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macOS Installer Local Privilege Escalation

Medium

Synopsis

The macOS Installer copies files into the desired "/Applications/<app>" or other path as described in the .pkg configuration files. These files are readable/writable by the user installing the application by default. It's not uncommon for installers (particularly those that require elevated permissions) to operate on files in these locations prior to finishing the installation. The most common use-case for this is to copy LaunchDaemons to the correct location or perform other privileged actions during a postinstall script.

It is possible for files in the applications directory to be modified prior to the postinstall script accessing them. In many cases, the postinstall script is used to implement stricter permissions on files of this nature. There is a small window of time between the copying of application files to the "/Applications/<app>" folder and the running of a postinstall script where a malicious actor could modify files that are later used at a higher privilege level. In many instances we've observed this happening in, overwriting these files generally results in local privilege escalation.

This issue is something that can be addressed by individual developers by validating the integrity of files during the installation process. This could, however, also be addressed within Installer itself by using stricter file ownerships and permissions. For example, when installing an application that requires elevated privileges to install (such as Microsoft Teams), the files extracted and moved to "/Applications/<app>" could be made to be owned by root initially rather than relying on the postinstall script to modify these permissions manually. This would greatly enhance the security posture of Installer.

Please note that this issue is very similar to an issue reported in July 2020 by NCC Group: https://research.nccgroup.com/2020/07/02/technical-advisory-macos-installer-local-root-privilege-escalation-cve-2020-9817/.

Solution

Developers are encouraged to pay special attention to the directories and files being used during the installation process when creating distribution packages. In general, it's best practice to use an installer sandbox whenever possible. When that isn't possible, verifying the integrity of files as well as enforcing proper permissions on the directories and files being operated on is enough to mitigate these issues.

Disclosure Timeline

March 16, 2021 - Tenable discloses to vendor.
March 16, 2021 - Vendor sends automated acknowledgment.
March 17, 2021 - Vendor requests clarification; Tenable provides clarification.
March 18, 2021 - Vendor requests additional PoC; Tenable provides.
April 6, 2021 - Apple states that mitigations for this issue are best left up to individual vendors.
April 7, 2021 - Tenable requests further explanation.
April 16, 2021 - Tenable requests status update.
April 16, 2021 - Apple states that they do not consider this a security issue.
April 16, 2021 - Tenable informs intent to publish advisory. Requests information regarding difference in treatment from CVE-2020-9817.
April 16, 2021 - Apple requests advance copy of advisory. Tenable states that the information to be published can be found in the original disclosure email.

All information within TRA advisories is provided “as is”, without warranty of any kind, including the implied warranties of merchantability and fitness for a particular purpose, and with no guarantee of completeness, accuracy, or timeliness. Individuals and organizations are responsible for assessing the impact of any actual or potential security vulnerability.

Tenable takes product security very seriously. If you believe you have found a vulnerability in one of our products, we ask that you please work with us to quickly resolve it in order to protect customers. Tenable believes in responding quickly to such reports, maintaining communication with researchers, and providing a solution in short order.

For more details on submitting vulnerability information, please see our Vulnerability Reporting Guidelines page.

If you have questions or corrections about this advisory, please email [email protected]

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