CVE-2021-1723Network attack vector

Service interruption via malformed HTTP/2 requests

Published Jan 12, 2021 · Updated May 28, 2026

HTTP/2 parsing in Microsoft ASP.NET Core 3.1 through 3.1.10 and 5.0 through 5.0.1 allows remote attackers to interrupt Kestrel. Microsoft identifies Kestrel's HTTP/2 request parser as the locus but has not published the malformed field or incorrect parsing operation. Any network client that can reach an HTTP/2 endpoint can trigger the availability-only condition; affected runtimes can be standalone, included with Visual Studio 2019, or deployed self-contained.

CVSS severity7.5
High
EPSS probability4.91%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP/2 parsing in Microsoft ASP.NET Core 3.1 through 3.1.10 and 5.0 through 5.0.1 allows remote attackers to interrupt Kestrel. Microsoft identifies Kestrel's HTTP/2 request parser as the locus but has not published the malformed field or incorrect parsing operation. Any network client that can reach an HTTP/2 endpoint can trigger the availability-only condition; affected runtimes can be standalone, included with Visual Studio 2019, or deployed self-contained.

The record

CVE
CVE-2021-1723
Published
Jan 12, 2021
Updated
May 28, 2026
Vendor
Microsoft
Product
Microsoft Visual Studio 2019
Classifications
T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 12, 2021CVE publishedPublication date reported by the CVE source.
  2. May 28, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=16.0.0 <publication
  2. Affected versionversion=16.0 <publication

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Microsoft · Microsoft Visual Studio 2019version=16.0.0 <publication; version=16.0 <publication
Microsoft · ASP.NET Coreversion=3.0 <publication; version=5.0 <publication

Published CVSS scores

7.5Microsoft CorporationCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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