Original release date: July 28, 2021
This Joint Cybersecurity Advisory was coauthored by the U.S. Cybersecurity and Infrastructure Security Agency (CISA), the Australian Cyber Security Centre (ACSC), the United Kingdom’s National Cyber Security Centre (NCSC), and the U.S. Federal Bureau of Investigation (FBI).
This advisory provides details on the top 30 vulnerabilities—primarily Common Vulnerabilities and Exposures (CVEs)—routinely exploited by malicious cyber actors in 2020 and those being widely exploited thus far in 2021.
Cyber actors continue to exploit publicly known—and often dated—software vulnerabilities against broad target sets, including public and private sector organizations worldwide. However, entities worldwide can mitigate the vulnerabilities listed in this report by applying the available patches to their systems and implementing a centralized patch management system.
Click here for a PDF version of this report.
In 2020, cyber actors readily exploited recently disclosed vulnerabilities to compromise unpatched systems. Based on available data to the U.S. Government, a majority of the top vulnerabilities targeted in 2020 were disclosed during the past two years. Cyber actor exploitation of more recently disclosed software flaws in 2020 probably stems, in part, from the expansion of remote work options amid the COVID-19 pandemic. The rapid shift and increased use of remote work options, such as virtual private networks (VPNs) and cloud-based environments, likely placed additional burden on cyber defenders struggling to maintain and keep pace with routine software patching.
Four of the most targeted vulnerabilities in 2020 affected remote work, VPNs, or cloud-based technologies. Many VPN gateway devices remained unpatched during 2020, with the growth of remote work options challenging the ability of organization to conduct rigorous patch management.
CISA, ACSC, the NCSC, and FBI consider the vulnerabilities listed in table 1 to be the topmost regularly exploited CVEs by cyber actors during 2020.
Table 1:Top Routinely Exploited CVEs in 2020
In 2021, malicious cyber actors continued to target vulnerabilities in perimeter-type devices. Among those highly exploited in 2021 are vulnerabilities in Microsoft, Pulse, Accellion, VMware, and Fortinet.
CISA, ACSC, the NCSC, and FBI assess that public and private organizations worldwide remain vulnerable to compromise from the exploitation of these CVEs. Malicious cyber actors will most likely continue to use older known vulnerabilities, such as CVE-2017-11882 affecting Microsoft Office, as long as they remain effective and systems remain unpatched. Adversaries’ use of known vulnerabilities complicates attribution, reduces costs, and minimizes risk because they are not investing in developing a zero-day exploit for their exclusive use, which they risk losing if it becomes known.
Organizations are encouraged to remediate or mitigate vulnerabilities as quickly as possible to reduce the risk of exploitation. Most can be remediated by patching and updating systems. Organizations that have not remediated these vulnerabilities should investigate for the presence of IOCs and, if compromised, initiate incident response and recovery plans. See the Contact Information section below for how to reach CISA to report an incident or request technical assistance.
CISA, ACSC, the NCSC, and FBI have identified the following as the topmost exploited vulnerabilities by malicious cyber actors from 2020: CVE-2019-19781, CVE-2019-11510, CVE-2018-13379, CVE-2020-5902, CVE-2020-15505, CVE-2020-0688, CVE-2019-3396, CVE-2017-11882, CVE-2019-11580, CVE-2018-7600, CVE 2019-18935, CVE-2019-0604, CVE-2020-0787, CVE-2020-1472.[1][2][3] Among these vulnerabilities, CVE-2019-19781 was the most exploited flaw in 2020, according to U.S. Government technical analysis.CVE-2019-19781 is a recently disclosed critical vulnerability in Citrix’s Application Delivery Controller (ADC)—a load balancing application for web, application, and database servers widely use throughout the United States.[4][5] Nation-state and criminal cyber actors most likely favor using this vulnerability because it is easy to exploit, Citrix servers are widespread, and exploitation enables the actors to perform unauthorized RCE on a target system.[6]
Identified as emerging targets in early 2020,[7] unremediated instances of CVE-2019-19781 and CVE-2019-11510 continued to be exploited throughout the year by nation-state advanced persistent threat actors (APTs) who leveraged these and other vulnerabilities, such as CVE-2018-13379[8][9], in VPN services[10][11] to compromise an array of organizations, including those involved in COVID-19 vaccine development.[12][13]
The CVE-2019-11510 vulnerability in Pulse Connect Secure VPN was also frequently targeted by nation-state APTs. Actors can exploit the vulnerability to steal the unencrypted credentials for all users on a compromised Pulse VPN server and retain unauthorized credentials for all users on a compromised Pulse VPN server and can retain unauthorize access after the system is patched unless all compromised credentials are changed. Nation-state APTs also commonly exploited CVE-2020-15505 and CVE-2020-5902.[14][15][16][17]
In 2021, cyber actors continued to target vulnerabilities in perimeter-type devices. In addition to the 2020 CVEs listed above, organizations should prioritize patching for the following CVEs known to be exploited.
