CVE-2026-40372: Microsoft Security Advisory CVE-2026-40372 – ASP.NET Core Elevation of Privilege

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 0.8%, percentile 57: FIRST’s model estimates a 0.8% chance of exploitation activity in the next 30 days, higher than 57% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes Microsoft.AspNetCore.DataProtection (NuGet) >= 10.0.0, <= 10.0.6, fixed in 10.0.7. Check your lockfile, including transitive dependencies.

What it is

Improper verification of cryptographic signature in ASP.NET Core allows an unauthorized attacker to elevate privileges over a network.

From the GitHub advisory

Executive Summary:

A bug in Microsoft.AspNetCore.DataProtection 10.0.0-10.0.6 NuGet packages can give an attacker the opportunity to execute an Elevation of Privilege attack by forging authentication cookies, and also allows some protected payloads to be decrypted.

If an attacker used forged payloads to authenticate as a privileged user during the vulnerable window, they may have induced the application to issue legitimately-signed tokens (session refresh, API key, password reset link, etc.) to themselves. Those tokens remain valid after upgrading to 10.0.7 unless the DataProtection key ring is rotated.

This is comparable in capability to MS10-070, which exploited a similar padding-oracle condition in ASP.NET's legacy encryption infrastructure.

Announcement

Weakness type: CWE-347.

Product: Microsoft.AspNetCore.DataProtection (NuGet) · Assigned by: microsoft · Published: 2026-04-21

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation none, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.82% probability of exploitation activity in the next 30 days; percentile 57.

Timeline

• 2026-04-21 — CVE record published by microsoft
• 2026-04-23 — GitHub-reviewed advisory GHSA-9mv3-2cwr-p262 published (critical)
• 2026-10-15 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. ASP.NET Core Elevation of Privilege Vulnerability msrc.microsoft.com

Vulnerability records

1. GitHub Advisory GHSA-9mv3-2cwr-p262 github.com
2. CVE.org record CVE-2026-40372 cve.org
3. NVD entry CVE-2026-40372 nvd.nist.gov

Other references from the CVE record and advisory

1. access.redhat.com access.redhat.com
2. RHBZ#2460224 bugzilla.redhat.com
3. security.access.redhat.com security.access.redhat.com
4. github.com github.com
5. github.com github.com

Critical NuGet advisories · Today’s Security Digest