Threat actors were using Windows Arbitrary File Deletion to perform Denial-of-service attacks on systems affected by this vulnerability. However, recent reports indicate that this Windows Arbitrary file deletion can be used for a full compromise.

The possibility of this attack depends on the CVE-2023-27470 arbitrary file deletion vulnerability combining it with a Time-of-Check to Time-of-Use (TOCTOU) race condition, which enables the deletion of files on a Windows system and subsequently creates an elevated Command Prompt.

CVE-2023-27470 & TOCTOU – Technical Analysis

CVE-2023-27470 affects N-Able’s Take Control Agent, which can lead to an arbitrary file deletion vulnerability. This vulnerability analysis was done using Microsoft’s Process Monitor, often called ProcMon. 

This vulnerability exists due to insecure file operations conducted by NT AUTHORITY\SYSTEM processes that were detected with the help of ProcMon filters.

The process that was analyzed during this vulnerability was BASupSrvcUpdater.exe, belonging to Take Control Agent 7.0.41.1141.

Race Condition

BASupSrvcUpdater.exe attempts every 30 seconds to a non-existent folder under the C:\ProgramData\GetSupportService_N-Central\PushUpdates as an NT AUTHORITY\SYSTEM process. For further research, this PushUpdates folder and a dummy file aaa.txt were created.

BASupSrvcUpdater.exe made an attempt to read the contents of the folder and performed a deletion, which was logged in the C:\ProgramData\GetSupportService_N-Central\Logs\BASupSrvcUpdater_[DATE].log log file. 

This particular action gives rise to a race condition, as a threat actor can exploit this condition by utilizing the timeframe between the deletion and logging.

To exploit this condition and perform a full system compromise, an attacker must replace a file in the PushUpdates folder with a pseudo-symlink.

A complete report about this attack has been published, which provides detailed information about the exploitation, techniques, process, and method of complete system compromise.

To prevent this attack, it is recommended for organizations using N-able to upgrade to version 7.0.43 to fix this vulnerability.

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