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Mitigation: ZipArchiveEntry.FullName Path Separator

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Starting with apps that target the .NET Framework 4.6.1, the path separator used in the ZipArchiveEntry.FullName property has changed from the backslash ("\") used in previous versions of .NET Framework to a forward slash ("/"). System.IO.Compression.ZipArchiveEntry objects are created by calling one of the overloads of the ZipFile.CreateFromDirectory method.

Impact

The change brings the .NET implementation into conformity with section 4.4.17.1 of the .ZIP File Format Specification and allows .ZIP archives to be decompressed on non-Windows systems.

Decompressing a zip file created by an app that targets a previous version of .NET Framework on non-Windows operating systems such as MacOS fails to preserve the directory structure. For example, on MacOS, it creates a set of files whose file name concatenates the directory path, any backslash ("\") characters, and the filename. As a result, the directory structure of decompressed files is not preserved.

The impact of this change on .ZIP files that are decompressed on the Windows operating system by APIs in .NET Framework System.IO namespace should be minimal, since these APIs can seamlessly handle either a slash ("/") or a backslash ("\") as the path separator character.

Mitigation

If this behavior is undesirable, you can opt out of by adding a configuration setting to the <runtime> section of your application configuration file. The following shows both the <runtime> section and the opt-out switch.

<runtime>  
   <AppContextSwitchOverrides value="Switch.System.IO.Compression.ZipFile.UseBackslash=true" />  
</runtime>

In addition, apps that target previous versions of .NET Framework but are running on .NET Framework 4.6.1 and later versions can opt in to this behavior by adding a configuration setting to the <runtime> section of the application configuration file. The following shows both the <runtime> section and the opt-in switch.

<runtime>  
   <AppContextSwitchOverrides value="Switch.System.IO.Compression.ZipFile.UseBackslash=false" />  
</runtime>

See also