Security readout for executives and security teams
Plain-English summary
This is a denial-of-service flaw in Windows Hyper-V. If exploited, it could make affected virtualization hosts or services unavailable, but the provided sources do not indicate data theft or code execution. Business urgency is highest where affected Windows Server or Windows 10 systems run Hyper-V for production workloads.
Executive priority
Treat as high priority for environments relying on Hyper-V availability. The main business risk is service disruption to virtualized workloads, not confirmed data compromise. Patch exposed or production Hyper-V hosts first, then close remaining affected Windows systems through normal change control.
Technical view
Microsoft identifies CVE-2021-31977 as a Windows Hyper-V denial-of-service vulnerability with CVSS 8.6: network attack vector, low complexity, no privileges, no user interaction, changed scope, and high availability impact. Listed affected platforms include Windows 10 versions 1507 through 21H1 and Windows Server 2016, 2019, 2004, and 20H2 variants.
Likely exposure
Exposure is most likely on affected Windows systems with Hyper-V enabled, especially Windows Server 2016/2019 and listed Server Core installations supporting production virtualization. Systems not running Hyper-V are lower priority, but version and feature-state validation is still needed.
Exploitation context
The source bundle marks KEV as false and provides no cited evidence of active exploitation. CVSS indicates exploitation could be remote and unauthenticated, with availability impact only. No public exploit details or operational attacker behavior are included in the supplied evidence.
Researcher notes
The bundle lacks CWE detail, exploit narrative, and product-specific mitigation beyond Microsoft guidance. Analysis should stay anchored to CVSS availability impact and the enumerated affected CPEs. Validate exposure by OS build and Hyper-V role state rather than assuming every listed Windows system is materially vulnerable.
Mitigation direction
- Apply Microsoft's official fix referenced by the MSRC advisory.
- Prioritize production Hyper-V hosts and Server Core installations.
- Check Microsoft guidance for any version-specific prerequisites or supersedence.
- Plan maintenance windows for hosts supporting critical virtual machines.
- Document exceptions where affected systems cannot be updated promptly.
Validation and detection
- Inventory Windows hosts against the listed affected versions and builds.
- Confirm whether Hyper-V is enabled on each in-scope system.
- Verify installed Microsoft security updates cover CVE-2021-31977.
- Review virtualization monitoring for unexplained Hyper-V availability events.
- Track remediation status separately for production and non-production hosts.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CVE-2021-31977 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 8.6 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:U/RL:O/RC:C
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:U/RL:O/RC:C3.94Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
8.6HighVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:U/RL:O/RC:C
Source materials
- CVE List V5 sourceCVE List V5
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31977CVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.