.CAT file extension
To open .CAT files on Windows, if you installed the Windows SDK, open Command Prompt and use SignTool to verify the catalog’s signature (for example, the verification command used in Windows driver signing workflows).
To open a .CAT file, treat it as a Windows security catalog: on Windows you can inspect or verify it using built-in tools like SignTool (Windows SDK) rather than a regular viewer. On macOS, Linux, iOS, and Android there is usually no standard app support, so the practical approach is to transfer the file to a Windows PC for verification.
Last updated: April 29, 2026 · Reviewed by Julian Stricker
Open on your device
Choose your operating system for a dedicated step-by-step opening guide.
How to open .CAT files
Use these platform-specific instructions to open .CAT files safely.
Windows
- If you installed the Windows SDK, open Command Prompt and use SignTool to verify the catalog’s signature (for example, the verification command used in Windows driver signing workflows).
- If the .CAT came with a driver package, keep it with the rest of the package files and let Windows/driver installation validate it; don’t rename it or try to “open” it in an editor.
Mac
- There is no common native macOS app for Windows security catalog (.CAT) files; transfer the file to a Windows machine to inspect or verify it using Windows tooling.
- If you only need a quick check, view the file as raw bytes/text in a hex viewer or text editor, but expect it to be non-human-readable and not reliably interpretable.
Linux
- Linux desktops may recognize the MIME mapping for .cat (often associated with application/vnd.ms-pki.seccat), but there is usually no standard viewer; transfer the file to Windows for meaningful verification.
- If needed for troubleshooting only, open it in a hex viewer to confirm it is not plain text, and keep it unchanged with its related driver/package files.
iOS
- iOS typically cannot preview Windows catalog (.CAT) files; save it in Files and transfer it to a Windows PC for verification/inspection with Windows tools.
Android
- Android typically cannot preview Windows catalog (.CAT) files; download/store it and transfer it to a Windows PC for verification/inspection with Windows tools.
Security notes
- .CAT files are security-related metadata used for integrity/signature validation; if a catalog is missing, mismatched, or replaced, it may indicate a tampered or improperly packaged driver/software set.
- Avoid editing or “fixing” a .CAT file with a hex editor or text editor; changing it will typically invalidate hashes/signatures and can break driver/package validation.
- Be cautious when a .CAT file is delivered together with executables or driver installers: the catalog itself is not usually executed, but it may be part of a distribution that you should only trust if it comes from a reputable source.
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Can't open this file?
These are the most common causes and fixes when .CAT files fail to open.
Common reasons
- Windows asks what app to use / file won’t open
- Catalog signature verification fails
- Wrong file type: .CAT is not a Windows security catalog
Fix steps
- On Windows, use Windows signing/verification tools (for example, SignTool as referenced in Windows driver signing documentation) rather than trying to open it in a document app.
- If the file came with a driver or installer, keep it in the same folder as the rest of the package so Windows can validate it during installation.
OS-specific troubleshooting
What is a .CAT file?
A .CAT file (catalog) is used in Windows security workflows to associate a set of files with cryptographic hashes and a digital signature. Microsoft documentation describes creating catalog files with MakeCat and signing them as part of driver and installation validation workflows. The commonly referenced MIME type for this format is application/vnd.ms-pki.seccat (seen in MIME mapping lists), but it is not something most end-user apps can display meaningfully.
Background
In Windows, catalog (.CAT) files are part of the system’s trust and integrity model: they help Windows verify that a collection of files (often driver packages) match expected hashes and are properly signed. This is why .CAT files frequently appear inside driver packages or alongside installation files, and why they matter for driver installation and signature enforcement.
Microsoft’s documentation covers how catalogs are created (for example, using MakeCat) and how they are signed (for example, with signing workflows used for drivers). In practice, users do not “edit” these files; they are produced by build/signing pipelines and then validated by Windows.
Because .CAT files are security artifacts, most platforms outside Windows do not provide common, user-friendly viewers. Even on Windows, the most useful actions are verification and signature inspection rather than opening it as text. If you only need to know what it is, basic file properties and signature verification are typically the right approach.
Common MIME types: application/vnd.ms-pki.seccat
Further reading
Authoritative resources for more details on the .CAT format.
Common .CAT issues
Windows asks what app to use / file won’t open
.CAT files are not general-purpose documents; most systems do not have a default viewer. They are intended for security verification workflows (especially for drivers).
- On Windows, use Windows signing/verification tools (for example, SignTool as referenced in Windows driver signing documentation) rather than trying to open it in a document app.
- If the file came with a driver or installer, keep it in the same folder as the rest of the package so Windows can validate it during installation.
Catalog signature verification fails
Verification can fail if the catalog was modified, is incomplete, or is not the correct catalog for the files it is supposed to cover; it can also fail if it is unsigned or signed in a way your system does not trust.
- Re-download or re-extract the driver/package to ensure the .CAT and related files are intact and unmodified.
- Confirm you are verifying the correct .CAT for the associated files (catalogs are typically specific to a file set).
Wrong file type: .CAT is not a Windows security catalog
The .cat extension can be used by different products, but the most common meaning in Windows contexts is a security catalog. If it did not come from a Windows/driver/security workflow, it may be something else.
- Check where the file came from (for example, inside a Windows driver package strongly suggests a security catalog).
- If it did not come from a Windows security context, ask the sender or source application what created it before attempting to process it.
FAQ
What is a .CAT file used for on Windows?
It is commonly a Windows security catalog used to store hashes and a signature for a set of files (often drivers), enabling Windows to validate integrity and trust during installation and use.
Can I convert or open a .CAT file in a text editor?
You can open it as raw data, but it is not designed to be human-readable. The practical approach is signature/hash verification using Windows tools rather than conversion.
Is application/vnd.ms-pki.seccat the official MIME type?
It is a commonly referenced MIME mapping for .cat in MIME-type tables, but you should not rely on it as universally registered/recognized across all platforms; behavior varies by OS and desktop environments.
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