CVE-2021-21995 in ESXi
Summary
by MITRE • 07/13/2021
OpenSLP as used in ESXi has a denial-of-service vulnerability due a heap out-of-bounds read issue. A malicious actor with network access to port 427 on ESXi may be able to trigger a heap out-of-bounds read in OpenSLP service resulting in a denial-of-service condition.
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Analysis
by VulDB Data Team • 10/31/2025
The vulnerability CVE-2021-21995 represents a critical heap out-of-bounds read flaw within the OpenSLP service component of VMware ESXi hypervisor systems. This issue stems from improper input validation and memory management within the Service Location Protocol implementation that ESXi utilizes for network service discovery. The vulnerability specifically affects the OpenSLP daemon running on port 427, which is part of VMware's implementation of the Service Location Protocol used for locating network services. The flaw manifests when the service processes malformed or specially crafted network packets that trigger memory access violations beyond the allocated heap boundaries.
The technical exploitation of this vulnerability occurs through network-based attacks targeting the exposed port 427 on ESXi systems. When a malicious actor sends carefully constructed packets to this port, the OpenSLP service fails to properly validate incoming data structures before accessing memory locations, resulting in a heap out-of-bounds read condition. This memory access violation causes the service to crash or terminate unexpectedly, leading to a denial-of-service state that affects the entire ESXi host's ability to provide virtualization services. The vulnerability falls under CWE-125: Out-of-bounds Read, which is classified as a memory safety issue that can lead to system instability and service disruption.
The operational impact of this vulnerability extends beyond simple service interruption as it can compromise the availability of critical virtualized environments. ESXi hosts running vulnerable versions become susceptible to remote exploitation without authentication requirements, making them attractive targets for attackers seeking to disrupt virtualized infrastructure. The denial-of-service condition can affect multiple virtual machines running on the compromised host, potentially causing cascading failures in larger virtualized deployments. According to ATT&CK framework, this vulnerability maps to T1499.004: Endpoint Denial of Service, where adversaries leverage service-level vulnerabilities to disrupt system availability. The impact is particularly severe in enterprise environments where ESXi hosts form the foundation of virtualized data centers and cloud infrastructure.
Mitigation strategies for CVE-2021-21995 primarily involve applying VMware's official security patches and updates that address the heap memory handling issues within the OpenSLP service. System administrators should immediately upgrade to patched versions of ESXi that contain fixes for this vulnerability, as VMware has released security bulletins specifically addressing this issue. Network segmentation and firewall rules can provide temporary protection by blocking access to port 427 from untrusted networks, though this approach does not eliminate the underlying vulnerability. Additionally, implementing network monitoring solutions that can detect anomalous traffic patterns on port 427 may help identify potential exploitation attempts. The vulnerability demonstrates the importance of maintaining up-to-date security patches in virtualization environments, as unpatched hypervisor components represent significant attack vectors for adversaries targeting enterprise infrastructure. Regular vulnerability assessments and penetration testing should be conducted to identify and remediate similar issues across the entire virtualized environment.