What is the ZZZ file format?
A .zzz file has no single, fixed format - the extension is an informal placeholder label that can be applied to any file rather than a defined file type. Because .zzz sorts last alphabetically, developers, scripts, and users commonly rename files to this extension to disable them, mark them as junk, or park them temporarily, in the same spirit as a .BAK backup or a .TMP temporary file.
The extension gained its widest association through CCleaner, a system optimization and cleaning software for Windows, macOS, and Android. CCleaner is capable of removing unnecessary files that take up disk space or negatively affect system performance. In some versions, .zzz files are generated by CCleaner to store resources and files it has processed, in case they might need to be restored later.
Because .zzz carries no inherent format information, opening one correctly requires identifying what the file originally contained. A hex editor such as HxD or a file-identification utility such as TrID can inspect the raw bytes and reveal the original type. The content may be binary or plain text depending entirely on which program or user created the file. The extension itself carries no magic bytes or recognizable signature, so automated type detection typically falls back to application/octet-stream.
Security & safety
RISK: MEDIUMThe extension itself is harmless, but two cautions apply. First, a .zzz can be any kind of file, including an executable disguised by renaming - never run a .zzz; inspect it first. Second, and most important, several ransomware strains append .zzz to encrypted files: if many of your files were renamed to .zzz at once and won't open, treat it as a possible infection, disconnect/back up, and check ID-Ransomware/No More Ransom for a decryptor rather than paying.
Format details
in a nutshell- Application-specific temp/backup data - Some programs have written .zzz for transient or backup data (e.g. WordPerfect-era temp, various utilities); the format is whatever that app defined.
- Encrypted/ransomware-renamed files - Some ransomware strains append .zzz to encrypted files; such a .zzz is an encrypted original, not openable without the decryptor.
- Deliberately renamed / disabled files - Users rename a file to .zzz to disable it or push it to the end of a sorted list; the real type is the original extension.
Programs that open ZZZ files
Windows3 apps
TrID / TrIDNet (file type identifier)
Free Run TrID on the .zzz to detect its real type from the byte signature, then rename to that extension.
HxD (hex editor)
Free Open the .zzz in a hex viewer; check the first bytes against known magic numbers to see what it really is.
Notepad++ (if it's text)
Open-source Open to check whether the .zzz is readable text; if so, rename to .txt. 
macOS1 app
file command / Hex Fiend
Open-source Run file yourfile.zzz in Terminal to detect the type, or inspect bytes in Hex Fiend, then rename. 
Linux1 app
file command
Open-source Run file yourfile.zzz to identify the real format from its magic, then rename accordingly. Technical details
deep spec| Format type | No fixed format - any file can be given the .zzz extension; it is an informal label, not a file specification |
| Magic bytes / file signature | None; .zzz files carry no identifying byte sequence at any offset |
| MIME type | application/octet-stream (generic fallback); text/plain when content is known to be text |
| Encoding | Depends entirely on the original file - may be binary or UTF-8 / ASCII text |
| Structure | Undefined; content is determined by whatever program or user created or renamed the file |
| Compression | Not inherent - inherits any compression of the original file if present |
| Encryption | None by convention - content is not encrypted by the .zzz extension itself |
| Typical use cases | Placeholder, temporarily disabled file, junk/discard marker, renamed file to prevent auto-loading by applications |
| Alphabetical sort position | Sorts after all standard A-Z extensions, placing files at the bottom of directory listings |
| Associated platforms | Windows, macOS, Linux (cross-platform - a naming convention, not an OS-specific format) |
| Related extensions | .tmp, .bak, .old, .000 |
| Typical file size | Any size - equals the size of the underlying or renamed file |
| Identification approach | Inspect raw bytes with a hex editor (e.g. HxD) or file-type tool (e.g. TrID) to determine the true format |
| Owning developer | None - an informal, application-agnostic convention used by many programs and users |
| Released | N/A (informal convention, not a defined format) |
ZZZ conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about ZZZ files.
Frequently asked questions
What is a ZZZ file?
How do I open a ZZZ file?
file command on macOS/Linux, or check the first bytes in a hex viewer. Then rename it to the correct extension and open it with the matching program.
