Embattled VMware ESXi Hypervisor Flaw Exploitable in Myriad Ways

Security teams working to secure their organizations against a nearly two-year-old vulnerability in VMware's ESXi hypervisor technology that attackers suddenly began exploiting en masse last week must pay attention to all ESXi hosts in the environment, not just Internet-accessible ones.

That's the advice of security vendor Bitdefender after it analyzed the threat and discovered that attackers can exploit it in multiple ways.

Two-Year-Old Flaw

The vulnerability in question, CVE-2021-21974, is present in VMware's implementation of a service delivery protocol in ESXi called Open Service Location Protocol (OpenSLP). The vulnerability gives unauthenticated attackers the ability to remotely execute malicious code on affected systems without any user interaction.

VMware disclosed the vulnerability in February 2021 and issued a patch for it at the same time. Since then, attackers have targeted it heavily and made CVE-2021-29174 one of the most exploited vulnerabilities in 2021 and 2022. On Feb. 3, France's computer emergency response team warned about bad actors exploiting CVE-2021-21974 to distribute a ransomware variant dubbed ESXiArgs ransomware on ESXi hosts around the world.

The widespread nature of the attacks prompted the US Cybersecurity and Infrastructure Security Agency (CISA) to release a recovery script that victims of ESXiArgs could use to try to recover their systems.

Martin Zugec, technical solutions director at Bitdefender, says though the initial compromise vector remains unknown, a popular theory is that it is via direct exploitation through Internet-exposed port 427. VMware itself has recommended that if organizations cannot patch immediately, they should block access to port 427.

While that measure can slow down an adversary, it does not eliminate risk from the flaw entirely because attackers can exploit the vulnerability in other ways as well, Zugec says. If an organization blocks port 427, for instance, an attacker could still compromise one of the virtual machines running on an ESXi host via any existing vulnerability.

They could then escape the compromised virtual machine to exploit the vulnerability in OpenSLP and gain root access to the host, he says.

Other Ways to Exploit Flaw

"Threat actors can use any existing vulnerability to compromise a virtual machine — whether it's Linux or Windows-based," Zugec notes.

A threat actor can also relatively easily buy on the Dark Web access to a previously compromised virtual machine and attempt OpenSLP remote code execution against the hosting hypervisor, he says.

"If successful, the threat actor can gain access not only to the hypervisor host, but also to all other machines running on the same server," Zugec says. "The OpenSLP exploit in this case would allow a threat actor to escalate their access and move laterally to other — potentially more valuable — machines."

Zugec says Bitdefender has so far seen no evidence of attackers exploiting the VMware ESXi vulnerability in this manner. But, given the major focus on direct exploitation via port 427, Bitdefender wanted to warn the public about other methods to exploit this vulnerability, he says. In addition to blocking access to port 427, VMware has also recommended that organizations that cannot patch CVE-2021-21974 simply disable SLP where possible.

Shades of WannaCry

Bitdefender said its analysis of the latest attacks targeting CVE-2021-21974 suggest that the threat actors behind them are opportunistic and not very sophisticated. Many of the attacks appear completely automated in nature, from initial scans for vulnerable systems to ransomware deployment.

"We can compare this to WannaCry," Zugec notes. "While these attacks can reach a wide range of machines, the impact remains limited."

But more sophisticated threat actors would use the flaw in ESXi to conduct a much larger operation, he says. Initial access brokers, for instance, could deploy a remote Web shell and disable SLP service so other threat actors cannot exploit the same flaw. They could then simply lie in wait for the best opportunity to monetize their access. Potential options could include data theft, surveillance, and cryptojacking.

To fully address the risk of a cyberattack exploiting the VMware vuln, Bitdefender — like VMware and others — recommends that organizations apply the patch for it immediately.