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This joint Cybersecurity Advisory (CSA) was coauthored by cybersecurity authorities of the United States, Australia, Canada, New Zealand, and the United Kingdom: the Cybersecurity and Infrastructure Security Agency (CISA), National Security Agency (NSA), Federal Bureau of Investigation (FBI), Australian Cyber Security Centre (ACSC), Canadian Centre for Cyber Security (CCCS), New Zealand National Cyber Security Centre (NZ NCSC), and United Kingdom’s National Cyber Security Centre (NCSC-UK). This advisory provides details on the top 15 Common Vulnerabilities and Exposures (CVEs) routinely exploited by malicious cyber actors in 2021, as well as other CVEs frequently exploited.
U.S., Australian, Canadian, New Zealand, and UK cybersecurity authorities assess, in 2021, malicious cyber actors aggressively targeted newly disclosed critical software vulnerabilities against broad target sets, including public and private sector organizations worldwide. To a lesser extent, malicious cyber actors continued to exploit publicly known, dated software vulnerabilities across a broad spectrum of targets.
The cybersecurity authorities encourage organizations to apply the recommendations in the Mitigations section of this CSA. These mitigations include applying timely patches to systems and implementing a centralized patch management system to reduce the risk of compromise by malicious cyber actors.
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Globally, in 2021, malicious cyber actors targeted internet-facing systems, such as email servers and virtual private network (VPN) servers, with exploits of newly disclosed vulnerabilities. For most of the top exploited vulnerabilities, researchers or other actors released proof of concept (POC) code within two weeks of the vulnerability’s disclosure, likely facilitating exploitation by a broader range of malicious actors.
To a lesser extent, malicious cyber actors continued to exploit publicly known, dated software vulnerabilities—some of which were also routinely exploited in 2020 or earlier. The exploitation of older vulnerabilities demonstrates the continued risk to organizations that fail to patch software in a timely manner or are using software that is no longer supported by a vendor.
Top 15 Routinely Exploited Vulnerabilities
Table 1 shows the top 15 vulnerabilities U.S., Australian, Canadian, New Zealand, and UK cybersecurity authorities observed malicious actors routinely exploiting in 2021, which include:
- CVE-2021-44228. This vulnerability, known as Log4Shell, affects Apache’s Log4j library, an open-source logging framework. An actor can exploit this vulnerability by submitting a specially crafted request to a vulnerable system that causes that system to execute arbitrary code. The request allows a cyber actor to take full control over the system. The actor can then steal information, launch ransomware, or conduct other malicious activity. Log4j is incorporated into thousands of products worldwide. This vulnerability was disclosed in December 2021; the rapid widespread exploitation of this vulnerability demonstrates the ability of malicious actors to quickly weaponize known vulnerabilities and target organizations before they patch.
- CVE-2021-26855, CVE-2021-26858, CVE-2021-26857, CVE-2021-27065. These vulnerabilities, known as ProxyLogon, affect Microsoft Exchange email servers. Successful exploitation of these vulnerabilities in combination (i.e., “vulnerability chaining”) allows an unauthenticated cyber actor to execute arbitrary code on vulnerable Exchange Servers, which, in turn, enables the actor to gain persistent access to files and mailboxes on the servers, as well as to credentials stored on the servers. Successful exploitation may additionally enable the cyber actor to compromise trust and identity in a vulnerable network.
- CVE-2021-34523, CVE-2021-34473, CVE-2021-31207. These vulnerabilities, known as ProxyShell, also affect Microsoft Exchange email servers. Successful exploitation of these vulnerabilities in combination enables a remote actor to execute arbitrary code. These vulnerabilities reside within the Microsoft Client Access Service (CAS), which typically runs on port 443 in Microsoft Internet Information Services (IIS) (e.g., Microsoft’s web server). CAS is commonly exposed to the internet to enable users to access their email via mobile devices and web browsers.
- CVE-2021-26084. This vulnerability, affecting Atlassian Confluence Server and Data Center, could enable an unauthenticated actor to execute arbitrary code on vulnerable systems. This vulnerability quickly became one of the most routinely exploited vulnerabilities after a POC was released within a week of its disclosure. Attempted mass exploitation of this vulnerability was observed in September 2021.
Three of the top 15 routinely exploited vulnerabilities were also routinely exploited in 2020: CVE-2020-1472, CVE-2018-13379, and CVE-2019-11510. Their continued exploitation indicates that many organizations fail to patch software in a timely manner and remain vulnerable to malicious cyber actors.
