Critical VMware ESXi VM Escape: Patch CVE-2025-41236 Now

Broadcom/VMware patches CVE-2025-41236 in ESXi 7.x/8.x, but Shadowserver counts show slow remediation as thousands of internet-reachable hosts remain on vulnerable builds.

A critical integer-overflow flaw (CVE-2025-41236) in VMware’s VMXNET3 adapter enables guest-to-host code execution on ESXi. Broadcom released fixes in July, yet internet scans still see more than 16,000–17,000 exposed, unpatched servers. Security teams are urged to patch to ESXi 7.0 U3w or 8.0 U2e/U3f immediately. Support PortalCoSecurity

A month after Broadcom/VMware shipped patches for a critical ESXi hypervisor flaw, thousands of organizations are still running vulnerable builds exposed to the internet—leaving virtualized infrastructure open to guest-to-host takeovers and data center-level compromise, according to new counts shared by researchers. Support PortalCoSecurity

• What happened: A critical integer-overflow vulnerability CVE-2025-41236 (CVSS 9.3) affects VMware ESXi, Workstation and Fusion through the VMXNET3 virtual NIC. Successful exploitation lets an attacker who has control of a guest VM execute code on the host. Broadcom rates the issue Critical and provides patches. Support PortalNVD
• How many are still exposed: Shadowserver-based tallies show 17,238 vulnerable ESXi IPs on July 19, with little progress by August 10 (~16,330). That suggests remediation is lagging despite available fixes. X (formerly Twitter)CoSecurity
• Fixed versions: Broadcom lists ESXi 7.0 U3w (build 24784741), ESXi 8.0 U2e (24789317), and ESXi 8.0 U3f (24784735) as remediated builds. Workstation 17.6.4 and Fusion 13.6.4 are also fixed. No workarounds are offered. Support Portal

“A malicious actor with local administrative privileges on a virtual machine with VMXNET3 may exploit this issue to execute code on the host.” — Broadcom VMSA-2025-0013. Support Portal

“Exploitation… can lead to code execution on the host.” — CISA advisory language describing CVE-2025-41236. CISA

“An integer-overflow vulnerability exists in the VMXNET3 virtual network adapter … CVSS 9.3.” — Qualys threat advisory. threatprotect.qualys.com

Technical Analysis

Vulnerability class & scope. CVE-2025-41236 is an integer overflow in VMXNET3 affecting ESXi 7.x/8.x, Workstation 17.x, Fusion 13.x. The flaw enables a VM escape path: crafted operations inside a guest with VMXNET3 can corrupt memory and achieve host-level code execution. Non-VMXNET3 adapters are not affected. Support Portalthreatprotect.qualys.com

Exploit conditions. The attack requires control of a guest VM (local admin in the guest is sufficient). In cloud and MSP environments, a compromised tenant VM can be leveraged to target the hypervisor, threatening co-tenants and management planes. Support Portal

Patching & versions. Hypervisor fixes are shipping as ESXi 7.0 U3w, 8.0 U2e, 8.0 U3f; desktop hypervisors require Workstation 17.6.4 / Fusion 13.6.4. VMware Tools updates also accompany the releases for vSockets info-leak (CVE-2025-41239). Support Portal

Why the big exposure numbers? Shadowserver’s daily internet scans detect unpatched ESXi builds still reachable over management surfaces; while CVE-2025-41236 itself is a guest-to-host vector (not an external unauthenticated HTTP bug), internet-facing ESXi often correlates with weak hardening and delayed patch cycles—raising the chance a threat actor first gets a foothold in a guest VM, then pivots to the host via this flaw. shadowserver.orgCoSecurity

Impact & Response

Who’s at risk:
Hosts running ESXi 7.x/8.x with VMXNET3 on any guest—especially multi-tenant clouds, VDI farms, and MSPs—face blast-radius compromise if a guest VM is controlled by an attacker. Internet-reachable management planes add additional risk. Support Portal

Immediate actions for defenders:

1. Patch now to ESXi 7.0 U3w / 8.0 U2e / 8.0 U3f (and Workstation 17.6.4 / Fusion 13.6.4). There are no vendor workarounds. Support Portal
2. Reduce attack surface: remove public exposure of ESXi management, enforce VPN + MFA, and restrict API access. shadowserver.org
3. Harden guests: ensure least-privilege in VMs; monitor for privilege escalation and unusual packet patterns hitting VMXNET3.
4. Threat hunt: review for anomalous vmx process behavior, unexpected hostd/vpxa restarts, and cross-tenant lateral movement attempts; validate firmware and tools are current.
5. Segment & contain: isolate tenant networks; consider pausing high-risk workloads until patched.

Business implications: A successful VM escape undermines isolation guarantees, enabling data theft, extortion, and downtime across clusters—particularly damaging for providers with co-located tenants and regulated workloads. Insurance and compliance reviews will scrutinize ESXi patch SLAs.

Background

Broadcom disclosed CVE-2025-41236 alongside three related issues (41237/41238/41239) on July 15, 2025, crediting Pwn2Own researchers. Fixed builds were released the same day with guidance for Cloud Foundation and telco platforms. External telemetry indicates slow global remediation despite the severity. Support PortalSecure-ISSCoSecurity

Conclusion

CVE-2025-41236 is a textbook “guest-to-host” breakout with critical impact and mature patches. The risk now hinges on time-to-patch. If you run ESXi with VMXNET3, treat this like a safety recall: update hypervisors, update desktop hypervisors, lock down management access—and verify. Support Portal