One of the most effective best practices to mitigate many vulnerabilities is to update software versions once patches are available and as soon as is practicable. If this is not possible, consider applying temporary workarounds or other mitigations, if provided by the vendor. If an organization is unable to update all software shortly after a patch is released, prioritize implementing patches for CVEs that are already known to be exploited or that would be accessible to the largest number of potential attackers (such as internet-facing systems). This advisory highlights vulnerabilities that should be considered as part of the prioritization process. To further assist remediation, automatic software updates should be enabled whenever possible.
Focusing scarce cyber defense resources on patching those vulnerabilities that cyber actors most often use offers the potential of bolstering network security while impeding our adversaries’ operations. For example, nation-state APTs in 2020 extensively relied on a single RCE vulnerability discovered in the Atlassian Crow, a centralized identity management and application (CVE-2019-11580) in its reported operations. A concerted focus on patching this vulnerability could have a relative broad impact by forcing the actors to find alternatives, which may not have the same broad applicability to their target set.
Additionally, attackers commonly exploit weak authentication processes, particularly in external-facing devices. Organizations should require multi-factor authentication to remotely access networks from external sources, especially for administrator or privileged accounts.
Tables 2–14 provide more details about, and specific mitigations for, each of the top exploited CVEs in 2020.
Note: The lists of associated malware corresponding to each CVE below are not meant to be exhaustive but intended to identify a malware family commonly associated with exploiting the CVE.
Table 2: CVE-2019-19781 Vulnerability Details
Summary
This Joint Cybersecurity Advisory was coauthored by the U.S. Cybersecurity and Infrastructure Security Agency (CISA), the Australian Cyber Security Centre (ACSC), the United Kingdom’s National Cyber Security Centre (NCSC), and the U.S. Federal Bureau of Investigation (FBI).
This advisory provides details on the top 30 vulnerabilities—primarily Common Vulnerabilities and Exposures (CVEs)—routinely exploited by malicious cyber actors in 2020 and those being widely exploited thus far in 2021.
Cyber actors continue to exploit publicly known—and often dated—software vulnerabilities against broad target sets, including public and private sector organizations worldwide. However, entities worldwide can mitigate the vulnerabilities listed in this report by applying the available patches to their systems and implementing a centralized patch management system.
Click here for a PDF version of this report.
Technical Details
Key Findings
In 2020, cyber actors readily exploited recently disclosed vulnerabilities to compromise unpatched systems. Based on available data to the U.S. Government, a majority of the top vulnerabilities targeted in 2020 were disclosed during the past two years. Cyber actor exploitation of more recently disclosed software flaws in 2020 probably stems, in part, from the expansion of remote work options amid the COVID-19 pandemic. The rapid shift and increased use of remote work options, such as virtual private networks (VPNs) and cloud-based environments, likely placed additional burden on cyber defenders struggling to maintain and keep pace with routine software patching.
Four of the most targeted vulnerabilities in 2020 affected remote work, VPNs, or cloud-based technologies. Many VPN gateway devices remained unpatched during 2020, with the growth of remote work options challenging the ability of organization to conduct rigorous patch management.
CISA, ACSC, the NCSC, and FBI consider the vulnerabilities listed in table 1 to be the topmost regularly exploited CVEs by cyber actors during 2020.
Table 1:Top Routinely Exploited CVEs in 2020
Vendor | CVE | Type |
---|---|---|
Citrix | CVE-2019-19781 | arbitrary code execution |
Pulse | CVE 2019-11510 | arbitrary file reading |
Fortinet | CVE 2018-13379 | path traversal |
F5- Big IP | CVE 2020-5902 | remote code execution (RCE) |
MobileIron | CVE 2020-15505 | RCE |
Microsoft | CVE-2017-11882 | RCE |
Atlassian | CVE-2019-11580 | RCE |
Drupal | CVE-2018-7600 | RCE |
Telerik | CVE 2019-18935 | RCE |
Microsoft | CVE-2019-0604 | RCE |
Microsoft | CVE-2020-0787 | elevation of privilege |
Netlogon | CVE-2020-1472 | elevation of privilege |
In 2021, malicious cyber actors continued to target vulnerabilities in perimeter-type devices. Among those highly exploited in 2021 are vulnerabilities in Microsoft, Pulse, Accellion, VMware, and Fortinet.
CISA, ACSC, the NCSC, and FBI assess that public and private organizations worldwide remain vulnerable to compromise from the exploitation of these CVEs. Malicious cyber actors will most likely continue to use older known vulnerabilities, such as CVE-2017-11882 affecting Microsoft Office, as long as they remain effective and systems remain unpatched. Adversaries’ use of known vulnerabilities complicates attribution, reduces costs, and minimizes risk because they are not investing in developing a zero-day exploit for their exclusive use, which they risk losing if it becomes known.
Organizations are encouraged to remediate or mitigate vulnerabilities as quickly as possible to reduce the risk of exploitation. Most can be remediated by patching and updating systems. Organizations that have not remediated these vulnerabilities should investigate for the presence of IOCs and, if compromised, initiate incident response and recovery plans. See the Contact Information section below for how to reach CISA to report an incident or request technical assistance.
