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PsExec Local Privilege Escalation

Medium
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Synopsis

Local Privilege Escalation

This is due to the PsExec service component (PsExecSvc.exe) and how it uses its "\\.\pipe\PSEXESVC" named pipe. A low privileged attacker is able to create a named pipe with the same name before PsExecSvc starts on the machine. Once PsExecSvc starts and calls "CreateNamedPipe," a handle of the existing named pipe is obtained which still has the ACL's from the low privileged attacker, thus allowing the low privileged attacker to read/write to this pipe being used by PsExecSvc (which should be Administrator/System only). In this case, a low privileged attacker can execute any process locally as SYSTEM through this pipe. This vulnerability can be exploited on the target machine in either local or remote PsExec usage scenarios.

Solution

There is no known solution or mitigation for this issue.

Proof of Concept

https://github.com/tenable/poc/blob/master/Microsoft/Sysinternals/PsExecEscalate.cpp

Disclosure Timeline

9-10-2020 - Tenable discloses issue to Microsoft MSRC.
9-10-2020 - Automated MSRC response.
9-16-2020 - MSRC confirms behavior, continues to investigate solution.
9-30-2020 - Tenable follows up asking if there is an expected patch date.
10-08-2020 - Microsoft responds that they are still looking into possible solution.
10-22-2020 - Tenable asks for update on solution.
10-22-2020 - MSRC responds that they are figuring out a way to automatically mitigate the issue.
11-02-2020 - Tenable asks if patch will be for Windows or PsExec.
11-02-2020 - MSRC expects patch to be for PsExec.
11-13-2020 - Tenable asks if there is a planned CVE for this issue.
11-23-2020 - Tenable reminds that disclosure will be December 9, 2020 and asks for update on previous email.
12-04-2020 - Tenable follows up again, asking if there will be CVE assigned and latest patch status.
12-08-2020 - MSRC says they do not plan to issue a CVE, as they don't typically release them for affected product. Asks if Tenable could share advisory draft before release.
12-08-2020 - Tenable asks Microsoft why they does not plan to issue a CVE for this particular issue. Tenable explains we do not typically share drafts before disclosure dates.

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]

Risk Information

Tenable Advisory ID: TRA-2020-68
Credit:
David Wells
CVSSv2 Base / Temporal Score:
6.8
CVSSv2 Vector:
AV:L/AC:L/Au:S/C:C/I:C/A:C
Affected Products:
PsExec v2.32 and below
Risk Factor:
Medium

Advisory Timeline

December 9, 2020 - Initial release.
January 15, 2021 - Updated versions affected
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