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Date: Mon, 23 Apr 2018 05:43:53 -0400
From: Nightwatch Cybersecurity Research <research@...htwatchcybersecurity.com>
To: fulldisclosure@...lists.org
Subject: Re: [FD] Auto-detection of Compressed Files in Apple’s macOS

As a follow-up on this, Cisco has issued a public advisory to address
this issue in their AMP appliance. It is tracked under CVE-2018-0237:
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180418-amp
https://nvd.nist.gov/vuln/detail/CVE-2018-0237

Thanks

On Sun, Feb 25, 2018 at 9:45 PM, Nightwatch Cybersecurity Research
<research@...htwatchcybersecurity.com> wrote:
> [On the web here:
> https://wwws.nightwatchcybersecurity.com/2018/02/25/research-compressed-files-auto-detection-on-macos/]
>
> [NOTE: This was originally discovered as a result of a different set
> of bugs in Google’s Chrome browser, details of which will be posted
> soon. While the impact of this particular issue isn’t high, it was
> interesting enough for us to pursue a coordinated disclosure process.
> Because of the large number of parties involved, the disclosure
> coordination process took a long time which is why this article took
> almost two years to publish.]
>
> SUMMARY
>
> Compressed files on macOS are autodetected by the operating system
> even if they are renamed to certain other extensions. This can be used
> to fool users and antivirus software that relies on file extensions by
> packaging malicious code inside compressed files with different
> extensions. The vendor (Apple) does not consider this to be a security
> issue. Most anti-virus vendors for macOS are not affected by this
> issue. This was originally discovered in macOS v10.11 (El Capitan) and
> v10.12 (Sierra), but the latest version of macOS v10.13 (High Sierra)
> was not tested.
>
> BACKGROUND
>
> On Microsoft Windows, files are identified by their extensions, which
> appears after the “.” in the filename. On macOS metadata about the
> file maybe available separately and either a creator code, a type code
> or a Uniform Type Identifier is used. However, on the Internet (in
> browsers and email clients) instead of filenames, MIME media types are
> used with a registry maintained by IANA on behalf of the IETF. Linux
> systems use a mix of extensions and media types, with some
> auto-detection / “sniffing” of media types based on file content. Some
> mappings do exists across the various systems as well.
>
> For example, a ZIP archive would be identified as follows:
> - Windows – .zip extension
> - Internet/Linux – application/zip media type
> - macOS UTI – com.pkware.zip-archive
>
> Additionally, on most desktop OSes, an association exists between a
> file type and an application that will open it by default. Those
> associations are maintained differently from OS to OS, but at their
> core they associate a particular identifier about a file type such as
> an extension (Windows) or a media type (browsers), and a program
> assigned to open it by default. Users are used to this arrangement and
> many security utilities such as antivirus programs will only look
> inside files that maybe dangerous. For example, a ZIP file on Windows
> if renamed to a different extension may not necessarily be scanned by
> default because double clicking on it will not open it.
>
> Another important point is that malware authors may sometimes try to
> disguise malicious code by compressing it inside an archive such as a
> ZIP file. The expectation is that when a user downloads it, they will
> double click and open it using the default program on that platform,
> and then will execute the malicious code. This is another reason why
> this functionality deserves a closer look.
>
> DETAILS
>
> The following two things were discovered:
> 1. The compression utility that is part of macOS will open any file
> extension associated with that program and will try to “sniff” /
> auto-detect the original file type used. The following file extensions
> were tested:
>
> ZIP Files when renamed as:
> - .AS
> - .CPGZ
> - .PAX
> - .XIP (a Gatekeeper warning will be shown for non-signed files)
>
> DMG files when renamed as:
> - .CDR
> - .DART
> - .DC42
> - .DISKCOPY42
> - .DMG
> - .DMGPART
> - .DVDR
> - .IMG
> - .IMGPART
> - .ISO
> - .NDIF
> - .SMI
> - .SPARSEBUNDLE
> - .SPARSEIMAGE
> - .TOAST
> - .UDIF
>
> 2. The OS itself (macOS) itself will open and execute some file
> formats even when renamed to a different extension. Gatekeeper
> protection is not bypassed. The following extensions are affected:
> - PKG
> - MPKG
>
> To duplicate the first issue, create a ZIP file containing any content
> (we used the EICAR test file) and rename to include a file extension
> as any of the compression formats above for ZIP. (AS, CPGZ, PAX or
> XIP). Send this file to a macOS computer via USB or email or a link;
> download and double click. The ZIP file will open correctly.
>
