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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.

Click here for a PDF version of this report. 

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

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

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. 

Resources

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

CVE-2021-42237

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

 

CVE-2021-35464

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

CVE-2021-27104

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

 

 

CVE-2021-27103

FTA 9_12_411 and earlier

CVE-2021-27102

FTA versions 9_12_411 and earlier

CVE-2021-27101

FTA 9_12_370 and earlier

 

CVE-2021-21985

VMware

vCenter Server 7.0, 6.7, 6.5

Cloud Foundation (vCenter Server) 4.x and 3.x

VMware Advisory VMSA-2021-0010

CCCS VMware Security Advisory

CVE-2021-21972

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 VMware Security Advisory

CCCS Alert APT Actors Target U.S. and Allied Networks – Update 1

CVE-2021-20038

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

 

CVE-2021-44228

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

CVE-2021-40539

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

CCCS Zoho Security Advisory

CVE-2021-40444

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

 

CVE-2021-34527

Microsoft

Multiple Windows products; see Microsoft Security Update Guide: Windows Print Spooler Remote Code Execution Vulnerability, CVE-2021-34527

Microsoft Security Update Guide: Windows Print Spooler Remote Code Execution Vulnerability, CVE-2021-34527

Joint CSA Russian State-Sponsored Cyber Actors Gain Network Access by Exploiting Default Multifactor Authentication Protocols and “PrintNightmare” Vulnerability

CCCS Alert Windows Print Spooler Vulnerability Remains Unpatched – Update 3

CVE-2021-34523

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

Microsoft Security Update Guide: Microsoft Exchange Server Elevation of Privilege Vulnerability, CVE-2021-34523

Joint CSA Iranian Government-Sponsored APT Cyber Actors Exploiting Microsoft Exchange and Fortinet Vulnerabilities in Furtherance of Malicious Activities

ACSC Alert Microsoft Exchange ProxyShell Targeting in Australia

 

CVE-2021-34473

Microsoft

Multiple Exchange Server versions; see: Microsoft Security Update Guide: Microsoft Exchange Server Remote Code Execution Vulnerability, CVE-2021-34473

Microsoft Security Update Guide: Microsoft Exchange Server Remote Code Execution Vulnerability, CVE-2021-34473

CVE-2021-31207

Microsoft

Multiple Exchange Server versions; see Microsoft Update Guide: Microsoft Exchange Server Security Feature Bypass Vulnerability, CVE-2021-31207

Microsoft Update Guide: Microsoft Exchange Server Security Feature Bypass Vulnerability, CVE-2021-31207

CVE-2021-3156

Sudo

Sudo before 1.9.5p2

Sudo Stable Release 1.9.5p2

 

CVE-2021-27852

Checkbox Survey

Checkbox Survey versions prior to 7

 

 

CVE-2021-27065

Microsoft Exchange Server

Multiple versions; see: Microsoft Security Update Guide: Microsoft Exchange Server Remote Code Execution Vulnerability, CVE-2021-27065

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

CVE-2021-26858

Microsoft

Exchange Server, multiple versions; see Microsoft Security Update Guide: Microsoft Exchange Server Remote Code Execution Vulnerability, CVE-2021-26858

Microsoft Security Update Guide: Microsoft Exchange Server Remote Code Execution Vulnerability, CVE-2021-26858

CVE-2021-26857

Microsoft

Exchange Server, multiple versions; see Microsoft Security Update Guide: Microsoft Exchange Server Remote Code Execution Vulnerability, CVE-2021-26857

Microsoft Security Update Guide: Microsoft Exchange Server Remote Code Execution Vulnerability, CVE-2021-26857

CVE-2021-26855

Microsoft

Exchange Server, multiple versions; see Microsoft Security Update Guide: Microsoft Exchange Server Remote Code Execution Vulnerability, CVE-2021-26855

Microsoft Security Update Guide: Microsoft Exchange Server Remote Code Execution Vulnerability, CVE-2021-26855

