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Windows/DOS executable file (.dll)

.dll file signature | application/x-msdownload

The Dynamic Link Library (DLL) format is a Microsoft Windows component file format used by the Windows operating system and maintained as part of the Windows platform. DLL files provide shared functions, resources, and code modules that can be loaded by applications and system services at run time. Because DLLs can execute code within a process, they are commonly associated with software loading, compatibility issues, and malware abuse; unsigned or unexpected files should be treated cautiously.

High

Magic Bytes

Offset 0
4D 5A

Sources: Apache Tika, Wikipedia, Gary Kessler

Extension

.dll

MIME Type

application/x-msdownload

Byte Offset

0

Risk Level

High

Validation Code

How to validate .dll files in Python

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

How to validate .dll files in Node.js

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

How to validate .dll files in Go

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

API Endpoint

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

See the full API documentation for all endpoints and parameters.

Related Formats

.com .exe .sys .a .elf .wasm

Frequently Asked Questions

What is a .dll file?

A .dll file is a Windows/DOS executable file file. The Dynamic Link Library (DLL) format is a Microsoft Windows component file format used by the Windows operating system and maintained as part of the Windows platform. DLL files provide shared functions, resources, and code modules that can be loaded by applications and system services at run time. Because DLLs can execute code within a process, they are commonly associated with software loading, compatibility issues, and malware abuse; unsigned or unexpected files should be treated cautiously.

What are the magic bytes for .dll files?

The magic bytes for Windows/DOS executable 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 .dll file?

To validate a .dll 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 .dll files?

The primary MIME type for .dll files is application/x-msdownload.

Is it safe to open .dll files?

Windows/DOS executable file (.dll) files are high risk because they can contain executable code. Never open .dll files from untrusted sources. Always scan with antivirus software, verify the source, and consider running in a sandboxed environment.