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Microsoft Windows task scheduler contains a local privilege escalation vulnerability in the ALPC interface

Vulnerability Note VU#906424

Original Release Date: 2018-08-28 | Last Revised: 2018-09-13

Overview

Microsoft Windows task scheduler contains a local privilege escalation vulnerability in the Advanced Local Procedure Call (ALPC) interface, which can allow a local user to obtain SYSTEM privileges.

Description

The Microsoft Windows task scheduler SchRpcSetSecurity API contains a vulnerability in the handling of ALPC, which can allow an authenticated user to overwrite the contents of a file that should be protected by filesystem ACLs. This can be leveraged to gain SYSTEM privileges. We have confirmed that the public exploit code works on 64-bit Windows 10 and Windows Server 2016 systems. We have also confirmed compatibility with 32-bit Windows 10 with minor modifications to the public exploit code. Compatibility with other Windows versions is possible with further modifications.

This vulnerability is being exploited in the wild.

Impact

An authenticated local user may be able to gain elevated (SYSTEM) privileges.

Solution

Apply an update

This issue is addressed in the Microsoft update for CVE-2018-8440.

Deploy Microsoft Sysmon Detection Rules

Kevin Beaumont has provided guidance for creating rules to detect exploitation of this vulnerability.

Set ACLs on the C:\Windows\Tasks directory

Karsten Nilsen has provided a mitigation for this vulnerability. Caution: This mitigation has not been approved by Microsoft. However, in our testing it does block exploits for this vulnerability. It also appears to let scheduled tasks to continue to run, and users can continue to create new scheduled tasks as necessary. However, this change will reportedly break things created by the legacy task scheduler interface. This can include things like SCCM and the associated SCEP updates. Please ensure that you have tested this mitigation to ensure that it does not cause unacceptable consequences in your environment.

To apply this mitigation, run the following commands in an elevated-privilege prompt,:

icacls c:\windows\asks /remove:g "Authenticated Users"
icacls c:\windows\asks /deny system:(OI)(CI)(WD,WDAC)

Note that when a fix is made available for this vulnerability, these changes should be undone. This can be done by executing the following commands:

icacls c:\windows\asks /remove:d system
icacls c:\windows\asks /grant:r "Authenticated Users":(RX,WD)

Vendor Information

906424
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Microsoft

Notified:  August 27, 2018 Updated:  September 11, 2018

Status

  Affected

Vendor Statement

No statement is currently available from the vendor regarding this vulnerability.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

Addendum

There are no additional comments at this time.

If you have feedback, comments, or additional information about this vulnerability, please send us email.


CVSS Metrics

Group Score Vector
Base 6.8 AV:L/AC:L/Au:S/C:C/I:C/A:C
Temporal 6.5 E:F/RL:U/RC:C
Environmental 6.4 CDP:ND/TD:H/CR:ND/IR:ND/AR:ND

References

Credit

This issue was publicly disclosed by SandboxEscaper.

This document was written by Will Dormann.

Other Information

CVE IDs: CVE-2018-8440
Date Public: 2018-08-27
Date First Published: 2018-08-28
Date Last Updated: 2018-09-13 13:05 UTC
Document Revision: 67

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