Risk of SharePoint vulnerability to UK organisations

The NCSC is raising awareness of a new remote code execution vulnerability (CVE2020-16952) affecting Microsoft SharePoint.

21 October, 2020


The NCSC is raising awareness of a new remote code execution vulnerability (CVE-2020-16952) affecting Microsoft SharePoint. Successful exploitation of this vulnerability would allow an attacker to run arbitrary code and carry out security actions in the context of the local administrator on affected installations of SharePoint server.

The NCSC always recommends applying security updates promptly to mitigate the exploitation of all vulnerabilities but in this case the NCSC has previously seen a large number of exploitations of SharePoint vulnerabilities, such as CVE-2019-0604, against UK organisations. Two SharePoint CVEs also appear in the CISA Top 10 Routinely Exploited Vulnerabilities.

The NCSC is issuing this alert to ensure that system owners are aware of this vulnerability and to ensure remediation actions are taken.


The vulnerability is a caused by a validation issue in user-supplied data.1 This vulnerability can be exploited when a user uploads a specially crafted SharePoint application package to an affected version of SharePoint. This affects versions:

  • Microsoft SharePoint Foundation 2013 Service Pack 1

  • Microsoft SharePoint Enterprise Server 2016

  • Microsoft SharePoint Server 2019

  • SharePoint Online as part of Office 365 is not affected.

The October 2020 SharePoint security update addresses the vulnerability by correcting how SharePoint checks the source markup of application packages.


This vulnerability can be mitigated by ensuring that the relevant security updates are installed. Microsoft has published an advisory on this vulnerability which includes links to these updates:



Proof of concept code (PoC) is available here.

This PoC can be detected by identifying HTTP headers containing the string runat="server", as well as auditing SharePoint page creations.


The NCSC strongly advises that organisations refer to the Microsoft guidance referenced in this alert and ensure the necessary updates are installed in affected SharePoint products. It is also important to keep informed of any possible updated future updates to the guidance via this link.

The NCSC generally recommends following vendor best practice advice in the mitigation of vulnerabilities. In the case of this SharePoint vulnerability, it is important to install the latest updates as soon as practicable.


A variety of mitigations will be useful in defending against the campaigns detailed in this report:

Protect your devices and networks by keeping them up to date: use the latest supported versions, apply security updates promptly, use antivirus and scan regularly to guard against known malware threats. See NCSC Guidance:  https://www.ncsc.gov.uk/guidance/mitigating-malware-and-ransomware-attacks

Prevent and detect lateral movement in your organisation’s networks. See NCSC Guidance: https://www.ncsc.gov.uk/guidance/preventing-lateral-movement

Set up a security monitoring capability so you are collecting the data that will be needed to analyse network intrusions. See NCSC Guidance: https://www.ncsc.gov.uk/guidance/introduction-logging-security-purposes

Review and refresh your incident management processes. See NCSC Guidance:  https://www.ncsc.gov.uk/collection/incident-management

Further information: Invest in preventing malware-based attacks across various scenarios.  See NCSC Guidance:  https://www.ncsc.gov.uk/guidance/mitigating-malware-and-ransomware-attacks