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Microsoft Office Open XML Format (.xlsx)

.xlsx file signature | application/octet-stream

Microsoft Office Open XML Format (OOXML) DocumentNOTE:There is no subheader for MS OOXML files as there is with DOC, PPT, and XLS files. To better understand the format of these files,rename any OOXML file to have a .ZIP extension and then unZIP the file;look at the resultant file named[Content_Types].xmlto see the contenttypes. In particular, look for the<Override PartName=tag, where youwill findword,ppt, orxl, respectively.Trailer:Look for50 4B 05 06(PK..) followed by 18 additional bytesat the

Safe

Magic Bytes

Offset 0
50 4B 03 04 50 4B 05 06 50 4B 07 08

Sources: Wikipedia

All Known Signatures

2 signature variants are documented for .xlsx files across multiple sources.

Hex Signature Offset Sources
50 4B 03 04 50 4B 05 06 50 4B 07 08 0 Wikipedia
50 4B 03 04 14 00 06 00 0 Gary Kessler

Extension

.xlsx

MIME Type

application/octet-stream

Byte Offset

0

Risk Level

Safe

Validation Code

How to validate .xlsx files in Python

Python 
def is_xlsx(file_path: str) -> bool:
    """Check if file is a valid XLSX by magic bytes."""
    signature = bytes([0x50, 0x4B, 0x03, 0x04, 0x50, 0x4B, 0x05, 0x06, 0x50, 0x4B, 0x07, 0x08])
    with open(file_path, "rb") as f:
        return f.read(12) == signature

How to validate .xlsx files in Node.js

Node.js 
function isXLSX(buffer: Buffer): boolean {
  const signature = Buffer.from([0x50, 0x4B, 0x03, 0x04, 0x50, 0x4B, 0x05, 0x06, 0x50, 0x4B, 0x07, 0x08]);
  return buffer.subarray(0, 12).equals(signature);
}

How to validate .xlsx files in Go

Go 
func IsXLSX(data []byte) bool {
    signature := []byte{0x50, 0x4B, 0x03, 0x04, 0x50, 0x4B, 0x05, 0x06, 0x50, 0x4B, 0x07, 0x08}
    if len(data) < 12 {
        return false
    }
    return bytes.Equal(data[:12], signature)
}

API Endpoint

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

See the full API documentation for all endpoints and parameters.

Frequently Asked Questions

What is a .xlsx file?

A .xlsx file is a Microsoft Office Open XML Format file. Microsoft Office Open XML Format (OOXML) DocumentNOTE:There is no subheader for MS OOXML files as there is with DOC, PPT, and XLS files. To better understand the format of these files,rename any OOXML file to have a .ZIP extension and then unZIP the file;look at the resultant file named[Content_Types].xmlto see the contenttypes. In particular, look for the<Override PartName=tag, where youwill findword,ppt, orxl, respectively.Trailer:Look for50 4B 05 06(PK..) followed by 18 additional bytesat the

What are the magic bytes for .xlsx files?

The magic bytes for Microsoft Office Open XML Format files are 50 4B 03 04 50 4B 05 06 50 4B 07 08 at byte offset 0. These bytes uniquely identify the file format regardless of the file extension.

How do I validate a .xlsx file?

To validate a .xlsx file, read the first bytes of the file and compare them against the known magic bytes (50 4B 03 04 50 4B 05 06 50 4B 07 08) at offset 0. This is more reliable than checking the file extension alone, as extensions can be renamed.

What is the MIME type for .xlsx files?

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

Is it safe to open .xlsx files?

Microsoft Office Open XML Format (.xlsx) 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.