OSX.Keydnap is a MacOS X based

Trojan horse In Greek mythology, the Trojan Horse () was a wooden horse said to have been used by the Greeks during the Trojan War to enter the city of Troy and win the war. The Trojan Horse is not mentioned in Homer, Homer's ''Iliad'', with the poem ending ...

that steals passwords from the iCloud Keychain of the infected machine. It uses a

dropper An eye dropper, also called Pasteur pipette or simply dropper, is a device used to transfer small quantities of liquids. They are used in the laboratory and also to dispense small amounts of liquid medicines. A very common use is to dispense eye ...

to establish a permanent backdoor while exploiting MacOS vulnerabilities and security features like

Gatekeeper A gatekeeper is a person who controls access to something, for example via a city gate or bouncer, or more abstractly, controls who is granted access to a category or status. Gatekeepers assess who is "in or out", in the classic words of manage ...

, iCloud Keychain and the file naming system. It was first detected in early July 2016 by ESET researchers, who also found it being distributed through a compromised version of Transmission Bit Torrent Client.

Technical details

Download and installation

OSX.Keydnap is initially downloaded as a Zip archive. This archive contains a single

Mach-O Mach-O (Mach object) file format, is a file format for executables, object code, shared libraries, dynamically loaded code, and core dumps. It was developed to replace the a.out format. Mach-O is used by some systems based on the Mach kern ...

file and a

Resource fork A resource fork is a fork of a file on Apple's classic Mac OS operating system that is used to store structured data. It is one of the two forks of a file, along with the data fork, which stores data that the operating system treats as unstruct ...

containing an icon for the executable file, which is typically a JPEG or text file image. Additionally, the dropper takes advantage of how OS X handles file extensions by putting a space behind the extension of the file name for example – as “keydnap.jpg ” instead of “keydnap.jpg”. Usually commonly seen icon images and names are used to exploit users' willingness to click on benign looking files. When the file is opened, the Mach-O executable runs by default in the Terminal instead of an image viewer like the user would expect. This initial execution does three things. First, it downloads and executes the backdoor component. Second, it downloads and opens a decoy document to match what the dropper file is pretending to be. Finally, it quits the Terminal to cover up that it was ever open. The terminal is only opened momentarily.

Establishing the backdoor connection

Since the downloader is not persistent, the downloaded backdoor component spawns a process named "icloudsyncd" that runs at all times. It also adds an entry to the LaunchAgents directory to survive reboots. The icloudsyncd process is used to communicate with a command & control server via an onion.to address, establishing the backdoor. It then attempts to capture passwords from the iCloud Keychain, using the proof-of-concept Keychaindump, and transmits them back to the server. Keychaindump reads securityd’s memory and searches for the decryption key for the user’s keychain as described in “Keychain Analysis with Mac OS X Memory Forensics” by K. Lee and H. Koo.

Gatekeeper signing workaround

Mac OS uses Gatekeeper to verify if an application is signed with a valid Apple Developer ID certificate preventing OSX.Keydnap from running. Further, even if the user does have Gatekeeper turned off, they will see a warning that the file is an application downloaded from the Internet giving the user an option to not execute the application. However, by packing OSX.Keydnap with a legitimate signing key as in the case of the compromised Transmission app, it successfully bypasses Gatekeeper protection.

Detection and removal

Activating

is an easy way to prevent accidental installation of OSX.Keydnap. If the user's Mac has Gatekeeper activated, the malicious file will not be executed and a warning will be displayed to the user. This is because the malicious Mach-O file is unsigned, which automatically triggers a warning in Gatekeeper. Users who have been infected by the compromised Transmission app or disabling Gatekeeper are able to remove this malware infection with Spyware Doctor.{{Cite web, url=https://iboostup.com/spyware-doctor/research/infection/Trojan.Keydnap, title=Keydnap - Threat Details, last=Spyware Doctor, date=2016-08-30, website=www.iboostup.com, publisher=iBoostUp, first=macOS malware research, access-date=2024-12-05

References

MacOS malware Trojan horses