YAA: An Obscure MacOS Compressed File Format


MacOS introduced a new compression archive format in High Sierra (v10.13) called “YAA”. Because this format is new, it may not be supported correctly by security tools, thus allowing malware authors a way to bypass existing controls in such tools. It is recommended that vendors add support for this format to their tools. Users should NOT accept or open YAA archives received from unknown sources.


Early this year we ran across a new file format specific to MacOS: YAA. Apparently it was originally released in the fall of 2017 as part of MacOS High Sierra (v10.13) (see this article from MacKungFu). As per this forum post, apparently it is used as part of compressing the “Content” section of signed executables for MacOS. According to another post, this is an LZFSE tool. This should not be confused with an older tool with the same name called “Yet Another Assembler”.

It is supported via a command line utility called “yaa” as well as the Archive Utility in the GUI – but in the GUI you can only uncompress the archive. For details regarding the CLI, run “yaa” or “man yaa” (also see here).

Our previous research around novel file formats found issues in how Google Chrome (here and here), and various anti-virus vendors for MacOS handle compressed files (here). Specifically, handling a novel compressed archive format is something that existing antivirus software, browsers and other utilities may not be dong well since they are not aware of the format. On the other hand, malware authors can easily package malware inside a compressed archive, which will decompressed by a user via double-clicking the file in Finder. This results from the fact that the Archive utility supports decompression of a lot more file formats that vendors maybe aware of.

For YAA, we have done some testing back in January against a handful of security tools and found some to be vulnerable. Due to lack of resources, we didn’t pursue a more extensive investigation into additional tools but did report whichever issues were found back to the relevant vendors. Because of lack of resources, we are publishing this post to increase awareness of this issue.

To replicate this issue on a particular tool, do the following:

  1. Download the EICAR test file from here into a folder.
  2. Create a YAA file archive as follows:
    • yaa -d folder -o archive.yaa
  3. Test the resulting “archive.yaa” with the security tool of your choice.
  4. To uncompress the archive, double click on it in Finder.

Vendor Responses

This section contains responses we received for this issue from specific vendors:

  • Chromium: Safe browsing pings not sent back for YAA files (issue # 1039128)
    • The default behavior has been set to FULL_PING, so unknown types (such as YAA) should now be sending pings to Safe Browsing.” (also see our blog post)
  • Google/Gmail: YAA archives are not scanned for malware by GMail (issue # 147190746)
    • “Not all file formats are supported and this is expected.”

Most A/V vendors responded either with the fact that once expanded the contents will be scanned, or that this will add this as a feature in the future.


Written by Y. Shafranovich


Majority of this research and responsible disclosure was done in January 2020. Blog post was published on June 14th, 2020.