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Apple Patches USB Code Execution Flaw in macOS

One of the vulnerabilities addressed by Apple in its latest set of security patches for macOS is an arbitrary code execution flaw, which could be exploited via malicious USB devices.

One of the vulnerabilities addressed by Apple in its latest set of security patches for macOS is an arbitrary code execution flaw, which could be exploited via malicious USB devices.

Discovered by Trend Micro security researchers and reported to Apple in April this year, the issue resides in fsck_msdos, a system tool designed to check for and fix errors in devices formatted with the FAT filesystem.

The security researchers discovered that because the tool is automatically invoked by macOS when a device using the FAT filesystem (such as a USB disk or an SD card) is inserted, a security bug could allow malicious devices to execute arbitrary code when they are connected to a Mac.

The vulnerability is created by a memory corruption issue and its exploitation could lead to an attacker taking full control of a vulnerable system, Trend Micro says.

“We do not believe that this attack has been used in the wild. We strongly recommend that users update their software to address this flaw, as well as the others that were part of this update cycle,” the security researchers note.

Trend Micro found that malicious code could modify a byte containing the high bits of a memory address with an arbitrary value and set it to point to another address.

“If the target address is sprayed with a malformed dosDirEntry structure, arbitrary code execution is now possible. This can potentially allow an attacker to take over the vulnerable device,” the security researchers note.

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Tracked as CVE-2017-13811, the vulnerability was addressed by Apple with the release of macOS High Sierra 10.13.1 (and Security Update 2017-001 Sierra, and Security Update 2017-004 El Capitan), which resolved nearly 150 vulnerabilities, including three KRACK-related flaws.

Trend Micro explains that fsck_msdos is used in other BSD-based operating systems, as well as in Android. Because of that, other vendors were also informed of the vulnerability, including Google.

However, it appears that the issue won’t be resolved in Android, because “fsck_msdos runs under a very restricted SELinux domain.” Nevertheless, Google is apparently looking into addressing the bug in a future release of the operating system, the researchers note.

To mitigate the impact of this vulnerability, IT administrators are advised to restrict USB access to devices, especially considering that this is a method frequently used by malware to enter targeted systems. They should also consider physical controls for especially sensitive devices.

Related: Apple Patches Dangerous KRACK Wi-Fi Vulnerabilities

Related: Apple Silently Patched macOS Security Bypass Flaw

Written By

Ionut Arghire is an international correspondent for SecurityWeek.

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