15 most exploited vulnerabilities of 2021

Global cybersecurity authorities have published a joint advisory on the 15 Common Vulnerabilities and Exposures (CVEs) most routinely exploited by malicious cyber actors in 2021. The advisory is co-authored by the U.S. Cybersecurity and Infrastructure Security Agency (CISA), U.S. National Security Agency (NSA), U.S. 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).

The advisory warned that malicious cyber actors aggressively targeted newly disclosed critical software vulnerabilities against broad target sets, including public and private sector organizations worldwide, last year. What’s more, malicious actors also continued to exploit publicly known, dated software vulnerabilities.

Attackers target internet-facing systems with new exploits and dated vulnerabilities

“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,” the advisory read. 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, it continued.

To a lesser extent, malicious cyber actors also continued to exploit publicly known, dated software vulnerabilities, some of which were routinely exploited in 2020 or earlier, the advisory continued. “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.”

The UK’s NCSC CEO, Lindy Cameron, said, “NCSC and our allies are committed to raising awareness of vulnerabilities and presenting actionable solutions to mitigate them. This advisory places the power in the hands of network defenders to fix the most common cyber weaknesses in the public and private sector ecosystem.”

15 most exploited vulnerabilities in 2021

Among the 15 most targeted vulnerabilities of 2021 are infamous exploits Log4Shell, ProxyShell and ProxyLogon, which impact Apache Log4j and Microsoft Exchange Server. Others include vulnerabilities in products from VMware, Fortinet and Pulse Secure. The 15 most targeted vulnerabilities of 2021 were:

1. CVE-2021-44228 (Log4Shell): Remote code execution (RCE) vulnerability in Apache Log4j
2. CVE-2021-40539: RCE vulnerability in Zoho ManageEngine AD SelfService Plus
3. CVE-2021-34523 (ProxyShell): Elevation of privilege vulnerability in Microsoft Exchange Server
4. CVE-2021-34473 (ProxyShell): RCE vulnerability in Microsoft Exchange Server
5. CVE-2021-31207 (ProxyShell): Security feature bypass in Microsoft Exchange Server
6. CVE-2021-27065 (ProxyLogon): RCE vulnerability in Microsoft Exchange Server
7. CVE-2021-26858 (ProxyLogon): RCE vulnerability in Microsoft Exchange Server
8. CVE-2021-26857 (ProxyLogon): RCE vulnerability in Microsoft Exchange Server
9. CVE-2021-26855 (ProxyLogon): RCE vulnerability in Microsoft Exchange Server
10. CVE-2021-26084: Arbitrary code execution vulnerability in Atlassian Confluence Server and Data Center
11. CVE-2021-21972: RCE vulnerability in VMware vSphere Client
12. CVE-2020-1472 (ZeroLogon): Elevation of privilege vulnerability in Microsoft Netlogon Remote Protocol (MS-NRPC)
13. CVE-2020-0688: RCE vulnerability in Microsoft Exchange Server
14. CVE-2019-11510: Arbitrary file reading vulnerability in Pulse Secure Pulse Connect Secure
15. CVE-2018-13379: Path traversal vulnerability in Fortinet FortiOS and FortiProxy

In this list are three vulnerabilities that were 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, the advisory stated.

To mitigate the risks of falling victim to attacks that exploit such vulnerabilities, the advisory urged organizations to implement vulnerability and configuration management, identity and access management, and protective controls and architecture.