Security Advisory 2021-01

Summary Local privilege escalation in Octopus Server (CVE-2021-26556)
Advisory Number 2021-01
Discovery Date 01 Jan 2021
Patch Release Date 07 Mar 2021
Advisory Release Date 10 Mar 2021
Product Octopus Server
Operating System Windows
Severity High
CVE ID CVE-2021-26556

Customers who have downloaded and installed any of the Octopus Server versions listed below ("Details") are affected.

Please upgrade your Octopus Server immediately to fix this vulnerability.

Customers using Octopus Cloud are not affected.

Customers who have upgraded Octopus Server to version 2020.6.4671 or higher are not affected.


Octopus Deploy has given this vulnerability a high rating. This rating was given according to the Octopus Deploy severity levels, which ranks vulnerabilities as critical, high, medium, or low severity.

This is our assessment and you should evaluate its applicability to your own environment.


When Octopus Server is installed using a custom folder location, folder ACLs are not set correctly and could lead to an unprivileged user using DLL side-loading to gain privileged access.

The versions of Octopus Server affected by this vulnerability are:

  • All 0.x.x, 1.x.x, 2.x.x, 3.x.x, 4.x.x versions

  • All 2018.x.x, 2019.x.x, 2020.1.x, 2020.2.x, 2020.3.x versions

  • All 2020.4.x versions before 2020.4.229

  • All 2020.5.x versions before 2020.5.256

This issue can be tracked here:


To address this vulnerability, we have released Octopus Server versions:

  • 2020.4.229

  • 2020.5.256

  • 2020.6.4671

The latest versions of Octopus Deploy products can be downloaded from and previous versions can be downloaded from

What You Need to Do

Octopus Deploy recommends that you upgrade to the latest version (2020.6.4671). You can download the latest version of from

If you can't upgrade to the latest version (2020.6.4671):

If you have feature version... ...then upgrade to this version
2018.x.x, 2019.x.x, 2020.1.x, 2020.2.x, 2020.3.x 2020.4.229 or greater
2020.4.x 2020.4.229 or greater
2020.5.x 2020.5.256 or greater

For Octopus Server versions: 0.xx, 1.x.x, 2.x.x, 3.x.x, 4.x.x please contact our support team for advice on upgrading to a supported version.


If you are unable to patch your Octopus Server installation you can effectively mitigate this vulnerability by removing any “CreateFiles” or “AppendData” permissions for “BUILTIN\Users” on the Octopus Server installation directory.

The following links are for the PowerShell commands that can be used to check folder ACLs and update them:


If you have any questions or concerns regarding this advisory, please contact our support team

Exploitation and Public Announcements

The Octopus Deploy security team is not aware of any malicious use of the vulnerability that is described in this advisory.

A public announcement has been made by DeepSurface Security at


This vulnerability was found by Robert Chen from DeepSurface Security.