CVE-2026-70188: Microsoft Security Advisory CVE-2026-70188 - ASP.NET Core Security Feature Bypass Vulnerability

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.App.Runtime.linux-arm (NuGet) >= 11.0.0-preview.1, <= 11.0.0-rc.1, fixed in 11.0.0-rc.2; Microsoft.AspNetCore.App.Runtime.linux-arm64 (NuGet) >= 11.0.0-preview.1, <= 11.0.0-rc.1, fixed in 11.0.0-rc.2; Microsoft.AspNetCore.App.Runtime.linux-musl-arm (NuGet) >= 11.0.0-preview.1, <= 11.0.0-rc.1, fixed in 11.0.0-rc.2; Microsoft.AspNetCore.App.Runtime.linux-musl-arm64 (NuGet) >= 11.0.0-preview.1, <= 11.0.0-rc.1, fixed in 11.0.0-rc.2. Check your lockfile, including transitive dependencies.

What it is

Inconsistent interpretation of http requests ('http request/response smuggling') in ASP.NET Core allows an unauthorized attacker to bypass a security feature over a network.

From the GitHub advisory

Executive summary

Microsoft is releasing this security advisory to provide information about a vulnerability in Microsoft.AspNetCore.Server.Kestrel.Core. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability.

Kestrel accepts newline characters in HTTP/2 request trailers and values retrieved from the dynamic HPACK table instead of treating them as malformed.

Announcement

Announcement for this issue can be found at https://github.com/dotnet/announcements/issues/462

Weakness type: CWE-444.

Product: Microsoft.AspNetCore.App.Runtime.linux-arm (NuGet) · Assigned by: microsoft · Published: 2026-10-13

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): 0.83% probability of exploitation activity in the next 30 days; percentile 57.

Timeline

• 2026-10-13 — CVE record published by microsoft
• 2026-10-13 — GitHub-reviewed advisory GHSA-fpq8-4vhr-rcxx published (critical)
• 2026-10-21 — 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 Security Feature Bypass Vulnerability msrc.microsoft.com

Vulnerability records

1. GitHub Advisory GHSA-fpq8-4vhr-rcxx github.com
2. CVE.org record CVE-2026-70188 cve.org
3. NVD entry CVE-2026-70188 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com

Critical NuGet advisories · Today’s Security Digest