ACE (WinAce Archive)
A Windows-era archive format known for tight compression ratios that briefly rivaled RAR in the late 1990s.
| Full name | WinAce Archive |
| Extension | .ace |
| MIME type | application/x-ace-compressed |
| Developer | Marcel Lemke / e-merge GmbH |
| Released | 1998 (format); WinAce 1.0 released July 1999) |
| Type | Lossless compressed archive |
| Compression method | Proprietary dictionary + statistical coding |
| Encryption | Password-based (Blowfish in later versions) |
What is a ACE file?
ACE is a proprietary compressed archive format created by German programmer Marcel Lemke. It was later maintained by e-merge GmbH under the WinAce application. The format gained a following in the late 1990s and early 2000s for producing smaller archives than ZIP on many file types.
An ACE file is a compressed archive that can hold one or more files and folders, much like ZIP or RAR. The format supports solid compression, which compresses all files together to improve ratios on collections of similar files. It also supports multi-volume splitting, recovery records for repairing damaged archives, and optional password protection. ACE archives use the .ace extension and the application/x-ace-compressed MIME type.
History
Marcel Lemke developed the ACE compression format in 1998, and the first public release of WinAce appeared in July 1999. The format peaked in popularity between 1999 and 2001, when it offered slightly better compression than RAR on certain data types. Development stalled after e-merge GmbH acquired the project, and in February 2019, security researchers at Check Point discovered a critical path-traversal vulnerability (CVE-2018-20250) in unacev2.dll, the shared library used to decompress ACE files. Because the source code was unavailable and the library had not been updated since 2005, WinRAR and most other tools dropped ACE support entirely.
How it works
ACE archives begin with a signature header that identifies the format version and compression settings. Each file inside the archive is preceded by a local header containing the filename, original size, compressed size, CRC checksum, and timestamp. The compressed data stream follows immediately after each header. Recovery records, when present, are stored as separate blocks near the end of the archive and allow partial reconstruction of damaged files.
What it is used for
- Extracting old ACE archives downloaded before 2005 when the format was common on file-sharing sites
- Preserving or migrating legacy software distributed in ACE format
- Converting ACE archives to modern formats like ZIP or 7Z for safer long-term storage
- Studying archive format history and compression algorithm design
How to open it
Modern tools such as 7-Zip (with a plugin), PeaZip, and The Unarchiver can open ACE files on Windows, macOS, and Linux. WinRAR dropped ACE support in version 5.70 (2019) due to the unacev2.dll security vulnerability, so it can no longer open ACE files.
Pros and cons
Strengths
- Strong compression ratios that matched or beat RAR on many file types in its era
- Solid archive mode improves ratios further when archiving many similar files
- Recovery records allow partial repair of corrupted or incomplete archives
- Multi-volume splitting makes large archives easier to transfer across size-limited media
Trade-offs
- Proprietary format with no open specification, limiting third-party support
- Affected by CVE-2018-20250, a critical path-traversal vulnerability in unacev2.dll that was never patched
- WinRAR and most mainstream tools dropped native ACE support after the 2019 vulnerability disclosure
- Effectively abandoned since the mid-2000s with no ongoing development
Convert ACE files
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ACE FAQ
Is it safe to open ACE files?
Only if you use a tool that does not rely on the vulnerable unacev2.dll library. Tools like 7-Zip with its ACE plugin handle extraction without that library. Avoid opening ACE files from untrusted sources regardless of the tool.
Why did WinRAR stop supporting ACE files?
In February 2019, Check Point researchers found CVE-2018-20250, a path-traversal bug in unacev2.dll that allowed arbitrary code execution. Because the source code was not available and the library had not been updated since 2005, the only fix was to remove ACE support entirely starting with WinRAR 5.70.
How do I convert an ACE file to ZIP?
Extract the ACE archive using a compatible tool such as 7-Zip or PeaZip, then recompress the extracted files into a ZIP archive. Some online converters handle this in one step.
Is ACE better than ZIP or RAR?
ACE offered slightly better compression than ZIP and was competitive with RAR in the early 2000s. By current standards, 7Z (7-Zip) outperforms all three. ACE is no longer a practical choice because of its abandoned status and security history.