Original release date: April 27, 2022 | Last revised: April 28, 2022
Summary
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.
Download the Joint Cybersecurity Advisory: 2021 top Routinely Exploited Vulnerabilities (pdf, 777kb).
Technical Details
Key Findings
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.[1] 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
Log4Shell
Apache Log4j
Remote code execution (RCE)
Zoho ManageEngine AD SelfService Plus
RCE
ProxyShell
Microsoft Exchange Server
Elevation of privilege
ProxyShell
Microsoft Exchange Server
RCE
ProxyShell
Microsoft Exchange Server
Security feature bypass
ProxyLogon
Microsoft Exchange Server
RCE
ProxyLogon
Microsoft Exchange Server
RCE
ProxyLogon
Microsoft Exchange Server
RCE
ProxyLogon
Microsoft Exchange Server
RCE
Atlassian Confluence Server and Data Center
Arbitrary code execution
VMware vSphere Client
RCE
ZeroLogon
Microsoft Netlogon Remote Protocol (MS-NRPC)
Elevation of privilege
Microsoft Exchange Server
RCE
Pulse Secure Pulse Connect Secure
Arbitrary file reading
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
Sitecore XP
RCE
ForgeRock OpenAM server
RCE
Accellion FTA
OS command execution
Accellion FTA
Server-side request forgery
Accellion FTA
OS command execution
Accellion FTA
SQL injection
VMware vCenter Server
RCE
SonicWall Secure Mobile Access (SMA)
RCE
Microsoft MSHTML
RCE
Microsoft Windows Print Spooler
RCE
Sudo
Privilege escalation
Checkbox Survey
Remote arbitrary code execution
Pulse Secure Pulse Connect Secure
Remote arbitrary code execution
SonicWall SSLVPN SMA100
Improper SQL command neutralization, allowing for credential access
Windows Print Spooler
RCE
QNAP QTS and QuTS hero
Remote arbitrary code execution
Citrix Application Delivery Controller (ADC) and Gateway
Arbitrary code execution
Progress Telerik UI for ASP.NET AJAX
Code execution
Cisco IOS Software and IOS XE Software
Remote arbitrary code execution
Microsoft Office
RCE
Microsoft Office
RCE
Mitigations
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.
CISA Insights Risk Considerations for Managed Service Provider Customers
CISA Insights Mitigations and Hardening Guidance for MSPs and Small- and Mid-sized Businesses
ACSC advice on How to Manage Your Security When Engaging a Managed Service Provider
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.
Resources
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.
Disclaimer
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.
Purpose
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.
References
[1] CISA’s Apache Log4j Vulnerability Guidance
Appendix: Patch Information and Additional Resources for Top Exploited Vulnerabilities
CVE
Vendor
Affected Products
Patch Information
Resources
Sitecore
Sitecore XP 7.5.0 – Sitecore XP 7.5.2
Sitecore XP 8.0.0 – Sitecore XP 8.2.7
Sitecore Security Bulletin SC2021-003-499266
ACSC Alert Active Exploitation of vulnerable Sitecore Experience Platform Content Management Systems
ForgeRock
Access Management (AM) 5.x, 6.0.0.x, 6.5.0.x, 6.5.1, 6.5.2.x and 6.5.3
OpenAM 9.x, 10.x, 11.x, 12.x and 13.x
ForgeRock AM Security Advisory #202104
ACSC Advisory Active exploitation of ForgeRock Access Manager / OpenAM servers
CCCS ForgeRock Security Advisory
Accellion
FTA 9_12_370 and earlier
Accellion Press Release: Update to Recent FTA Security Incident
Joint CSA Exploitation of Accellion File Transfer Appliance
ACSC Alert Potential Accellion File Transfer Appliance compromise
FTA 9_12_411 and earlier
FTA versions 9_12_411 and earlier
FTA 9_12_370 and earlier
VMware
vCenter Server 7.0, 6.7, 6.5
Cloud Foundation (vCenter Server) 4.x and 3.x
VMware Advisory VMSA-2021-0010
VMware
vCenter Server 7.0, 6.7, 6.5
Cloud Foundation (vCenter Server) 4.x and 3.x
VMware Advisory VMSA-2021-0002
ACSC Alert VMware vCenter Server plugin remote code execution vulnerability
CCCS Alert APT Actors Target U.S. and Allied Networks – Update 1
SonicWall
SMA 100 Series (SMA 200, 210, 400, 410, 500v), versions 10.2.0.8-37sv, 10.2.1.1-19sv, 10.2.1.2-24sv
SonicWall Security Advisory SNWLID-2021-0026
ACSC Alert Remote code execution vulnerability present in SonicWall SMA 100 series appliances
CCCS SonicWall Security Advisory
Apache
Log4j, all versions from 2.0-beta9 to 2.14.1
For other affected vendors and products, see CISA’s GitHub repository.
