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Font file (.fon)

.fon file signature | application/octet-stream

The FON file format is a Windows font resource format developed and maintained by Microsoft for early versions of Windows. It is used to store bitmap fonts and font resources for system interfaces, legacy applications, and display rendering in older Windows environments. FON files are a legacy format and are structurally based on executable containers, so they should be handled cautiously when obtained from untrusted sources.

Safe

Magic Bytes

Offset 0
4D 5A

Sources: Wikipedia, Gary Kessler

Extension

.fon

MIME Type

application/octet-stream

Byte Offset

0

Risk Level

Safe

Validation Code

How to validate .fon files in Python

Python 
def is_fon(file_path: str) -> bool:
    """Check if file is a valid FON by magic bytes."""
    signature = bytes([0x4D, 0x5A])
    with open(file_path, "rb") as f:
        return f.read(2) == signature

How to validate .fon files in Node.js

Node.js 
function isFON(buffer: Buffer): boolean {
  const signature = Buffer.from([0x4D, 0x5A]);
  return buffer.subarray(0, 2).equals(signature);
}

How to validate .fon files in Go

Go 
func IsFON(data []byte) bool {
    signature := []byte{0x4D, 0x5A}
    if len(data) < 2 {
        return false
    }
    return bytes.Equal(data[:2], signature)
}

API Endpoint

GET /api/v1/fon
curl https://filesignature.org/api/v1/fon

See the full API documentation for all endpoints and parameters.

Related Formats

.otf .ttf .woff .woff2 .afm .dfont

Frequently Asked Questions

What is a .fon file?

A .fon file is a Font file file. The FON file format is a Windows font resource format developed and maintained by Microsoft for early versions of Windows. It is used to store bitmap fonts and font resources for system interfaces, legacy applications, and display rendering in older Windows environments. FON files are a legacy format and are structurally based on executable containers, so they should be handled cautiously when obtained from untrusted sources.

What are the magic bytes for .fon files?

The magic bytes for Font file files are 4D 5A at byte offset 0. These bytes uniquely identify the file format regardless of the file extension.

How do I validate a .fon file?

To validate a .fon file, read the first bytes of the file and compare them against the known magic bytes (4D 5A) at offset 0. This is more reliable than checking the file extension alone, as extensions can be renamed.

What is the MIME type for .fon files?

There is no officially registered MIME type for .fon files. Systems typically use application/octet-stream as a generic fallback when handling this format.

Is it safe to open .fon files?

Font file (.fon) 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.