Critical RCE Flaws Affect VMware ESXi and vSphere Client — Patch Now

CVE-2019-19781 is a path-traversal flaw (classified CWE-22, though CISA's description calls it unspecified) in Citrix ADC (formerly NetScaler ADC), Citrix Gateway, and Citrix SD-WAN WANOP appliances that lets an unauthenticated remote attacker traverse directories via crafted requests and execute arbitrary commands on the appliance, typically with root privileges. It is triggered by sending specially crafted directory-traversal requests (crafted URLs/requests to the appliance's management or VPN endpoints), which lets the attacker write files and run commands with no credentials. Successful exploitation yields arbitrary code execution on the appliance, enabling theft of VPN/ADC credentials, lateral movement into the corporate network, and installation of persistent backdoors. Any organization running affected ADC, Gateway, or SD-WAN WANOP firmware is affected, with internet-facing gateways used for remote access at the highest risk. Exploitation is confirmed in the wild: the vulnerability is on CISA's KEV (added 2021-11-03) with known ransomware use, EPSS assigns near-certain (100.0%) probability of exploitation within 30 days, and no public PoC is listed despite confirmed abuse.

Do: Upgrade Citrix ADC, Gateway, and SD-WAN WANOP appliances to the fixed firmware builds listed in Citrix advisory CTX267020; if patching cannot be done immediately, apply Citrix's published interim mitigation and restrict internet exposure to the appliance. Because exploitation grants root code execution and persistence, after patching hunt for indicators of compromise (unexpected nsroot account, modified system files, crontab/scheduled entries), kill all active and inactive sessions, and rotate appliance and VPN credentials. Prioritize internet-facing gateways and comply with CISA's required action to apply vendor updates.

CVE-2020-3992 is a use-after-free (CWE-416) in the OpenSLP service used by VMware ESXi, rated critical at CVSS 9.8. An unauthenticated attacker with access to port 427 on an ESXi host's management network can send crafted SLP traffic that triggers the memory-reuse flaw and gains remote code execution on the hypervisor. Successful exploitation grants full control of the ESXi host, and attackers have used these OpenSLP flaws in the ESXiArgs ransomware campaign to encrypt the disks of hosted virtual machines. Affected products are ESXi 7.0, 6.7 and 6.5 prior to the October 2020 patch releases (as well as the related VMware Cloud Foundation). The flaw is in CISA's KEV catalog (added 2021-11-03) with ransomware use confirmed, and EPSS assigns an 83% probability of exploitation within 30 days (100th percentile); no public PoC is listed in the source data.

Do: Apply VMware's ESXi security patches: ESXi_7.0.1-0.0.16850804 (7.0), ESXi670-202010401-SG (6.7), or ESXi650-202010401-SG (6.5), or the corresponding VMware Cloud Foundation update, per CISA's required action. As interim mitigation, restrict or disable the SLP service and firewall port 427 so ESXi management interfaces are not reachable from the internet. Because ransomware use is confirmed, check hosts for signs of compromise; CISA has published an ESXiArgs recovery script for affected deployments.

CVE-2021-21972 is a remote code execution vulnerability in a plugin of the vSphere Client in VMware vCenter Server, underpinned by a path traversal flaw (CWE-23) in the plugin's file-upload functionality. It is triggered over the network through port 443: an attacker who can reach the vCenter web interface can submit crafted file-upload requests, traverse to arbitrary filesystem paths, and plant executable files on the underlying operating system. Successful exploitation yields unrestricted privileges on the vCenter host OS, giving attackers control of the central management platform for an organization's entire VMware vSphere virtualized estate. Any organization running an affected vCenter Server release whose port 443 is reachable from untrusted networks is exposed. The flaw is listed in CISA's Known Exploited Vulnerabilities catalog (added 2021-11-03) with known ransomware use and a 99.9% EPSS score (100th percentile), indicating active, widespread exploitation in the wild, though no public PoC is recorded in this data.

Do: Apply the vCenter Server updates published in VMware's advisory for CVE-2021-21972 as soon as possible, prioritizing internet-facing or partner-reachable vCenter instances. Until patched, restrict access to vCenter on port 443 to trusted management networks and review access logs for unauthenticated file-upload activity against the vSphere Client upload endpoint. Because ransomware operators have actively exploited this bug, hunt for signs of compromise such as webshells or unexpected new local accounts on vCenter appliances.

CVE-2021-21973 is a server-side request forgery (SSRF, CWE-918) in the vSphere Client (HTML5), caused by improper validation of URLs in a vCenter Server plugin. An unauthenticated attacker with network access to TCP port 443 can trigger it by sending a crafted POST request to the affected vCenter Server plugin, causing vCenter to make attacker-influenced internal requests. The direct impact is information disclosure (confidentiality only, CVSS 3.1 base 5.3), but SSRF in vCenter is frequently chained with other vCenter flaws to reach remote code execution, as reflected in the related advisories. Everyone running VMware vCenter Server 7.x/6.7/6.5 or VMware Cloud Foundation 4.x/3.x before the fixed releases is affected. The flaw is actively exploited: it is listed in CISA KEV (added 2022-03-07) and carries a very high EPSS (87.6%), with reporting of coordinated SSRF exploitation campaigns involving hundreds of source IPs.

Do: Upgrade vCenter Server to 7.0 U1c, 6.7 U3l, or 6.5 U3n, and Cloud Foundation to 4.2 or 3.10.1.2, per VMware's instructions. Until patched, restrict access to vCenter's port 443 from untrusted networks and review access logs for suspicious unauthenticated POST requests to the vSphere Client plugin endpoints. Given the KEV listing and reports of coordinated SSRF exploitation, treat patching as urgent even though this flaw alone yields information disclosure.

OpenSLP as used in ESXi (7.0 before ESXi70U1c-17325551, 6.7 before ESXi670-202102401-SG, 6.5 before ESXi650-202102101-SG) has a heap-overflow vulnerability. A malicious actor residing within the same network segment as ESXi who has access to port 427 may be able to trigger the heap-overflow issue in OpenSLP service resulting in remote code execution.

vSphere Replication 8.3.x prior to 8.3.1.2, 8.2.x prior to 8.2.1.1, 8.1.x prior to 8.1.2.3 and 6.5.x prior to 6.5.1.5 contain a post-authentication command injection vulnerability which may allow an authenticated admin user to perform a remote code execution.