The Top 15 Routinely Exploited Vulnerabilities of 2021

The Top 15 Routinely Exploited Vulnerabilities of 2021

The Top 15 Routinely Exploited Vulnerabilities of 2021


Today, the Cybersecurity and Infrastructure Agency (CISA) published a joint advisory with the cybersecurity authorities of the United States, Australia, Canada, New Zealand, and the United Kingdom detailing 2021’s top routinely exploited vulnerabilities.

CISA noted that while some threat actors continued to “play the hits” by exploiting older vulnerabilities from 2020 and earlier, newer vulnerabilities were generally more popular targets.

Two conclusions can be drawn from this data: Older vulnerabilities are still targeted and organizations must patch these. There are no excuses here. 

However, patching older vulnerabilities is not enough as threat actors are detecting and exploiting new vulnerabilities faster than ever. Organizations must match or exceed the speed of threat actors to mitigate the risk of breaches or incidents.

CISA also notes that proof of concept (POC) code for most of the top exploited vulnerabilities was released within two weeks of the vulnerability disclosure. We strongly recommend that your patching policy reflect this speed – any critical vulnerabilities should aim to be patched within a 24/72 hour threshold due to the already existing weaponization of these vulnerabilities.  

Top 15 Routinely Exploited Vulnerabilities of 2021

*Patchable with Automox

Vulnerability Spotlights

CVE-2021-44228Perhaps the most well-documented vulnerability of 2021 was “Log4Shell,” a remote code execution vulnerability in the Apache Log4j library, a widely used open-source logging framework.

Disclosed in December of 2021, the vulnerability was quickly weaponized by threat actors, and when exploited gave them full control over the system to steal information, deploy ransomware or cryptocurrency miners, or conduct other malicious activity.

CVE-2021-26855, CVE-2021-26858, CVE-2021-26857, CVE-2021-27065These vulnerabilities, collectively referred to as ProxyLogon, affect Microsoft Exchange email servers. When chained, exploitation of these vulnerabilities allows for an unauthenticated attacker to execute arbitrary code on the affected server and gain access to the files, mailboxes, and credentials stored on the servers.

Advanced Persistent Threats (APT), such as Hafnium, weaponized these vulnerabilities quickly to gain access to vulnerable Exchange servers.

CVE-2021-34523, CVE-2021-34473, CVE-2021-31207Microsoft Exchange email servers were a popular target for threat actors in 2021. These vulnerabilities are collectively referred to as “ProxyShell” and when chained together allow attackers to execute arbitrary code on the server.

Recommended Mitigation

CISA included additional recommendations to help mitigate the above vulnerabilities and improve your security posture for the inevitable threats in 2022 and beyond:

Improve your vulnerability and configuration management 

  • Update software, operating systems, applications, and firmware with a centralized patch management system in a timely manner

  • Prioritize patching known exploited vulnerabilities first

  • Replace end-of-life software that is no longer supported by the vendors

  • If you’re unable to scan and patch internet-facing assets promptly, consider moving the services to a reputable cloud service provider or managed service provider (MSP)

Implement identity and access management

  • Enforce multi-factor authentication (MFA) for all users, especially those on VPN connections – no exceptions

  • Incorporate a regular (at least annual) review, update, and removal of privileged accounts

  • Adopt and configure access control with a “least privilege” principle

Implement protective controls and secure architecture

  • Configure and secure internet-facing network devices, disable unused and nonessential ports and protocols, encrypt network traffic, and disable unused network services and devices

  • Segment networks to help limit or block lateral movement by threat actors

  • Use private virtual local area networks (LANs)

  • Implement continuous monitoring and investigate abnormal activity

  • Reduce third-party applications with unique system or application builds, using them only when business-critical

  • Implement application allowlisting

And if you’re not already using Automox for patching, you can start a free trial to patch your Microsoft and Log4j vulnerabilities today. 

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