Log4j: Apache Log4j Security Vulnerabilities
For additional information, see joint CSA: Mitigating Log4Shell and Other Log4j-Related Vulnerabilities
CISA webpage Apache Log4j Vulnerability Guidance
CCCS Active exploitation of Apache Log4j vulnerability – Update 7
Zoho ManageEngine
ADSelfService Plus version 6113 and prior
Zoho ManageEngine: ADSelfService Plus 6114 Security Fix Release
Joint CSA APT Actors Exploiting Newly Identified Vulnerability in ManageEngine ADSelfService Plus
Microsoft
Multiple Windows products; see Microsoft Security Update Guide: MSHTML Remote Code Execution Vulnerability, CVE-2021-40444
Microsoft Security Update Guide: MSHTML Remote Code Execution Vulnerability, CVE-2021-40444
Microsoft
Multiple Windows products; see Microsoft Security Update Guide: Windows Print Spooler Remote Code Execution Vulnerability, CVE-2021-34527
CCCS Alert Windows Print Spooler Vulnerability Remains Unpatched – Update 3
Microsoft
Microsoft Exchange Server 2013 Cumulative Update 23
Microsoft Exchange Server 2016 Cumulative Updates 19 and 20
Microsoft Exchange Server 2019 Cumulative Updates 8 and 9
ACSC Alert Microsoft Exchange ProxyShell Targeting in Australia
Microsoft
Multiple Exchange Server versions; see: Microsoft Security Update Guide: Microsoft Exchange Server Remote Code Execution Vulnerability, CVE-2021-34473
Microsoft
Multiple Exchange Server versions; see Microsoft Update Guide: Microsoft Exchange Server Security Feature Bypass Vulnerability, CVE-2021-31207
Sudo
Sudo before 1.9.5p2
Checkbox Survey
Checkbox Survey versions prior to 7
Microsoft Exchange Server
Multiple versions; see: Microsoft Security Update Guide: Microsoft Exchange Server Remote Code Execution Vulnerability, CVE-2021-27065
CISA Alert: Mitigate Microsoft Exchange Server Vulnerabilities
ACSC Advisory Active exploitation of Vulnerable Microsoft Exchange servers
CCCS Alert Active Exploitation of Microsoft Exchange Vulnerabilities – Update 4
Microsoft
Exchange Server, multiple versions; see Microsoft Security Update Guide: Microsoft Exchange Server Remote Code Execution Vulnerability, CVE-2021-26858
Microsoft
Exchange Server, multiple versions; see Microsoft Security Update Guide: Microsoft Exchange Server Remote Code Execution Vulnerability, CVE-2021-26857
Microsoft
Exchange Server, multiple versions; see Microsoft Security Update Guide: Microsoft Exchange Server Remote Code Execution Vulnerability, CVE-2021-26855
Jira Atlassian
Confluence Server and Data Center, versions 6.13.23, from version 6.14.0 before 7.4.11, from version 7.5.0 before 7.11.6, and from version 7.12.0 before 7.12.5.
