High

.NET System.Security.Cryptography.Cose spoofing / security feature bypass

IdentifiersCVE-2026-21218CWE-345

CVE-2026-21218 is a spoofing vulnerability in .NET, including .NET and Visual Studio-related distributions, caused by improper handling of a missing special element during processing of COSE-related data in applications using the System.Security.Cryptography.Cose package. The available advisory context indicates that affected applications on .NET 8.0, 9.0, and 10.0 may incorrectly accept a message that should be rejected because critical-header validation can be bypassed when the expected special element is absent. Microsoft classifies the issue as a spoofing vulnerability; the flaw is also described in advisory material as a security feature bypass affecting applications that rely on this package for message validation.

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 network attacker to spoof trusted data by bypassing critical-header validation in affected .NET applications. As a result, a service may accept a crafted COSE message it should have rejected, which can undermine message authenticity and integrity checks and may enable unauthorized access decisions or downstream data manipulation depending on how the application uses the validated content.

Mitigation

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

Identify applications that reference System.Security.Cryptography.Cose and prioritize patching those deployments. If immediate patching is not possible, reduce exposure by avoiding or disabling use of affected package versions where feasible, and apply compensating controls at trust boundaries so untrusted COSE messages are not accepted without additional validation. Applications that do not use System.Security.Cryptography.Cose are not affected.

Remediation

Patch, then assume compromise.

Update the affected System.Security.Cryptography.Cose package to a fixed version and redeploy the application. The provided advisory context identifies fixed versions as 8.0.2 for .NET 8, 9.0.13 for .NET 9, and 10.0.3 for .NET 10. Microsoft also recommends updating the corresponding .NET runtimes and/or SDKs, although the advisory states that package update is sufficient to remediate the vulnerability. Recompilation and redeployment of applications using the package are required after updating.

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

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

11 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