Kerberos KDC Security Feature Bypass Vulnerability

CVE-2020-17049

Security Vulnerability

Released: 10 de nov. de 2020

Last updated: 13 de jul. de 2021

Assigning CNA: Microsoft

CVE.org link: CVE-2020-17049

Impact: Security Feature Bypass

Max Severity: Important

CVSS Source: Microsoft

Vector String: CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C

Metrics: CVSS:3.1 6.6 / 5.8

Metric

Value

Base score metrics(8)

Attack Vector

This metric reflects the context by which vulnerability exploitation is possible. The Base Score increases the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable component.

Network

The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).

Attack Complexity

This metric describes the conditions beyond the attacker’s control that must exist in order to exploit the vulnerability. Such conditions may require the collection of more information about the target or computational exceptions. The assessment of this metric excludes any requirements for user interaction in order to exploit the vulnerability. If a specific configuration is required for an attack to succeed, the Base metrics should be scored assuming the vulnerable component is in that configuration.

High

A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).

Privileges Required

This metric describes the level of privileges an attacker must possess before successfully exploiting the vulnerability.

High

The attacker is authorized with (i.e., requires) privileges that provide significant (e.g., administrative) control over the vulnerable component that could affect component-wide settings and files.

User Interaction

This metric captures the requirement for a user, other than the attacker, to participate in the successful compromise the vulnerable component. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.

None

The vulnerable system can be exploited without any interaction from any user.

Scope

Does a successful attack impact a component other than the vulnerable component? If so, the Base Score increases and the Confidentiality, Integrity and Authentication metrics should be scored relative to the impacted component.

Unchanged

An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.

Confidentiality

This metric measures the impact to the confidentiality of the information resources managed by a software component due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.

High

There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.

Integrity

This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information.

High

There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.

Availability

This metric measures the impact to the availability of the impacted component resulting from a successfully exploited vulnerability. It refers to the loss of availability of the impacted component itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of an impacted component.

High

There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Temporal score metrics(3)

Exploit Code Maturity

This metric measures the likelihood of the vulnerability being attacked, and is typically based on the current state of exploit techniques, public availability of exploit code, or active, 'in-the-wild' exploitation.

Unproven

No publicly available exploit code is available, or an exploit is theoretical.

Remediation Level

The Remediation Level of a vulnerability is an important factor for prioritization. The typical vulnerability is unpatched when initially published. Workarounds or hotfixes may offer interim remediation until an official patch or upgrade is issued. Each of these respective stages adjusts the temporal score downwards, reflecting the decreasing urgency as remediation becomes final.

Official Fix

A complete vendor solution is available. Either the vendor has issued an official patch, or an upgrade is available.

Report Confidence

This metric measures the degree of confidence in the existence of the vulnerability and the credibility of the known technical details. Sometimes only the existence of vulnerabilities are publicized, but without specific details. For example, an impact may be recognized as undesirable, but the root cause may not be known. The vulnerability may later be corroborated by research which suggests where the vulnerability may lie, though the research may not be certain. Finally, a vulnerability may be confirmed through acknowledgement by the author or vendor of the affected technology. The urgency of a vulnerability is higher when a vulnerability is known to exist with certainty. This metric also suggests the level of technical knowledge available to would-be attackers.

Confirmed

Detailed reports exist, or functional reproduction is possible (functional exploits may provide this). Source code is available to independently verify the assertions of the research, or the author or vendor of the affected code has confirmed the presence of the vulnerability.

Please see Common Vulnerability Scoring System for more information on the definition of these metrics.

Executive Summary

A security feature bypass vulnerability exists in the way Key Distribution Center (KDC) determines if a service ticket can be used for delegation via Kerberos Constrained Delegation (KCD).

To exploit the vulnerability, a compromised service that is configured to use KCD could tamper with a service ticket that is not valid for delegation to force the KDC to accept it.

The update addresses this vulnerability by changing how the KDC validates service tickets used with KCD.

Exploitability

The following table provides an exploitability assessment for this vulnerability at the time of original publication.

Publicly disclosed: No
Exploited: No
Exploitability assessment: Exploitation Less Likely

FAQ

Do I need to take further steps to be protected from this vulnerability?

Yes. As of April 13, 2021, customers who have already installed the November 10, 2020 security updates need to do the following:

This update assumes that all Domain Controllers are updated with the December 8, 2020 updates or later updates. The December 8, 2020 security updates include fixes for all known issues originally introduced by the November 10, 2020 release of CVE-2020-17049. This update also adds support for Windows Server 2008 SP2 and Windows Server 2008 R2.
Install the updates released on April 13, 2021. These updates remove the PerformTicketSignature setting 0. Setting PerformTicketSignature to 0 after this update is installed will have the same effect as setting PerformTicketSignature to 1. The Domain Controllers (DC)s will be in Deployment mode.

For more information and further steps to enable full protection on domain controller servers see Managing deployment of Kerberos S4U changes for CVE-2020-17049.

Acknowledgements

Jake Karnes of NetSPI

Microsoft recognizes the efforts of those in the security community who help us protect customers through coordinated vulnerability disclosure. See Acknowledgements for more information.

Security Updates

To determine the support lifecycle for your software, see the Microsoft Support Lifecycle.

