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.
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