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files with double extensions

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Double, multiple, and hidden file extension



double file extension



Legitimate use of double file extension


PeaZip can handle and create files with double extension; most common cases for having have a file with two extensions are

what are TAR.GZ, TAR.BZ, TAR.XZ archives

Compressed containers

When a compression only algorithm - most common ones are GZip Deflate compression, BZip2 compression or 7-Zip's lzma XZ compression - is applied to multiple files, which consequnetly needs to be consolidated into a single archive - most times a TAR file - before the compression step (also known as solid mode compression).

This result in the first step archiving extension (e.g. creating a TAR archive file) being pre-pended to second step compression extension (e.g. XZ).

Extracting compressed tar files (TAR.GZ / TAR.BZ / TAR.XZ) could be treated as atomic, single step operation, but usually (as in PeaZip, 7-Zip and other archival utilities) extraction of compressed TAR files is a two step process which firstly uncompress the TAR archive, and then unarchive the contained files and folder structure.

multiple file extensions

Spanned multi-volume archives

File spanning is applied (often in a single pass) to output archive, creating a set of volumes of desired size, with a numerator in the file extension to declare what is the first volume and the progressive order of data chunks, in example .R01, .Z01, etc
Some splitting standards adds the numerator information in a separate extension, before or after usual archive extension, in example .7Z.001, .7Z.002, .7Z.003 etc.

Risks, security issues of "Hide known file extensions" option


A completely unrelated use of double file extension spread after Micorsoft set "Hide known file types extensions" option enabled by default for Windows XP and newer systems - this is still the default behavior on Vista, 7, 8, 10 - opening the ground for attack exploiting hidden files extensions.

files with second extension

This option allows an attacker to trivially add a file extension before the true one in order to mask the real nature of the file - being the last file extension hidden by default to end users by the system in file browser and most applications following system's file browser policies.

In example, an executable virus named attachment.exe can be renamed in attachment.doc.exe.
By deafult, end user would be prompted "attachment.doc" (or any other harmless file extension used by the attacker, i.e. .jpeg, .mpg), but once clicked the file would be executed as .exe file (true file extension) by the system.
In this way an executable file that should trigger a great level of awareness and caution from user (e.g. .exe, .scr, .bat, .vb, .js...) can be easily masked as harmless, common, file type to mislead end user.

hidden file extension

PeaZip file and archive browser never hide file extension, avoiding this type of forgery.
Moreover PeaZip warns each time an executable or script file is being executed from an archive, in this way 1) the user is made aware of the potentially harmful nature of the file 2) the user can evaluate to extract the whole archive before, as executable and script files could need some archived resources (i.e. dll) to be available in uncompressed form before properly running.

Sometimes files with double extension are treated as suspicious ones, but if it is the right case for executable ones (exe or script file type as last extension), it is definitely NOT the case for archive files with double extension, being TAR.something very common file types - especially on *x systems.

Read more about this topic: what are file extensionsfile extension wiki, list of known file typesfilename extension, TARtwo file extensions archive format, which usually comes with a second extension (tar.gz, tar.bz) declaring the compression scheme applied to the tar container.


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FAQ > Security > Files with double extensions

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