is a multipart message');
}

parseMimeParts(content)

Parses MIME message into individual parts with headers and body.

Parameters:

• content: string - MIME message content

Returns: Array<{ headers: string; body: string }>

Example:

const parts = parseMimeParts(mimeMessage);
console.log(`Found ${parts.length} parts`);

Validation Rules

Boundary Validation (boundary-validator)

Validates multipart boundaries according to RFC 2046:

• Boundary parameter presence in Content-Type
• Boundary format and characters
• Boundary length (max 70 characters)
• Boundary markers in message body
• Closing boundary marker presence

Content-Type Validation (content-type-validator)

Validates Content-Type headers according to RFC 2045:

• MIME type format (type/subtype)
• Token validity for type and subtype
• Parameter format and syntax
• Required boundary for multipart types
• Recommended charset for text types

Transfer Encoding Validation (transfer-encoding-validator)

Validates Content-Transfer-Encoding according to RFC 2045:

• Valid encoding values (7bit, 8bit, binary, quoted-printable, base64)
• Base64 format and padding
• Quoted-printable escape sequences
• Line length requirements
• Character restrictions for 7bit encoding

Part Structure Validation (part-structure-validator)

Validates MIME message part structure:

• Header-body separation (blank line)
• Header format and continuation
• Nested multipart messages
• Maximum nesting depth
• Empty body warnings

Boundary Mismatch Detection (boundary-mismatch-validator)

Detects boundary mismatches and inconsistencies:

• Declared boundaries used in body
• Undeclared boundary markers
• Multiple closing markers
• Incorrect boundary marker format
• Duplicate boundary declarations

Examples

Validating Email with Attachments

const email = `Content-Type: multipart/mixed; boundary="outer"

--outer
Content-Type: multipart/alternative; boundary="inner"

--inner
Content-Type: text/plain; charset=utf-8

Plain text version
--inner
Content-Type: text/html; charset=utf-8

<html><body>HTML version</body></html>
--inner--
--outer
Content-Type: application/pdf; name="document.pdf"
Content-Transfer-Encoding: base64

JVBERi0xLjQK
--outer--`;

const problems = await validateMimeStructure(email);
console.log(`Found ${problems.length} validation issues`);

Selective Validation

// Only validate boundaries and content-type
const problems = await validateMimeStructure(content, {
  checkBoundaries: true,
  checkContentType: true,
  checkTransferEncoding: false,
  checkPartStructure: false,
  checkBoundaryMismatch: false
});

Custom Nesting Depth

// Allow deeper nesting for complex email structures
const problems = await validateMimeStructure(content, {
  maxNestingDepth: 15
});

Integration Status

• Logger: not-applicable - Primitive validator with no internal logging
• Docs-Suite: ready - Complete API documentation and examples
• NeverHub: not-applicable - Stateless validation utility

See Also

• @bernierllc/validators-core - Core validation framework
• @bernierllc/validators-runner - Validation orchestration
• @bernierllc/validators-html-syntax - HTML validation
• @bernierllc/validators-json-structure - JSON validation

License

Copyright (c) 2025 Bernier LLC. All rights reserved.