> You can also do the same thing but with a DMG file for any of the DMG
> file formats listed above (DC42, ISO, etc).
>
> To duplicate the second issue, create a PKG file containing some code
> or take an existing one, rename as .MPKG and transfer to a macOS
> computer. Double click to execute.
>
> All testing was done in May 2016 on a MacBook Pro running MacOS
> v10.11.3 (El Capitan), and re-tested again in April 2017 on a MacBook
> running MacOS v10.12.04 (Sierra). It is unclear whether later versions
> of MacOS are affected since we did not perform testing on versions
> past v10.12.04 (Sierra).
>
> RECOMMENDATIONS
>
> There are two issues:
> 1. Human users and anti-malware software are not aware that macOS
> supports a large number of legacy compression file types and may not
> be properly looking out for them or scanning them.
> Because of the “sniffing” behavior, it would be trivial for an
> attacker to package malware inside a well known format like ZIP or DMG
> rename it to one of these extensions.
> 2. Anti-virus software may fail to scan such archives because they do
> not expect a ZIP file to be packaged that way.
>
> The information in this article was originally discovered while
> analyzing a non-Apple application running on macOS (a separate
> advisory will be published in the future).
>
> Our recommendations are as follows:
> - Apple should consider deprecating or adding a warning for these
> extensions and removing the “sniffing” support.
> - Anti-malware software for macOS should support all of these formats,
> as well as accounting for the possibility of one format being renamed
> as another
>
> VENDOR RESPONSES
>
> The vendor (Apple) does not consider this to be a security issue as follows:
>>> After examining your report we do not see any actual security implications.
>>> All of the extensions provided in your report are supported disk image formats and will be treated equally.
>
>>> After examining your report we do not see any actual security implications. Archive Utility opens archive files and the extensions you provided are archive extensions.
>
>>> After examining your report we do not see any actual security implications. The Installer app makes it clear when executable code is running even if the file has been renamed.
>
> As per advice of Apple’s security team, we also contacted multiple
> antivirus vendors that provide AV software for macOS to check if they
> are affected by this issue. Here is what we got back:
>
> Vendors That Responded:
> - Avast – not affected
> - Avira – not affected
> - AVG – related bug for engine versions prior to 4668 has been fixed
> earlier (see CVE-2017-9977 and our blog post); other products not
> affected
> - BitDefender – not affected
> - Cisco – one product impacted, tracked by bug identifier CSCve34034 –
> no CVE has been issued:
> Cisco AMP Virtual Private Cloud Appliance – The Cisco AMP appliance
> does not rely on the file extension when processing ZIP archives or
> PKG install packages. However, older versions relied on file extension
> to detect DMG files and so is susceptible to one of the scan evasion
> problems described in the advisory. The DMG portion is now fixed in
> software release 1.4.5.
> - ClamXAV (Canimaan Software) – not affected
> - Comodo – not affected
> - CyberByte – not affected
> - Dr. Web – not affected
> - ESet – not affected
> - F-Secure – not affected
> - Intego – not affected
> - Kaspersky – not affected
> - Malware Bytes – not affected
> - Protect Works – not affected
> - QuickHeal – not affected
> - Sophos – not affected
> - Symantec – not affected
> - Trend Micro – not affected
> - Webroot – not affected
>
> Other Vendors:
> - 360 Total Security – pending
> - BullGuard – no response
> - EScanAV – no response
> - GData – pending
> - MacKeeper – no response
> - McAfee – no response
> - Panda – no response
> - QuikAV – pending
> - Total Defense – pending
>
> REFERENCES
>
> Apple Product Security Followup Numbers: 638059697,  640528823 and 640528841
> Cisco Ref # PSIRT-1814664974
>
> CREDITS
>
> Advisory written by Yakov Shafranovich.
>
> TIMELINE SUMMARY
>
> 2016-03-21: Report # 638059697 submitted
> 2016-05-04: Reports # 640528823 and 640528841 submitted
> 2016-05-21: Report # 640528823 rejected
> 2016-06-22: Report # 638059697 rejected
> 2016-06-23: Report # 640528841 rejected
>
> 2017-03-15: Advisory provided to the vendor for comment
> 2017-04-23: Retested on macOS Sierra, updated and resent to vendor for comment
> 2017-04-28: Reply from vendor received
> 2017-05-01: Retested on a fresh install of macOS Sierra, revised
> advisory sent to vendor for comment
> 2017-05-01: Notifications go out to AV vendors
>
> 2018-01-24: Second time that notifications go out to AV vendors
> 2018-02-10: Third and final time that notifications go out to AV vendors
> 2018-02-10: Final advisory shared with the vendor (Apple) for comment
> 2018-02-25: Public disclosure

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