High

.NET and Visual Studio Spoofing Vulnerability

IdentifiersCVE-2026-21218CWE-345

CVE-2026-21218 is a spoofing vulnerability affecting .NET and Visual Studio, specifically associated with the System.Security.Cryptography.Cose package in supported .NET 8.0, .NET 9.0, and .NET 10.0 versions. The flaw stems from improper handling of a missing special element during message processing, which can cause critical-header validation to be bypassed. As a result, an application or service may accept a crafted message that should have been rejected during verification.

For your environment

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation allows an unauthorized remote attacker to spoof trusted input by bypassing critical-header validation in affected message-processing logic. This can cause applications or services to accept maliciously crafted messages as valid, undermining trust decisions and potentially enabling unauthorized actions or data manipulation depending on how the application relies on COSE validation results.

Mitigation

If you can’t patch tonight, do this now.

Identify whether the application references and uses System.Security.Cryptography.Cose, as applications that do not use this package are not affected. Until patching is completed, reduce exposure by limiting acceptance of untrusted network-supplied COSE messages and enforcing additional application-layer validation of message structure and required protected elements before treating messages as trusted.

Remediation

Patch, then assume compromise.

Update the affected System.Security.Cryptography.Cose package to a fixed version. Reported patched versions are 8.0.2, 9.0.13, and 10.0.3. After updating the package, recompile and redeploy affected applications. Microsoft also recommends updating the corresponding runtime and/or SDKs, although the package update itself is identified as sufficient to address this vulnerability.

PUBLIC EXPLOITS

Exploits

No public exploits tracked yet. Mallory keeps watching.

VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

Microsoft CorporationNetapplication

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

12 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.

12 SOURCESView more in app

nsfocus globalNews

Mar 4, 2026

Microsoft’s February Security Update of High-Risk Vulnerability Notice for Multiple Products - NSFOCUS, Inc., a global network and cyber security leader, protects enterprises and carriers from advanced cyber attacks.

A denial-of-service vulnerability affecting .NET.

thecyberexpress com vulnerabilitiesNews

Feb 11, 2026

Microsoft Patch Tuesday February 2026 Fixes 6 Zero-Days

A vulnerability affecting .NET and Visual Studio with an 'Exploitation Unlikely' rating per the content.

bleeping computerNews

Feb 10, 2026

Microsoft February 2026 Patch Tuesday fixes 6 zero-days, 58 flaws

A spoofing vulnerability in .NET (no additional details provided in the content).

zerodayinitiative blogNews

Feb 10, 2026

Zero Day Initiative - The February 2026 Security Update Review

A spoofing vulnerability affecting .NET and Visual Studio.

What this page doesn’t show

The version that knows your environment.

This page is what’s public. Mallory adds the parts that aren’t: which of your assets are affected, which adversaries are exploiting it right now, which detections to deploy, and what to do tonight.

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Exposure mapping

Query your assets running an affected version, and investigate the blast radius.

Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity6

Community discussion across Reddit, Mastodon, and other social sources.

EXTERNAL REFERENCES

MITRE CVE

cve.org · CVE-2026-21218

NVD

nvd.nist.gov/vuln/detail/CVE-2026-21218

MITRE CWE · CWE-345

cwe.mitre.org

Vendor Advisory

msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21218

.NET and Visual Studio Spoofing Vulnerability

IdentifiersCVE-2026-21218CWE-345