Nearly 19,000 ESXi Servers Still Vulnerable to CVE-2021-21974

Last updated at Fri, 10 Feb 2023 16:55:49 GMT

Last week, multiple organizations issued warnings that a ransomware campaign dubbed “ESXiArgs” was targeting VMware ESXi servers, allegedly by leveraging CVE-2021-21974—a nearly two-year-old heap overflow vulnerability. Two years. And yet, Rapid7 research has found that a significant number of ESXi servers likely remain vulnerable. We believe, with high confidence, that there are at least 18,581 vulnerable internet-facing ESXi servers at the time of this writing.

That 18,581 number is based on Project Sonar telemetry. We leverage the TLS certificate Recog signature to determine that a particular server is a legitimate ESXi server. Then, after removing likely honeypots from the results, we checked the build ids of the scanned servers against a list of vulnerable build ids.

Project Sonar is a Rapid7 research effort aimed at improving security through the active analysis of public networks. As part of the project, we conduct internet-wide surveys across more than 70 different services and protocols to gain insights into global exposure to common vulnerabilities.

We have also observed additional incidents targeting ESXi servers, unrelated to the ESXiArgs campaign, that possibly also leverage CVE-2021-21974. RansomExx2—a relatively new strain of ransomware written in Rust and targeting Linux has been observed exploiting vulnerable ESXi servers. According to a recent IBM Security X-Force report, ransomware written in Rust has lower antivirus detection rates compared to those written in more common languages.

CISA issues fix, sort of

The U.S. Cybersecurity and Infrastructure Security Agency (CISA) on Wednesday released a ransomware decryptor to help victims recover from ESXiArgs attacks. However, it's important to note the script is not a cure all and requires additional tools for a full recovery. Moreover, reporting suggests that the threat actor behind the campaign has modified their attack to mitigate the decryptor.

The script works by allowing users to unregister virtual machines that have been encrypted by the ransomware and re-register them with a new configuration file. However, you still need to have a backup of the encrypted parts of the VM to make a full restore.

The main benefit of the decryptor script is that it enables users to bring virtual machines back to a working state while data restore from backup occurs in the background. This is particularly useful for users of traditional backup tools without virtualization-based disaster recovery capabilities.

Rapid7's recommendations

Deny access to servers. Unless a service absolutely needs to be on the internet, do not expose it to the internet. Some victims of these attacks had these servers exposed to the open internet, but could have gotten just as much business value out of them by restricting access to allowlisted IP addresses. If you are running an ESXi server, or any server, default to denying access to that server except from trusted IP space.

Patch vulnerable ESXi Servers. VMware issued a patch for CVE-2021-21974 nearly two years ago. If you have unpatched ESXi servers in your environment, click on that link and patch them now.

Develop and adhere to a patching strategy. Patching undoubtedly has challenges. However, this event illustrates perfectly why it’s essential to have a patching strategy in place and stick to it.

Back up virtual machines. Make sure you have a backup solution in place, even for virtual machines. As noted above, the decryptor script issued by the CIA is only a partial fix. The only way to completely recover from attacks associated with CVE-2021-21974 is via operational backups. There are a wide variety of backup solutions available to protect virtual machines today.

Drew Burton contributed to this article.