VU#287122: Parsec Remote Desktop App is prone to a local elevation of privilege due to a logical flaw in its code integrity verification process

Overview

Parsec updater for Windows was prone to a local privilege escalation vulnerability, this vulnerability allowed a local user with Parsec access to gain NT_AUTHORITY/SYSTEM privileges.

Description

The vulnerability is a time-of-check time–of-use (TOCTOU) vulnerability. There existed a small window between verifying the signature and integrity of the update DLL and the execution of DLL main.

By exploiting this race condition, a local attacker could swap out the officially signed Parsec DLL with a DLL that they created, which would subsequently be executed as the SYSTEM user as described in CVE-2023-37250.

CVE-2023-37250
The application launches DLLs from a User owned directory. Since the user owns both the DLL file and the directory, it is possible to (successfully) attempt tricking Parsec into loading an unsigned/arbitrary DLL file and execute its DllMain() method with SYSTEM privileges, creating a Local Privilege Escalation vulnerability.

Impact

By exploiting this race condition, a local attacker could swap out the officially signed Parsec DLL with a DLL that they created, which would subsequently be executed as the SYSTEM user.

Solution

The vulnerability applies to a “Per User” installation as opposed to a “Shared User”. There is an update that has been made available. To force an update, you can either completely quit, and re-open the application several times until the loader is updated (by confirming in the logs). Or you can download a special installer that only updates the files inside of the program files that can be downloaded from https://builds.parsec.app/package/parsec-update-executables.exe.

Acknowledgements

Thanks to the reporter, Julian Horoszkiewicz.This document was written by Timur Snoke.

Vendor Information

References

https://atos.net/en/lp/securitydive/roaming-and-racing-to-get-system-cve-2023-37250

https://support.parsec.app/hc/en-us/articles/18311425588237-CVE-2023-37250