CVE-2021-31957: ASP.NET Core Denial of Service Vulnerability

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MEDIUM 5.9REMOTEWATCHLISTUPGRADEUPGRADEUPGRADE

CVE-2021-31957 is a medium-severity flaw (CVSS 5.9) affecting .net, .net core, visual studio 2019. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

ASP.NET Core Denial of Service Vulnerability

Why this priority

EPSS top 8%: very likely to be exploited
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	5.9 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	5.1% (top 8%)
CISA exploited list	No
Weakness	CWE-400
Published	2021-06-08
Last modified	2026-10-07

Publisher (CNA): Microsoft Corporation · Vendor: fedoraproject · microsoft

Last updated 2026-10-07 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-06-08 23:15 UTCPublished CVE record published
2026-10-07 17:16 UTCUpdated Record last modified

microsoft .net UPGRADE

Affected: >= 5.0 and <= 5.0.6

Fixed in: newer than 5.0.6

Safe to downgrade to: a release before 5.0

1. Check installed version

apt-cache policy .net || rpm -q .net || .net --version

2. UPGRADE to newer than 5.0.6 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade .net

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade .net

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 5.0 (Debian / Ubuntu)

apt-cache madison .net sudo apt install .net=<VERSION_BEFORE_5.0> sudo apt-mark hold .net

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list .net sudo dnf downgrade .net-<VERSION_BEFORE_5.0> sudo dnf versionlock add .net

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

microsoft .net core UPGRADE

Affected: >= 3.1 and <= 3.1.15

Fixed in: newer than 3.1.15

Safe to downgrade to: a release before 3.1

1. Check installed version

apt-cache policy .net-core || rpm -q .net-core || .net-core --version

2. UPGRADE to newer than 3.1.15 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade .net-core

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade .net-core

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 3.1 (Debian / Ubuntu)

apt-cache madison .net-core sudo apt install .net-core=<VERSION_BEFORE_3.1> sudo apt-mark hold .net-core

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list .net-core sudo dnf downgrade .net-core-<VERSION_BEFORE_3.1> sudo dnf versionlock add .net-core

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

microsoft visual studio 2019 UPGRADE

Affected: >= 16.0 and <= 16.10

Fixed in: newer than 16.10

Safe to downgrade to: a release before 16.0

1. Check installed version

apt-cache policy visual-studio-2019 || rpm -q visual-studio-2019 || visual-studio-2019 --version

2. UPGRADE to newer than 16.10 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade visual-studio-2019

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade visual-studio-2019

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 16.0 (Debian / Ubuntu)

apt-cache madison visual-studio-2019 sudo apt install visual-studio-2019=<VERSION_BEFORE_16.0> sudo apt-mark hold visual-studio-2019

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list visual-studio-2019 sudo dnf downgrade visual-studio-2019-<VERSION_BEFORE_16.0> sudo dnf versionlock add visual-studio-2019

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

microsoft visual studio 2019 DOWNGRADE

Affected: 8.10

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.10

1. Check installed version

apt-cache policy visual-studio-2019 || rpm -q visual-studio-2019 || visual-studio-2019 --version

2. No fixed release: DOWNGRADE to a release before 8.10 (Debian / Ubuntu)

apt-cache madison visual-studio-2019 sudo apt install visual-studio-2019=<VERSION_BEFORE_8.10> sudo apt-mark hold visual-studio-2019

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list visual-studio-2019 sudo dnf downgrade visual-studio-2019-<VERSION_BEFORE_8.10> sudo dnf versionlock add visual-studio-2019

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

fedoraproject fedora MITIGATE

Affected: 33 · 34

Fixed in: no fixed release known

Safe to downgrade to: a release before 33

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-31957?

.net: upgrade to newer than 5.0.6; .net core: upgrade to newer than 3.1.15; visual studio 2019: upgrade to newer than 16.10. Exact commands for each OS are on this page.

Is CVE-2021-31957 being exploited?

CVE-2021-31957 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-31957 affect?

CVE-2021-31957 affects .net, .net core, visual studio 2019. Severity is medium (CVSS 5.9). ASP.NET Core Denial of Service Vulnerability

How urgent is CVE-2021-31957?

Priority 54/100. EPSS top 8%: very likely to be exploited; Remotely exploitable, no login, no user action; On your watchlist

Software pages

.NET vulnerabilities · Visual Studio vulnerabilities

Related CVEs

CVE-2026-71328 HIGH 8.8 UPGRADE UPGRADE Heap-based buffer overflow in Visual Studio allows an unauthorized attacker to execute code over a network.
CVE-2026-69806 HIGH 7.0 UPGRADE UPGRADE Exposure of sensitive information to an unauthorized actor in .NET allows an authorized attacker to elevate pr
CVE-2026-69439 HIGH 8.8 UPGRADE UPGRADE Heap-based buffer overflow in .NET and Visual Studio allows an unauthorized attacker to elevate privileges ove
CVE-2026-69304 MEDIUM 5.9 UPGRADE UPGRADE Improper handling of highly compressed data (data amplification) in ASP.NET Core allows an unauthorized attack
CVE-2026-58649 MEDIUM 6.5 DOWNGRADE UPGRADE Origin validation error in .NET allows an unauthorized attacker to disclose information over a network.

References

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