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macOS High Sierra Leaks APFS Volume Passwords via Hint

A developer from Brazil noticed that the recently launched macOS High Sierra 10.13 operating system leaks the passwords for encrypted Apple File System (APFS) volumes via the password hint.

APFS is a new file system introduced by Apple with macOS High Sierra. When High Sierra is installed on a computer with a solid-state drive (SSD), the startup volume is automatically converted to APFS and users cannot opt out of the transition. APFS promises strong encryption, fast directory sizing, space sharing, and improved file system fundamentals.

Developer Matheus Mariano discovered the password leakage after he used the Disk Utility in High Sierra to add a new encrypted APFS volume to the container. When users add a new volume, they are asked to enter a password and, optionally, write a hint for it.

When the new volume is mounted, the user is asked to enter the password. However, Mariano noticed that if the “Show Hint” button is pressed, the hint that is displayed is actually the password set by the user. The password is not disclosed if no information is entered into the “Password hint” field when creating a new volume, although Apple recommends adding a hint.

“I really don’t know how this went unnoticed by Apple (and anyone else),” Mariano said.

SecurityWeek can confirm that the password for encrypted APFS volumes is leaked via the password hint on High Sierra.

APFS password leak via hint

macOS developer Felix Schwarz pointed out that users who have set a hint via the Disk Utility can address the issue by changing the hint using the diskutil command line utility.

Mariano said he reported the issue to Apple before making his findings public. He also published a video showing the vulnerability:

SecurityWeek has reached out to Apple for comment and will update this article if the company responds.

This is not the first security hole discovered by researchers in High Sierra. Patrick Wardle, director of research at Synack, reported last month that unsigned apps can steal passwords from the macOS keychain, and that Apple’s new Secure Kernel Extension Loading (SKEL) security feature can be easily bypassed.

UPDATE. Apple told SecurityWeek that an update released on Thursday, October 5, for High Sierra addresses both the APFS password disclosure issue and the keychain vulnerability reported by Wardle.

The company has also published a knowledge base article that provides more guidance to users on the password disclosure bug.

Related: Mac Firmware Updates Are Failing and Leaving Systems Vulnerable

Related: Apple Patches Vulnerabilities in macOS, macOS Server

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.