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 5. CVE 2021 43905

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MICROSOFT OFFICE APP REMOTE CODE EXECUTION VULNERABILITY

CVE-2021-43905
On this page
Security Vulnerability

Released: Dec 14, 2021

Assigning CNA:
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Microsoft

MITRE CVE-2021-43905

CVSS:3.1 9.6 / 8.6
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Metric

Value

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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.
Low
Specialized access conditions or extenuating circumstances do not exist. An
attacker can expect repeatable success against the vulnerable component.
Privileges Required
This metric describes the level of privileges an attacker must possess before
successfully exploiting the vulnerability.
None
The attacker is unauthorized prior to attack, and therefore does not require any
access to settings or files to carry out an attack.
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.
Required
Successful exploitation of this vulnerability requires a user to take some
action before the vulnerability can be exploited.
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.
Changed
An exploited vulnerability can affect resources beyond the security scope
managed by the security authority of the vulnerable component. In this case, the
vulnerable component and the impacted component are different and managed by
different security authorities.
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).
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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, exploit code
availability, or active, 'in-the-wild' exploitation.
Proof-of-Concept
Proof-of-concept exploit code is available, or an attack demonstration is not
practical for most systems. The code or technique is not functional in all
situations and may require substantial modification by a skilled attacker.
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.


EXPLOITABILITY

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

Exploited

Exploitability Assessment

No
No
Exploitation More Likely


FAQ

Is the Preview Pane an attack vector for this vulnerability?

No, the Preview Pane is not an attack vector.

How do I get the update for Office app?

The Microsoft Store will automatically update affected customers. Alternatively,
customers can get the update immediately; see here for details. Be sure to
select the tab for the operating system installed on your device to search for
updates.

It is possible for customers to disable automatic updates for the Microsoft
Store. The Microsoft Store will not automatically install this update for those
customers. You can get the update through the store here:
ms-windows-store://pdp/?productid=9WZDNCRD29V9.

How can I check if the update is installed?

App versions 18.2110.13110.0 and later contain this update.


ACKNOWLEDGEMENTS

 * Felix Boulet
 * Fabian Bräunlein and Lukas Euler from Positive Security

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.
Updates CVSS
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Dec 14, 2021
Office app
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Remote Code Execution
Critical
 * Release Notes

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CVE-2021-43905

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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

Revision Date

Description

1.0
Dec 14, 2021

Information published.