CVE-2021-26084

 

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

CVE-2021-22893

Pulse Secure

PCS 9.0R3/9.1R1 and Higher

Pulse Secure SA44784 – 2021-04: Out-of-Cycle Advisory: Multiple Vulnerabilities Resolved in Pulse Connect Secure 9.1R11.4

CCCS Alert  Active Exploitation of Pulse Connect Secure Vulnerabilities – Update 1

CVE-2021-20016

SonicWall

SMA 100 devices (SMA 200, SMA 210, SMA 400, SMA 410, SMA 500v)

SonicWall Security Advisory SNWLID-2021-0001

 

CVE-2021-1675

Microsoft

Multiple Windows products; see Microsoft Security Update Guide Windows Print Spooler Remote Code Execution Vulnerability, CVE-2021-1675

Microsoft Security Update Guide: Windows Print Spooler Remote Code Execution Vulnerability, CVE-2021-1675

CCCS Alert Windows Print Spooler Vulnerability Remains Unpatched – Update 3

CVE-2020-2509

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

 

CVE-2020-1472

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)

Joint CSA APT Actors Chaining Vulnerabilities Against SLTT, Critical Infrastructure, and Elections Organizations

CCCS Alert Microsoft Netlogon Elevation of Privilege Vulnerability – CVE-2020-1472 – Update 1

CVE-2020-0688

Microsoft

Exchange Server, multiple versions; see Microsoft Security Update Guide: Microsoft Exchange Validation Key Remote Code Execution Vulnerability, CVE-2020-0688

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

Joint CSA Russian State-Sponsored Cyber Actors Target Cleared Defense Contractor Networks to Obtain Sensitive U.S. Defense Information and Technology

CCCS Alert Microsoft Exchange Validation Key Remote Code Execution Vulnerability

CVE-2019-19781

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

Joint CSA APT Actors Chaining Vulnerabilities Against SLTT, Critical Infrastructure, and Elections Organizations

CISA Alert Chinese Ministry of State Security-Affiliated Cyber Threat Actor Activity

CCCS Alert Detecting Compromises relating to Citrix CVE-2019-19781

 

 

 

CVE-2019-18935

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

 

CVE-2019-11510

Pulse Secure

Pulse Connect Secure 8.2 before 8.2R12.1, 8.3 before 8.3R7.1, and 9.0 before 9.0R3.4

Pulse Secure: SA44101 – 2019-04: Out-of-Cycle Advisory: Multiple vulnerabilities resolved in Pulse Connect Secure / Pulse Policy Secure 9.0RX

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

Joint CSA APT Actors Chaining Vulnerabilities Against SLTT, Critical Infrastructure, and Elections Organizations

CCCS Alert APT Actors Target U.S. and Allied Networks – Update 1

CVE-2018-13379

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

Joint CSA Russian State-Sponsored Cyber Actors Target Cleared Defense Contractor Networks to Obtain Sensitive U.S. Defense Information and Technology

Joint CSA Iranian Government-Sponsored APT Cyber Actors Exploiting Microsoft Exchange and Fortinet Vulnerabilities in Furtherance of Malicious Activities

Joint CSA APT Actors Chaining Vulnerabilities Against SLTT, Critical Infrastructure, and Elections Organizations

ACSC Alert APT exploitation of Fortinet Vulnerabilities

CCCS Alert Exploitation of Fortinet FortiOS vulnerabilities (CISA, FBI) – Update 1

CVE-2018-0171

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

CVE-2017-11882

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

CVE-2017-0199

Microsoft

Multiple products; see Microsoft Security Update Guide: Microsoft Office/WordPad Remote Code Execution Vulnerability w/Windows, CVE-2017-0199

Microsoft Security Update Guide: Microsoft Office/WordPad Remote Code Execution Vulnerability w/Windows, CVE-2017-0199

CCCS Microsoft Security Updates

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