The top 5 most routinely exploited vulnerabilities of 2021

The top 5 most routinely exploited vulnerabilities of 2021

A joint Cybersecurity Advisory, coauthored by cybersecurity authorities of the United States (CISA, NSA, and FBI), Australia (ACSC), Canada (CCCS), New Zealand (NZ NCSC), and the United Kingdom (NCSC-UK) has detailed the top 15 Common Vulnerabilities and Exposures (CVEs) routinely exploited by malicious cyber actors in 2021, as well as other CVEs frequently exploited.

Publicly disclosed computer security flaws are listed in the Common Vulnerabilities and Exposures (CVE) database. Its goal is to make it easier to share data across separate vulnerability capabilities (tools, databases, and services). These are the CVEs that made it into the top 10.

1. Log4Shell

CVE-2021-44228, commonly referred to as Log4Shellor Logjam. This was a software flaw in the Apache Log4j logging utility. A logger is a piece of software that logs every event that happens in a computer system. The records it produces are useful for IT and security folks to trace errors or check any abnormal behavior within a system.

When Log4Shell emerged in December 2021, what caught many by surprise was the enormous number of applications and web services, including those offered by Twitter, Apple, Google, Amazon, Steam, and Microsoft, among others, that were relying on Log4j, many of which inherited the vulnerability.

This made for an exceptionally broad attack surface. Combine that with an incredibly easy to use exploit and there should be no surprise that this vulnerability made it to the top of the list.

The Cybersecurity and Infrastructure Security Agency (CISA) has launched an open source scanner to find applications that are vulnerable to the Log4j vulnerabilities listed as CVE-2021-44228 and CVE-2021-45046. The CISA Log4j scanneris based on other open source tools and supports scanning lists of URLs, several fuzzing options, DNS callback, and payloads to circumvent web-application firewalls.

2. CVE-2021-40539

CVE-2021-40539is a REST API authentication bypass vulnerability in ManageEngine’s single sign-on (SSO) solutionwith resultant remote code execution (RCE) that exists in Zoho ManageEngine ADSelfService Plus version 6113 and prior. When word of this vulnerability came out it was already clear that it was being exploited in the wild. Zoho remarked that it was noticing indications of this vulnerability being exploited. Other researchers chimed in saying the attacks had thus far been highly targeted and limited, and possibly the work of a single threat actor. It was clear from the start that APTthreat-actors were likely among those exploiting the vulnerability.

The vulnerability allows an attacker to gain unauthorized access to the product through REST API endpoints by sending a specially crafted request. This allows attackers to carry out subsequent attacks resulting in RCE.

For those that have never heard of this software, it’s a self-service password management and single sign-on (SSO) solution for Active Directory (AD) and cloud apps. Which means that any attacker that is able to exploit this vulnerability immediately has access to some of the most critical parts of a corporate network. A patch for this vulnerability was made available on September 7, 2021. Users were advised to update to ADSelfService Plus build 6114. The FBI, CISA, and CGCYBER also strongly urged organizations to make sure that ADSelfService Plus was not directly accessible from the Internet.

The ManageEngine site has specific instructions on how to identify and update vulnerable installations.

3. ProxyShell

Third on the list are 3 vulnerabilities that we commonly grouped together and referred to as ProxyShell. CVE-2021-34523, CVE-2021-34473, and CVE-2021-31207.

The danger lies in the fact that these three vulnerabilities can be chained together to allow a remote attacker to run code on an unpatched Microsoft Exchange server. Attackers use them as follows:

  • Get in with CVE-2021-31207, a Microsoft Exchange Server security feature bypass vulnerability. The vulnerability allows a remote user to bypass the authentication process.
  • Take control with CVE-2021-34523, a Microsoft Exchange Server elevation of privilege (EoP) vulnerability. The vulnerability allows a user to raise their permissions.
  • Do bad things with CVE-2021-34473, a Microsoft Exchange Server remote code execution (RCE) vulnerability. The vulnerability allows an authenticated user to execute arbitrary code in the context of SYSTEM and write arbitrary files.

The vulnerabilities were found in Microsoft Exchange Server, which has a large userbase and which is usually set up as an Internet-facing instance. Plus, many publications have provided proof-of-concept (PoC) methodologies which anyone can copy and use.

Microsoft’s Security Update from May 2021 remediates all three ProxyShell vulnerabilities.

4. ProxyLogon

After the ProxyShell entries we go straight to four vulnerabilities that are grouped under a similar name—ProxyLogon—for similar reasons. CVE-2021-26855, CVE-2021-26857, CVE-2021-2685, and CVE-2021-27065all share the same description—”This vulnerability is part of an attack chain. The initial attack requires the ability to make an untrusted connection to Exchange server port 443.”

While the CVE description is the same for the 4 CVE’s we have learned that CVE-2021-26855 is a server-side request forgery (SSRF) vulnerability in Exchange that was used to steal mailbox content. The RCE vulnerability CVE-2021-26857 was used to run code under the System account. The other two zero-day flaws—CVE-2021-26858 and CVE-2021-27065—would allow an attacker to write a file to any part of the server.

Together these four vulnerabilities form an attack chain that only requires the attacker to find the server running Exchange, and the account from which they want to extract email. After exploiting these vulnerabilities to gain initial access, threat actors deployed web shells on the compromised servers to gain persistence and make more changes. Web shells can allow attackers to steal data and perform additional malicious actions.

ProxyLogon started out as a limited and targeted attack method attributed to a group called Hafnium. Unfortunately it went from limited and targeted attacks to a full-size panic in no time. Attackers started using the Exchange bugs to access vulnerable servers before establishing web shells to gain persistence and steal information.

Microsoft has released a one-click mitigation tool for Exchange Server deployments. The Microsoft Exchange On-Premises Mitigation Tool will help customers who do not have dedicated security or IT teams to apply these security updates. Details, a download link, user instructions, and more information can be found in the Microsoft Security Response Center.

5. CVE-2021-26084

CVE-2021-26084is an Object-Graph Navigation Language (OGNL) injection vulnerability that exists in some versions of Confluence Server and Data Centerthat can allow an unauthenticated attacker to execute arbitrary code on a Confluence Server or Data Center instance. This was a zero-day vulnerability that was only patched after it was found to be actively exploited in the wild. An attacker could exploit the vulnerability by simply sending a specially crafted HTTP request containing a malicious parameter to a vulnerable install.

Shortly after the vulnerability was disclosed and a patch came out, researchers noticed massive scanning activity for vulnerable instances and crypto-miners started to use the vulnerability to run their code on unpatched servers.

On the Confluence Support websiteyou can find a list of affected versions, instructions to upgrade, and a workaround for those that are unable to upgrade.

Lessons learned

What does this list tell us to look out for in 2022?

Well, first off, if you haven’t patched one of the above we would urgently advise you to do so. And it wouldn’t hurt to continue working down the listprovided by CISA.

Second, you may have noticed a pattern in what made these vulnerabilities so popular to exploit:

  • A large attack surface. Popular and widely used software makes for a larger number of potential victims. The money is in the numbers.
  • Internet-facing instances. Remember, your Internet-connected software shares the Internet with every basement-dwelling criminal hacker in the world.
  • Easy exploitability. When vulnerabilities are easy to exploit, and PoCs are publicly available and easy to deploy, the number of potential threat actors goes up.

So, if you notice or hear about a vulnerability that meets these “requirements” move it to the top of your “to-patch” list.

Stay safe, everyone!


Pieter Arntz

Malware Intelligence Researcher

Was a Microsoft MVP in consumer security for 12 years running. Can speak four languages. Smells of rich mahogany and leather-bound books.