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OST (.ost)

.ost file signature | application/vnd.ms-outlook-pst

Outlook Offline Storage Table (OST) is a data file format developed and maintained by Microsoft for Microsoft Outlook. It is used to store a synchronized offline copy of mailbox content, including email, calendar items, contacts, and other Outlook data, particularly in Exchange and Microsoft 365 environments. OST files are generally safe to open in Outlook, but they are tied to a specific account and can become inaccessible if the profile or server connection changes.

Safe

Magic Bytes

Offset 0
21 42 44 4E 2E 2E 2E 2E 53 4D

Sources: Apache Tika

Extension

.ost

MIME Type

application/vnd.ms-outlook-pst

Byte Offset

0

Risk Level

Safe

Validation Code

How to validate .ost files in Python

Python 
def is_ost(file_path: str) -> bool:
    """Check if file is a valid OST by magic bytes."""
    signature = bytes([0x21, 0x42, 0x44, 0x4E, 0x2E, 0x2E, 0x2E, 0x2E, 0x53, 0x4D])
    with open(file_path, "rb") as f:
        return f.read(10) == signature

How to validate .ost files in Node.js

Node.js 
function isOST(buffer: Buffer): boolean {
  const signature = Buffer.from([0x21, 0x42, 0x44, 0x4E, 0x2E, 0x2E, 0x2E, 0x2E, 0x53, 0x4D]);
  return buffer.subarray(0, 10).equals(signature);
}

How to validate .ost files in Go

Go 
func IsOST(data []byte) bool {
    signature := []byte{0x21, 0x42, 0x44, 0x4E, 0x2E, 0x2E, 0x2E, 0x2E, 0x53, 0x4D}
    if len(data) < 10 {
        return false
    }
    return bytes.Equal(data[:10], signature)
}

API Endpoint

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

See the full API documentation for all endpoints and parameters.

Related Formats

.pst

Frequently Asked Questions

What is a .ost file?

A .ost file is identified by the magic bytes 21 42 44 4E 2E 2E 2E 2E 53 4D at byte offset 0. Outlook Offline Storage Table (OST) is a data file format developed and maintained by Microsoft for Microsoft Outlook. It is used to store a synchronized offline copy of mailbox content, including email, calendar items, contacts, and other Outlook data, particularly in Exchange and Microsoft 365 environments. OST files are generally safe to open in Outlook, but they are tied to a specific account and can become inaccessible if the profile or server connection changes.

What are the magic bytes for .ost files?

The magic bytes for OST files are 21 42 44 4E 2E 2E 2E 2E 53 4D at byte offset 0. These bytes uniquely identify the file format regardless of the file extension.

How do I validate a .ost file?

To validate a .ost file, read the first bytes of the file and compare them against the known magic bytes (21 42 44 4E 2E 2E 2E 2E 53 4D) at offset 0. This is more reliable than checking the file extension alone, as extensions can be renamed.

What is the MIME type for .ost files?

The primary MIME type for .ost files is application/vnd.ms-outlook-pst.

Is it safe to open .ost files?

OST (.ost) 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.