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CVE Record

CVE-2021-26423: .NET Core and Visual Studio Denial of Service Vulnerability

.NET Core and Visual Studio Denial of Service Vulnerability

HighCVSS 7.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

This is a Microsoft .NET Core, .NET 5.0, PowerShell Core, and Visual Studio denial-of-service vulnerability. The main business risk is service or developer-tool disruption, not data theft. Microsoft rates it high severity, with network access and no authentication required in the CVSS vector.

Executive priority

Prioritize remediation in the normal high-severity patch cycle, faster for internet-facing services or critical build systems. The urgency is availability protection, not confirmed breach containment, based on the provided evidence.

Technical view

CVE-2021-26423 is scored CVSS 7.5: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. The provided sources identify denial of service only and do not describe the vulnerable code path or request pattern. The CVSS metadata indicates official remediation exists, but the bundle does not name exact fixed build numbers.

Likely exposure

Exposure is most likely where affected .NET Core 2.1/3.1, .NET 5.0, PowerShell Core 7.0/7.1, or listed Visual Studio 2017/2019 versions remain installed, especially on build systems, developer workstations, servers, and automation hosts.

Exploitation context

The source bundle does not show CISA KEV listing or cited active exploitation. CVSS marks exploit code maturity as unproven. Treat this as a high-impact availability risk, but do not assume exploitation in the wild from the provided evidence.

Researcher notes

The public bundle is sparse: no CWE, root cause, attack mechanics, or fixed version table is included here. Use the MSRC advisory as the authoritative remediation source and avoid deriving exploitability beyond the CVSS vector and KEV status supplied.

Mitigation direction

  • Review Microsoft MSRC guidance for CVE-2021-26423.
  • Upgrade affected Microsoft products to vendor-fixed supported builds.
  • Prioritize internet-facing .NET workloads and shared build infrastructure.
  • Retire unsupported .NET Core and PowerShell Core versions where possible.
  • Track completion through asset inventory and vulnerability management records.

Validation and detection

  • Inventory installed .NET, PowerShell Core, and Visual Studio versions.
  • Compare discovered versions against the affected product list.
  • Confirm patch status using Microsoft guidance, not CPE names alone.
  • Check whether exposed services depend on affected runtimes.
  • Document any remaining exception with owner and target date.
Prepared
Confidence
medium
Sources
3

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

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 · low confidence lookup

CVE-2021-26423 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

Open ATT&CK lookup
Vulnerability profileCVE Program record
Severity
High
CVSS
7.5 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H/E:U/RL:O/RC:C

Official CVE source material

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.

1CVSS vectors
3Timeline events
2ADP providers
2Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: yesTechnical Impact: partial

CVSS vector scores

1 official score

We 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.

ScoreVersionSeverityVectorExploitImpactSource
7.5CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H/E:U/RL:O/RC:C3.93.6microsoft

Vulnerability scoring details

Base CVSS 3.1 score

7.5High
CVSS 3.1 vector shape for CVE-2021-26423Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H/E:U/RL:O/RC:C

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CVECVE Program Container
CISA-ADPCISA ADP Vulnrichment
other:ssvc

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
MicrosoftMicrosoft Visual Studio 2017 version 15.9 (includes 15.0 - 15.8)15.9.0Listed
MicrosoftMicrosoft Visual Studio 2019 version 16.4 (includes 16.0 - 16.3)16.0Listed
MicrosoftMicrosoft Visual Studio 2019 version 16.7 (includes 16.0 – 16.6)16.0.0Listed
MicrosoftMicrosoft Visual Studio 2019 version 16.9 (includes 16.0 - 16.8)15.0.0Listed
MicrosoftMicrosoft Visual Studio 2019 version 16.10 (includes 16.0 - 16.9)16.10.0Listed
MicrosoftVisual Studio 2019 for Mac version 8.108.1.0Listed
Microsoft.NET Core 2.12.1Listed
Microsoft.NET Core 3.13.1Listed
Microsoft.NET 5.05.0.0Listed
MicrosoftPowerShell Core 7.17.1.0Listed
MicrosoftPowerShell Core 7.07.0.0Listed
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.