What is the CVSS score of CVE-2026-21218?

CVE-2026-21218 has a CVSS 3.1 base score of 7.5 (High). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N. EPSS exploitation probability: 0.0%.

How to fix or mitigate CVE-2026-21218?

Within 24 hours: Identify and inventory all .NET applications and services in production; assess which systems process authentication or handle sensitive transactions. Within 7 days: Implement network segmentation to isolate critical .NET systems; enable enhanced logging for authentication events; brief incident response team on escalation procedures. Within 30 days: Establish daily monitoring for vendor patch release announcements; prepare deployment procedures for rapid patching; conduct security testing of affected applications with compensating controls in place.

.Net CVE-2026-21218

HIGH

Improper Handling of Missing Special Element (CWE-166)

2026-02-10 secure@microsoft.com

GHSA-qvhc-9v3j-5rfw

Microsoft .NET .Net Net

7.5

CVSS 3.1 · NVD

View

Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

Access

Craft malicious .NET serialized object
Delivery

Omit required special element
Exploit

Send over network
Execution

Application deserializes without validation
Impact

Spoof identity or data integrity

Vulnerability Assessment

Weakness	CWE CWE-166 OWASP Top 10:2025 A05:2025 - Injection ASVS 5.0 V1 - Encoding and Sanitization Whitespace handling
Exploitation Conditions	No special conditions — remote unauthenticated exploitation against .NET applications that deserialize untrusted network data without validating required special elements in serialized objects.
Impact	CVSS 7.5 (HIGH). Attack vector: Network. Complexity: Low. Privileges required: None. User interaction: None. Exploitation no authentication required. Exploitable via remote/network.
Exploit ScenarioDerived	A remote attacker without authentication could exploit this vulnerability to compromise the affected system.

This Week Schedule remediation this week. Why: HIGH severity (CVSS 7.0-8.9).

Patch available 0.0% exploitation probability (EPSS)

Get an e-mail when this CVE is exploited, patched or re-scored. Free account.

Alert me

AnalysisAIv1 – Mar 12, 2026

.NET applications are vulnerable to spoofing attacks due to improper validation of a required security element, allowing unauthenticated remote attackers to forge or manipulate application data over the network. This vulnerability affects multiple .NET versions and currently has no available patch, exposing organizations to authentication bypass and data integrity risks. The attack requires no user interaction and can be exploited directly from the network.

Earlier versions of this analysis (1)

v1 Mar 12, 2026 - 22:02 CVSS 7.5

Improper handling of missing special element in .NET allows an unauthorized attacker to perform spoofing over a network. [CVSS 7.5 HIGH]

RemediationAI

Monitor vendor advisories for a patch. Restrict network access to the affected service where possible.

Affected Packages

microsoft net_10_0_installed_on_linux_10_0_3 fix: .NET 10.0 installed on Linux 10.0.3

microsoft net_10_0_installed_on_mac_os_10_0_3 fix: .NET 10.0 installed on Mac OS 10.0.3

microsoft net_10_0_installed_on_windows_10_0_3 fix: .NET 10.0 installed on Windows 10.0.3

microsoft net_8_0_installed_on_linux_8_0_24 fix: .NET 8.0 installed on Linux 8.0.24

microsoft net_8_0_installed_on_mac_os_8_0_24 fix: .NET 8.0 installed on Mac OS 8.0.24

microsoft net_8_0_installed_on_windows_8_0_24 fix: .NET 8.0 installed on Windows 8.0.24

microsoft net_9_0_installed_on_linux_9_0_13 fix: .NET 9.0 installed on Linux 9.0.13

microsoft net_9_0_installed_on_mac_os_9_0_13 fix: .NET 9.0 installed on Mac OS 9.0.13

microsoft net_9_0_installed_on_windows_9_0_13 fix: .NET 9.0 installed on Windows 9.0.13

nuget system.security.cryptography.cose fix: 8.0.2, 9.0.13, 10.0.3

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 10 packages in microsoft, nuget

Checking this CVE against your own stacks and servers is part of the Basic plan and up.

Technical ContextAI · show details

Affects .Net. Improper handling of missing special element in .NET allows an unauthorized attacker to perform spoofing over a network.

Know when .Net gets its next vulnerability

Watch products and get an email when something new is published
Check this CVE against your own stack and servers
Exploit and PoC references, threat intelligence from 22 sources

Continue with Google Continue with GitHub

No credit card · 7-day full trial

Severity by source

NVD PRIMARY 7.5 HIGH

AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Primary rating from NVD · only source for this CVE.

CVSS VectorNVD

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

High

Availability

None

Lifecycle Timeline

Patch released

Mar 31, 2026 - 21:13 nvd

Patch available

CVE Published

Feb 10, 2026 - 18:16 nvd

HIGH 7.5

DescriptionCVE.org

Improper handling of missing special element in .NET allows an unauthorized attacker to perform spoofing over a network.

Executive ViewAI

Why It Matters

CVE-2026-21218 is a HIGH severity vulnerability affecting .NET applications that could allow attackers to spoof identities and bypass authentication controls over network communications. Organizations running unpatched .NET infrastructure face immediate risk of unauthorized access and potential data compromise, requiring urgent mitigation actions until an official patch becomes available.

Business Impact: High

Compensating Controls

Implement network segmentation to restrict .NET application communication to trusted networks only; deploy WAF rules to inspect and validate authentication tokens and session identifiers; enforce mutual TLS (mTLS) for all inter-service communications; enable strict input validation and output encoding on all user-supplied data; increase monitoring and alerting for failed authentication attempts and anomalous network activity; consider disabling or restricting the affected .NET feature if not critical to business operations; require multi-factor authentication for all privileged access; monitor vendor advisories daily for patch availability.

Threat Intelligence Overview

EPSS Score

0.0%

Top 92% exploitable

Data Quality

3 sources

Confidence: Low

NVD

CVE.org

EPSS

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue

Severity HIGH

CISA KEV No

Public POC No

EPSS 0.0%

Patch available Yes

Active threat No

Deciding rule

HIGH severity (CVSS 7.0-8.9)

What every action level means

Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.

Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.

This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.

This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.

Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.