Table 1: Top 15 Routinely Exploited Vulnerabilities in 2021
|CVE||Vulnerability Name||Vendor and Product||Type|
|CVE-2021-44228||Log4Shell||Apache Log4j||Remote code execution (RCE)|
|CVE-2021-40539||Zoho ManageEngine AD SelfService Plus||RCE|
|CVE-2021-34523||ProxyShell||Microsoft Exchange Server||Elevation of privilege|
|CVE-2021-34473||ProxyShell||Microsoft Exchange Server||RCE|
|CVE-2021-31207||ProxyShell||Microsoft Exchange Server||Security feature bypass|
|CVE-2021-27065||ProxyLogon||Microsoft Exchange Server||RCE|
|CVE-2021-26858||ProxyLogon||Microsoft Exchange Server||RCE|
|CVE-2021-26857||ProxyLogon||Microsoft Exchange Server||RCE|
|CVE-2021-26855||ProxyLogon||Microsoft Exchange Server||RCE|
|CVE-2021-26084||Atlassian Confluence Server and Data Center||Arbitrary code execution|
|CVE-2021-21972||VMware vSphere Client||RCE|
|CVE-2020-1472||ZeroLogon||Microsoft Netlogon Remote Protocol (MS-NRPC)||Elevation of privilege|
|CVE-2020-0688||Microsoft Exchange Server||RCE|
|CVE-2019-11510||Pulse Secure Pulse Connect Secure||Arbitrary file reading|
|CVE-2018-13379||Fortinet FortiOS and FortiProxy||Path traversal|
Additional Routinely Exploited Vulnerabilities
In addition to the 15 vulnerabilities listed in table 1, U.S., Australian, Canadian, New Zealand, and UK cybersecurity authorities identified vulnerabilities, listed in table 2, that were also routinely exploited by malicious cyber actors in 2021.
These vulnerabilities include multiple vulnerabilities affecting internet-facing systems, including Accellion File Transfer Appliance (FTA), Windows Print Spooler, and Pulse Secure Pulse Connect Secure. Three of these vulnerabilities were also routinely exploited in 2020: CVE-2019-19781, CVE-2019-18935, and CVE-2017-11882.
Table 2: Additional Routinely Exploited Vulnerabilities in 2021
|CVE||Vendor and Product||Type|
|CVE-2021-35464||ForgeRock OpenAM server||RCE|
|CVE-2021-27104||Accellion FTA||OS command execution|
|CVE-2021-27103||Accellion FTA||Server-side request forgery|
|CVE-2021-27102||Accellion FTA||OS command execution|
|CVE-2021-27101||Accellion FTA||SQL injection|
|CVE-2021-21985||VMware vCenter Server||RCE|
|CVE-2021-20038||SonicWall Secure Mobile Access (SMA)||RCE|
|CVE-2021-34527||Microsoft Windows Print Spooler||RCE|
|CVE-2021-27852||Checkbox Survey||Remote arbitrary code execution|
|CVE-2021-22893||Pulse Secure Pulse Connect Secure||Remote arbitrary code execution|
|CVE-2021-20016||SonicWall SSLVPN SMA100||Improper SQL command neutralization, allowing for credential access|
|CVE-2021-1675||Windows Print Spooler||RCE|
|CVE-2020-2509||QNAP QTS and QuTS hero||Remote arbitrary code execution|
|CVE-2019-19781||Citrix Application Delivery Controller (ADC) and Gateway||Arbitrary code execution|
|CVE-2019-18935||Progress Telerik UI for ASP.NET AJAX||Code execution|
|CVE-2018-0171||Cisco IOS Software and IOS XE Software||Remote arbitrary code execution|
Vulnerability and Configuration Management
- Update software, operating systems, applications, and firmware on IT network assets in a timely manner. Prioritize patching known exploited vulnerabilities, especially those CVEs identified in this CSA, and then critical and high vulnerabilities that allow for remote code execution or denial-of-service on internet-facing equipment. For patch information on CVEs identified in this CSA, refer to the appendix.
- If a patch for a known exploited or critical vulnerability cannot be quickly applied, implement vendor-approved workarounds.
- Use a centralized patch management system.
- Replace end-of-life software, i.e., software that is no longer supported by the vendor. For example, Accellion FTA was retired in April 2021.