2020 CVEs
CISA, ACSC, the NCSC, and FBI have identified the following as the topmost exploited vulnerabilities by malicious cyber actors from 2020: CVE-2019-19781, CVE-2019-11510, CVE-2018-13379, CVE-2020-5902, CVE-2020-15505, CVE-2020-0688, CVE-2019-3396, CVE-2017-11882, CVE-2019-11580, CVE-2018-7600, CVE 2019-18935, CVE-2019-0604, CVE-2020-0787, CVE-2020-1472.[1][2][3] Among these vulnerabilities, CVE-2019-19781 was the most exploited flaw in 2020, according to U.S. Government technical analysis.CVE-2019-19781 is a recently disclosed critical vulnerability in Citrix’s Application Delivery Controller (ADC)—a load balancing application for web, application, and database servers widely use throughout the United States.[4][5] Nation-state and criminal cyber actors most likely favor using this vulnerability because it is easy to exploit, Citrix servers are widespread, and exploitation enables the actors to perform unauthorized RCE on a target system.[6]
Identified as emerging targets in early 2020,[7] unremediated instances of CVE-2019-19781 and CVE-2019-11510 continued to be exploited throughout the year by nation-state advanced persistent threat actors (APTs) who leveraged these and other vulnerabilities, such as CVE-2018-13379[8][9], in VPN services[10][11] to compromise an array of organizations, including those involved in COVID-19 vaccine development.[12][13]
The CVE-2019-11510 vulnerability in Pulse Connect Secure VPN was also frequently targeted by nation-state APTs. Actors can exploit the vulnerability to steal the unencrypted credentials for all users on a compromised Pulse VPN server and retain unauthorized credentials for all users on a compromised Pulse VPN server and can retain unauthorize access after the system is patched unless all compromised credentials are changed. Nation-state APTs also commonly exploited CVE-2020-15505 and CVE-2020-5902.[14][15][16][17]
2021 CVEs
In 2021, cyber actors continued to target vulnerabilities in perimeter-type devices. In addition to the 2020 CVEs listed above, organizations should prioritize patching for the following CVEs known to be exploited.
- Microsoft Exchange: CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065
- See CISA’s Alert: Mitigate Microsoft Exchange Server Vulnerabilities for more information on identifying and mitigating malicious activity concerning these vulnerabilities.
- Pulse Secure: CVE-2021-22893, CVE-2021-22894, CVE-2021-22899, and CVE-2021-22900
- See CISA’s Alert: Exploitation of Pulse Connect Secure Vulnerabilities for more information on how to investigate and mitigate this malicious activity.
- Accellion: CVE-2021-27101, CVE-2021-27102, CVE-2021-27103, CVE-2021-27104
- See the Australia-New Zealand-Singapore-UK-U.S. Joint Cybersecurity Advisory: Exploitation of Accellion File Transfer Appliance for technical details and mitigations.
- VMware: CVE-2021-21985
- See CISA’s Current Activity: Unpatched VMware vCenter Software for more information and guidance.
- Fortinet: CVE-2018-13379, CVE-2020-12812, and CVE-2019-5591
- See the CISA-FBI Joint Cybersecurity Advisory: APT Actors Exploit Vulnerabilities to Gain Initial Access for Future Attacks for more details and mitigations.
Mitigations and Indicators of Compromise
One of the most effective best practices to mitigate many vulnerabilities is to update software versions once patches are available and as soon as is practicable. If this is not possible, consider applying temporary workarounds or other mitigations, if provided by the vendor. If an organization is unable to update all software shortly after a patch is released, prioritize implementing patches for CVEs that are already known to be exploited or that would be accessible to the largest number of potential attackers (such as internet-facing systems). This advisory highlights vulnerabilities that should be considered as part of the prioritization process. To further assist remediation, automatic software updates should be enabled whenever possible.
Focusing scarce cyber defense resources on patching those vulnerabilities that cyber actors most often use offers the potential of bolstering network security while impeding our adversaries’ operations. For example, nation-state APTs in 2020 extensively relied on a single RCE vulnerability discovered in the Atlassian Crow, a centralized identity management and application (CVE-2019-11580) in its reported operations. A concerted focus on patching this vulnerability could have a relative broad impact by forcing the actors to find alternatives, which may not have the same broad applicability to their target set.
Additionally, attackers commonly exploit weak authentication processes, particularly in external-facing devices. Organizations should require multi-factor authentication to remotely access networks from external sources, especially for administrator or privileged accounts.
Tables 2–14 provide more details about, and specific mitigations for, each of the top exploited CVEs in 2020.
Note: The lists of associated malware corresponding to each CVE below are not meant to be exhaustive but intended to identify a malware family commonly associated with exploiting the CVE.
Table 2: CVE-2019-19781 Vulnerability Details
Citrix Netscaler Directory Traversal (CVE-2019-19781) |
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