Active Exploitation - CVE-2021-21985

THE THREAT

Security researchers have observed the active exploitation of the VMware vCenter vulnerability CVE-2021-21985 (CVSS: 9.8). Attacks have been identified as early as of June 4th. CVE-2021-21985 was publicly released on May 25th, 2021; exploitation allows for Remote Code Execution on the underlying operating system. In observed attacks, threat actors deployed web-shells on vulnerable internet facing vCenter servers.

Organizations that had not applied security patches to internet facing vCenter servers prior to June 4th are at an increased risk of exploitation. It is critical that organizations patch vulnerable assets as soon as possible.

What we’re doing about it

• MVS has plugins in place to identify this vulnerability
• eSentire security teams continue to track this topic for additional detection measures
• eSentire produced an advisory on CVE-2021-21985 on May 26th

What you should do about it

• After performing a business impact review, apply the relevant security patches provided by VMware
  • If patching is not currently possible, apply the available workarounds recommended by VMware until patches can be deployed

• Ensure that vCenter servers are not directly exposed to the internet

Additional information

CVE-2021-21985 impacts VMware vCenter Servers. The vulnerability is due to the Virtual SAN Health Check plugin, which is enabled by default in vCenter Servers. An unauthenticated attacker with access to port 443 may exploit the vulnerability to execute code with elevated privileges on the underlying operating system that hosts a vulnerable vCenter server.

There is multiple functioning Proof-of-Concept (PoC) exploit code. The availability of both technical details and exploit code increases the likelihood of widespread exploitation. While initial attacks were identified on June 4th, scanning for vulnerable devices has been ongoing since at least May 3rd.

At this point, attacks exploiting CVE-2021-21985 are believed to be opportunistic in nature. Exposing vCenter servers directly to the internet is not standard practice, but there are a high number of devices that can be identified through internet scanning services such as Shodan. Currently, nearly 6,000 vCenter Servers show as internet facing on Shodan.

As attacks exploiting CVE-2021-21985 mature, it is expected that exploitation will move from opportunistic to targeted. If an organization’s vCenter server is not internet facing, threat actors that have already established a foothold on the network may still exploit the vulnerability to execute code with high privileges. This makes patching all vulnerable devices critical, even if they are not exposed to the internet.

Details on real-world attacks are still minimal. eSentire is actively tracking this topic for additional details and detection opportunities.

References:

[1] https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-21985
[2] https://twitter.com/GossiTheDog/status/1400868390726733831
[3] https://kb.vmware.com/s/article/83829