Release date Descending

Release date

Product

Platform

Impact

Max Severity

Article

Download

Build Number

10 de nov. de 2020

Windows Server, version 20H2 (Server Core Installation)

Security Feature Bypass

Important

5004237

Security Update

10.0.19041.1110

10 de nov. de 2020

Windows Server 2012 R2 (Server Core installation)

Security Feature Bypass

Important

6.3.9600.20069
6.3.9600.20069

10 de nov. de 2020

Windows Server 2012 R2

Security Feature Bypass

Important

6.3.9600.20069
6.3.9600.20069

10 de nov. de 2020

Windows Server 2012 (Server Core installation)

Security Feature Bypass

Important

6.2.9200.23409
6.2.9200.23409

10 de nov. de 2020

Windows Server 2012

Security Feature Bypass

Important

6.2.9200.23409
6.2.9200.23409

10 de nov. de 2020

Windows Server 2008 R2 for x64-based Systems Service Pack 1 (Server Core installation)

Security Feature Bypass

Important

6.1.7601.25661
6.1.7601.25661

10 de nov. de 2020

Windows Server 2008 R2 for x64-based Systems Service Pack 1

Security Feature Bypass

Important

6.1.7601.25661
6.1.7601.25661

10 de nov. de 2020

Windows Server 2008 for x64-based Systems Service Pack 2 (Server Core installation)

Security Feature Bypass

Important

6.0.6003.21167
6.0.6003.21167

10 de nov. de 2020

Windows Server 2008 for x64-based Systems Service Pack 2

Security Feature Bypass

Important

6.0.6003.21167
6.0.6003.21167

10 de nov. de 2020

Windows Server 2008 for 32-bit Systems Service Pack 2 (Server Core installation)

Security Feature Bypass

Important

6.0.6003.21167
6.0.6003.21167

10 de nov. de 2020

Windows Server 2008 for 32-bit Systems Service Pack 2

Security Feature Bypass

Important

6.0.6003.21167
6.0.6003.21167

10 de nov. de 2020

Windows Server 2016 (Server Core installation)

Security Feature Bypass

Important

5004238

Security Update

10.0.14393.4530

10 de nov. de 2020

Windows Server 2016

Security Feature Bypass

Important

5004238

Security Update

10.0.14393.4530

10 de nov. de 2020

Windows Server, version 2004 (Server Core installation)

Security Feature Bypass

Important

5004237

Security Update

10.0.19041.1110

10 de nov. de 2020

Windows Server, version 1903 (Server Core installation)

Security Feature Bypass

Important

4592449

Security Update

10 de nov. de 2020

Windows Server, version 1909 (Server Core installation)

Security Feature Bypass

Important

5001337

Security Update

10 de nov. de 2020

Windows Server 2019 (Server Core installation)

Security Feature Bypass

Important

5004244

Security Update

10.0.17763.2061

10 de nov. de 2020

Windows Server 2019

Security Feature Bypass

Important

5004244

Security Update

10.0.17763.2061

All results loaded

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Disclaimer

The information provided in the Microsoft Knowledge Base is provided "as is" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.

Revisions

version

revisionDate

description

6.0

13 de jul. de 2021

Microsoft is releasing security updates to deploy the enforcement phase for this vulnerability. Active Directory domain controllers are now capable of Enforcement mode. Going to Enforcement mode requires that all Active Directory domain controllers have the December 8, 2020 update or a later Windows update installed. At this time, the PerformTicketSignature registry key settings will be ignored and Enforcement mode cannot be overridden. See the FAQ section of this CVE and KB4598347 for more information.

5.0

13 de abr. de 2021

Microsoft is releasing security updates for the second deployment phase for this vulnerability. These updates remove the PerformTicketSignature setting 0. Setting PerformTicketSignature to 0 after this update is installed will have the same effect as setting PerformTicketSignature to 1. The Domain Controllers (DC)s will be in Deployment mode. See the FAQ section of this CVE and KB4598347 for more information.

3.0

8 de dic. de 2020

To comprehensively address CVE-2020-17049, Microsoft has released the following: December 2020 Security Updates for all affected Windows 10 servers, Windows Server 2012 R2, and Windows Server 2012; December 2020 Monthly Rollup updates and Security Only updates for all affected versions of Windows Server 2008 R2 and Windows Server 2008. These updates include fixes for all known issues originally introduced by the November 10, 2020 security updates for CVE-2020-17049. Microsoft strongly recommends that customers running any of these versions of Windows Server install the updates and then follow the steps outlined in https://support.microsoft.com/help/4598347 to enable full protection on domain controller servers.

4.0

8 de dic. de 2020

In the Security Updates table, corrected the Download and Article links for all affected Windows 10 servers, Windows Server 2012 R2, and Windows Server 2012 R2. Note that the December 2020 Security Updates supercede the security updates released on November 10, 2020 and the updates released between November 17, 2020 and November 19, 2020 to address this vulnerability.

2.0

19 de nov. de 2020

Added information to the FAQ regarding a non-security update to address a known issue for Domain Controllers (DC) Servers.

1.2

13 de nov. de 2020

To address a known issue with the Kdc registry subkey settings, Microsoft has revised the guidance for deploying this update. Please see the FAQ section for updated information.

1.1

12 de nov. de 2020

Added an Executive Summary and updated the FAQs to further clarify the information for this CVE. This is an informational change only.

1.0

10 de nov. de 2020

Information published.

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