- Organizations that are unable to perform rapid scanning and patching of internet-facing systems should consider moving these services to mature, reputable cloud service providers (CSPs) or other managed service providers (MSPs). Reputable MSPs can patch applications—such as webmail, file storage, file sharing, and chat and other employee collaboration tools—for their customers. However, as MSPs and CSPs expand their client organization’s attack surface and may introduce unanticipated risks, organizations should proactively collaborate with their MSPs and CSPs to jointly reduce that risk. For more information and guidance, see the following resources.
Identity and Access Management
- Enforce multifactor authentication (MFA) for all users, without exception.
- Enforce MFA on all VPN connections. If MFA is unavailable, require employees engaging in remote work to use strong passwords.
- Regularly review, validate, or remove privileged accounts (annually at a minimum).
- Configure access control under the concept of least privilege principle.
- Ensure software service accounts only provide necessary permissions (least privilege) to perform intended functions (non-administrative privileges).
Note: see CISA Capacity Enhancement Guide – Implementing Strong Authentication and ACSC guidance on Implementing Multi-Factor Authentication for more information on hardening authentication systems.
Protective Controls and Architecture
- Properly configure and secure internet-facing network devices, disable unused or unnecessary network ports and protocols, encrypt network traffic, and disable unused network services and devices.
- Harden commonly exploited enterprise network services, including Link-Local Multicast Name Resolution (LLMNR) protocol, Remote Desktop Protocol (RDP), Common Internet File System (CIFS), Active Directory, and OpenLDAP.
- Manage Windows Key Distribution Center (KDC) accounts (e.g., KRBTGT) to minimize Golden Ticket attacks and Kerberoasting.
- Strictly control the use of native scripting applications, such as command-line, PowerShell, WinRM, Windows Management Instrumentation (WMI), and Distributed Component Object Model (DCOM).
- Segment networks to limit or block lateral movement by controlling access to applications, devices, and databases. Use private virtual local area networks.
- Continuously monitor the attack surface and investigate abnormal activity that may indicate lateral movement of a threat actor or malware.
- Use security tools, such as endpoint detection and response (EDR) and security information and event management (SIEM) tools. Consider using an information technology asset management (ITAM) solution to ensure your EDR, SIEM, vulnerability scanner etc., are reporting the same number of assets.
- Monitor the environment for potentially unwanted programs.
- Reduce third-party applications and unique system/application builds; provide exceptions only if required to support business critical functions.
- Implement application allowlisting.
- For the top vulnerabilities exploited in 2020, see joint CSA Top Routinely Exploited Vulnerabilities
- For the top exploited vulnerabilities 2016 through 2019, see joint CSA Top 10 Routinely Exploited Vulnerabilities.
- See the appendix for additional partner resources on the vulnerabilities mentioned in this CSA.
The information in this report is being provided “as is” for informational purposes only. CISA, the FBI, NSA, ACSC, CCCS, NZ NCSC, and NCSC-UK do not endorse any commercial product or service, including any subjects of analysis. Any reference to specific commercial products, processes, or services by service mark, trademark, manufacturer, or otherwise, does not constitute or imply endorsement, recommendation, or favoring.
This document was developed by U.S., Australian, Canadian, New Zealand, and UK cybersecurity authorities in furtherance of their respective cybersecurity missions, including their responsibilities to develop and issue cybersecurity specifications and mitigations.
Appendix: Patch Information and Additional Resources for Top Exploited Vulnerabilities
U.S. organizations: all organizations should report incidents and anomalous activity to CISA 24/7 Operations Center at email@example.com or (888) 282-0870 and/or to the FBI via your local FBI field office or the FBI’s 24/7 CyWatch at (855) 292-3937 or CyWatch@fbi.gov. When available, please include the following information regarding the incident: date, time, and location of the incident; type of activity; number of people affected; type of equipment used for the activity; the name of the submitting company or organization; and a designated point of contact. For NSA client requirements or general cybersecurity inquiries, contact Cybersecurity_Requests@nsa.gov. Australian organizations: visit cyber.gov.au or call 1300 292 371 (1300 CYBER 1) to report cybersecurity incidents and access alerts and advisories. Canadian organizations: report incidents by emailing CCCS at firstname.lastname@example.org. New Zealand organizations: report cyber security incidents to email@example.com or call 04 498 7654. United Kingdom organizations: report a significant cyber security incident: ncsc.gov.uk/report-an-incident (monitored 24 hours) or, for urgent assistance, call 03000 200 973.