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VMware Fixes Fusion Vulnerability Introduced by Previous Patch

An update released last week by VMware for the macOS version of Fusion attempts to fix a serious privilege escalation vulnerability introduced by a previous patch.

VMware informed customers in mid-March that it had patched a high-severity privilege escalation vulnerability in Fusion, Remote Console (VMRC) and Horizon Client for Mac. The flaw, tracked as CVE-2020-3950, can be exploited by an attacker with regular user privileges to escalate privileges to root.

The researchers who independently reported the issue to VMware, Rich Mirch and Jeffball, immediately noted that the patch was incomplete. VMware confirmed that the patch was incomplete a few days later.

Roughly one week after the initial patch was released, VMware made another attempt at fixing the vulnerability, but this second fix introduced a new vulnerability.

This new flaw, tracked as CVE-2020-3957, is described as a time-of-check time-of-use (TOCTOU) bug that still makes it possible for an attacker with low permissions to execute arbitrary code with root privileges.

VMware attempted to patch the TOCTOU vulnerability in Fusion last week with the release of version 11.5.5, but patches for VMRC and Horizon Client for Mac are pending.

Mirch, who plans on publishing a blog post and a new proof-of-concept (PoC) exploit for the vulnerability in the upcoming days, told SecurityWeek that his initial tests showed that the patch works. He says he has yet to perform a full assessment.

In addition to this vulnerability, VMware informed customers last week that it has released updates for ESXi, Workstation and Fusion to address a couple of medium-severity denial-of-service (DoS) vulnerabilities.

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.