Jira Atlassian: Confluence Server Webwork OGNL injection – CVE-2021-26084
ACSC Alert Remote code execution vulnerability present in certain versions of Atlassian Confluence
CCCS Atlassian Security Advisory
Pulse Secure
PCS 9.0R3/9.1R1 and Higher
CCCS Alert Active Exploitation of Pulse Connect Secure Vulnerabilities – Update 1
SonicWall
SMA 100 devices (SMA 200, SMA 210, SMA 400, SMA 410, SMA 500v)
SonicWall Security Advisory SNWLID-2021-0001
Microsoft
Multiple Windows products; see Microsoft Security Update Guide Windows Print Spooler Remote Code Execution Vulnerability, CVE-2021-1675
CCCS Alert Windows Print Spooler Vulnerability Remains Unpatched – Update 3
QNAP
QTS, multiple versions; see QNAP: Command Injection Vulnerability in QTS and QuTS hero
QuTS hero h4.5.1.1491 build 20201119 and later
QNAP: Command Injection Vulnerability in QTS and QuTS hero
Microsoft
Windows Server, multiple versions; see Microsoft Security Update Guide: Netlogon Elevation of Privilege Vulnerability, CVE-2020-1472
Microsoft Security Update Guide: Netlogon Elevation of Privilege Vulnerability, CVE-2020-1472
ACSC Alert Netlogon elevation of privilege vulnerability (CVE-2020-1472)
CCCS Alert Microsoft Netlogon Elevation of Privilege Vulnerability – CVE-2020-1472 – Update 1
Microsoft
Exchange Server, multiple versions; see Microsoft Security Update Guide: Microsoft Exchange Validation Key Remote Code Execution Vulnerability, CVE-2020-0688
CISA Alert Chinese Ministry of State Security-Affiliated Cyber Threat Actor Activity
CCCS Alert Microsoft Exchange Validation Key Remote Code Execution Vulnerability
Citrix
ADC and Gateway version 13.0 all supported builds before 13.0.47.24
NetScaler ADC and NetScaler Gateway, version 12.1 all supported builds before 12.1.55.18; version 12.0 all supported builds before 12.0.63.13; version 11.1 all supported builds before 11.1.63.15; version 10.5 all supported builds before 10.5.70.12
SD-WAN WANOP appliance models 4000-WO, 4100-WO, 5000-WO, and 5100-WO all supported software release builds before 10.2.6b and 11.0.3b
Citrix Security Bulletin CTX267027
CISA Alert Chinese Ministry of State Security-Affiliated Cyber Threat Actor Activity
CCCS Alert Detecting Compromises relating to Citrix CVE-2019-19781
Progress Telerik
UI for ASP.NET AJAX through 2019.3.1023
Telerik UI for ASP.NET AJAX Allows JavaScriptSerializer Deserialization
ACSC Alert Active exploitation of vulnerability in Microsoft Internet Information Services
Pulse Secure
Pulse Connect Secure 8.2 before 8.2R12.1, 8.3 before 8.3R7.1, and 9.0 before 9.0R3.4
CISA Alert Continued Exploitation of Pulse Secure VPN Vulnerability
CISA Alert Chinese Ministry of State Security-Affiliated Cyber Threat Actor Activity
ACSC Advisory Recommendations to mitigate vulnerability in Pulse Connect Secure VPN Software
CCCS Alert APT Actors Target U.S. and Allied Networks – Update 1
Fortinet
FortiProxy 2.0.2, 2.0.1, 2.0.0, 1.2.8, 1.2.7, 1.2.6, 1.2.5, 1.2.4, 1.2.3, 1.2.2, 1.2.1, 1.2.0, 1.1.6
Fortinet FortiGuard Labs: FG-IR-20-233
ACSC Alert APT exploitation of Fortinet Vulnerabilities
CCCS Alert Exploitation of Fortinet FortiOS vulnerabilities (CISA, FBI) – Update 1
Cisco
See Cisco Security Advisory: cisco-sa-20180328-smi2
Cisco Security Advisory: cisco-sa-20180328-smi2
CCCS Action Required to Secure the Cisco IOS and IOS XE Smart Install Feature
Microsoft
Office, multiple versions; see Microsoft Security Update Guide: Microsoft Office Memory Corruption Vulnerability, CVE-2017-11882
Microsoft Security Update Guide: Microsoft Office Memory Corruption Vulnerability, CVE-2017-11882
CCCS Alert Microsoft Office Security Update
Microsoft
Multiple products; see Microsoft Security Update Guide: Microsoft Office/WordPad Remote Code Execution Vulnerability w/Windows, CVE-2017-0199
CCCS Microsoft Security Updates
Contact Information
U.S. organizations: all organizations should report incidents and anomalous activity to CISA 24/7 Operations Center at [email protected] 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 [email protected]. 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 [email protected]. 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 [email protected]. New Zealand organizations: report cyber security incidents to [email protected] 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.
Revisions
April 27, 2022: Initial Version
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