M4B (.m4b)

.m4b file signature | audio/mp4

M4B is an MPEG-4 audiobook file format based on the ISO/IEC MPEG-4 Part 14 container specification, maintained through the MPEG and ISO standards process. It is commonly used for spoken-word audiobooks, podcasts, and long-form audio distributed through media players and digital bookstores, often with chapter markers and bookmarking support. The format is generally safe, though, like other MP4-based media files, it can contain metadata or embedded content that should be handled with current, trusted software.

Safe

Magic Bytes

Offset 4


              66 74 79 70 4D 34 41 20

Sources: Apache Tika

All Known Signatures

4 signature variants are documented for .m4b files across multiple sources.

Hex Signature	Offset	Sources
66 74 79 70 4D 34 41 20	4	Apache Tika
66 74 79 70 4D 34 42 20	4	Apache Tika
66 74 79 70 46 34 41 20	4	Apache Tika
66 74 79 70 46 34 42 20	4	Apache Tika

Extension

.m4b

MIME Type

audio/mp4

Byte Offset

Risk Level

Safe

Validation Code

How to validate .m4b files in Python

Python

def is_m4b(file_path: str) -> bool:
    """Check if file is a valid M4B by magic bytes at offset 4."""
    signature = bytes([0x66, 0x74, 0x79, 0x70, 0x4D, 0x34, 0x41, 0x20])
    with open(file_path, "rb") as f:
        f.seek(4)
        return f.read(8) == signature

How to validate .m4b files in Node.js

Node.js

function isM4B(buffer: Buffer): boolean {
  const signature = Buffer.from([0x66, 0x74, 0x79, 0x70, 0x4D, 0x34, 0x41, 0x20]);
  if (buffer.length < 12) return false;
  return buffer.subarray(4, 12).equals(signature);
}

How to validate .m4b files in Go

func IsM4B(data []byte) bool {
    signature := []byte{0x66, 0x74, 0x79, 0x70, 0x4D, 0x34, 0x41, 0x20}
    if len(data) < 12 {
        return false
    }
    return bytes.Equal(data[4:12], signature)
}

API Endpoint

GET /api/v1/m4b


              curl https://filesignature.org/api/v1/m4b

See the full API documentation for all endpoints and parameters.

Related Formats

.m4a .aac .ac3 .aif .aifc .aiff

Frequently Asked Questions

What is a .m4b file?

A .m4b file is identified by the magic bytes 66 74 79 70 4D 34 41 20 at byte offset 4. M4B is an MPEG-4 audiobook file format based on the ISO/IEC MPEG-4 Part 14 container specification, maintained through the MPEG and ISO standards process. It is commonly used for spoken-word audiobooks, podcasts, and long-form audio distributed through media players and digital bookstores, often with chapter markers and bookmarking support. The format is generally safe, though, like other MP4-based media files, it can contain metadata or embedded content that should be handled with current, trusted software.

What are the magic bytes for .m4b files?

The magic bytes for M4B files are 66 74 79 70 4D 34 41 20 at byte offset 4. These bytes uniquely identify the file format regardless of the file extension.

How do I validate a .m4b file?

To validate a .m4b file, read the first bytes of the file and compare them against the known magic bytes (66 74 79 70 4D 34 41 20) at offset 4. This is more reliable than checking the file extension alone, as extensions can be renamed.

What is the MIME type for .m4b files?

The primary MIME type for .m4b files is audio/mp4.

Is it safe to open .m4b files?

M4B (.m4b) files are generally safe to open. They are classified as low risk because they primarily contain data rather than executable code. However, always ensure files come from a trusted source.