What is the EX_ file format?
.ex_ is a compressed .exe executable file produced by Microsoft compression tools. An .ex_ file stores a complete Windows executable in a space-saving compressed form, without any ability to run it directly.
The idea behind the .ex_ format
.ex_ files are used for installer distribution purposes and to reduce file size. After installation, the files are decompressed and renamed back to their original .exe names. The naming convention follows a consistent rule: the last character of the original extension is replaced by an underscore - so .exe becomes .ex_, .dll becomes .dl_, .sys becomes .sy_, and so on. The format originates from early 1990s Microsoft tooling designed to fit programs onto space-constrained floppy disks.
The compressed data uses Microsoft's SZDD or KWAJ format (both LZ77-based), identifiable by the SZDD magic bytes (53 5A 44 44 88 F0 27 33) at offset 0. Newer installation media sometimes deliver these compressed files as named members inside a .cab archive, where the inner entry retains the .ex_ name. An .ex_ file is not a runnable program - it must be decompressed to recover the original .exe before it can be executed.
Creating and extracting .ex_ files
.ex_ files are generated from original .exe files using the Windows makecab command-line tool (or the older compress.exe). Reversing the process is possible by using the built-in Windows expand.exe tool or third-party archivers such as 7-Zip. The archive takes the same base name as the original .exe file, with only the extension's last character replaced by an underscore.
Security & safety
RISK: LOWEX_ is a benign compression format, not malware and not encrypted - the '_' is just Microsoft's naming convention for a compressed file. The only real caution: after you expand it you get an EXE, which (like any executable) should only be run if you trust its source. Scan the recovered .exe before running if it came from an untrusted CD or download. The EX_ itself cannot execute until decompressed.
Format details
in a nutshellPrograms that open EX_ files
Windows3 apps
7-Zip
Open-source Right-click → 7-Zip → Open archive; 7-Zip reads SZDD/KWAJ and CAB and extracts the inner EXE. 
Microsoft Expand (built-in)
Built-in In Command Prompt: 'expand setup.ex_ setup.exe' - restores the original runnable EXE. 
macOS1 app
7-Zip (via Keka / p7zip)
Open-source Keka (p7zip backend) can extract SZDD/CAB-based .ex_ files to recover the EXE for inspection. 
Linux1 app
cabextract / 7-Zip (p7zip)
Open-source Use 'cabextract' for CAB-based files, or p7zip's 7z for SZDD; recovers the contained EXE. Technical details
deep spec| Encoding | Binary |
| Byte order | Little-endian |
| Developer | Microsoft |
| Container format | Microsoft SZDD or KWAJ single-file compressed stream; also appears as a named member inside a CAB archive |
| Compression algorithm | LZ77-based MS-Compress (SZDD/KWAJ); MSZIP or LZX when delivered as a CAB member |
| Magic bytes - SZDD variant | 53 5A 44 44 88 F0 27 33 at offset 0 (ASCII prefix: SZDD) |
| Magic bytes - KWAJ variant | 4B 57 41 4A 88 F0 27 D1 at offset 0 - second MS-Compress single-file variant |
| Naming convention | Last character of the original extension replaced by underscore: .exe → .ex_, .dll → .dl_, .sys → .sy_ |
| Files per container | Exactly 1 file in standalone SZDD/KWAJ; unlimited when embedded as members inside a CAB archive |
| Integrity / checksum | No CRC in standalone SZDD/KWAJ streams; CAB members carry per-folder checksums |
| MIME type | application/octet-stream |
| Associated operating systems | MS-DOS and all Windows versions |
| Typical compressed size | Smaller than the source .exe; common range 100 KB - 50 MB |
| Executability | Not directly runnable; must be expanded to .exe with expand.exe or 7-Zip before execution |
| Superseded by | CAB archives and modern Windows installers (MSI/MSIX); SZDD/KWAJ single-file compression is considered legacy |
| Released | early 1990s (MS-DOS/Windows install media; COMPRESS.EXE / EXPAND.EXE) |
| Latest version | MSZIP-based SZDD/KWAJ compression as used by MAKECAB/COMPRESS |
| Specification | learn.microsoft.com |
EX_ conversions
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