CVE-2026-23666: Improper input validation in .NET Framework allows an unauthorized attacker to deny service over a network.

This month

HIGH 7.5REMOTEWATCHLISTDOWNGRADE

CVE-2026-23666 is a high-severity flaw (CVSS 7.5) affecting .net framework. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Improper input validation in .NET Framework allows an unauthorized attacker to deny service over a network.

Why this priority

High severity
Remotely exploitable, no login, no user action
On your watchlist

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.3% (top 29%)
CISA exploited list	No
Weakness	CWE-755, CWE-366
Published	2026-04-14
Last modified	2026-10-26

Publisher (CNA): Microsoft Corporation · Vendor: microsoft

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-14 18:16 UTCPublished CVE record published
2026-10-26 21:10 UTCUpdated Record last modified

microsoft .net framework DOWNGRADE

Affected: 3.5 · 4.7.2 · 4.8 · 4.8.1 · 4.6.2 · 4.7 · 4.7.1

Fixed in: no fixed release known

Safe to downgrade to: a release before 3.5

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 3.5 (Debian / Ubuntu)

apt-cache madison .net-framework sudo apt install .net-framework=<VERSION_BEFORE_3.5> sudo apt-mark hold .net-framework

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

dnf --showduplicates list .net-framework sudo dnf downgrade .net-framework-<VERSION_BEFORE_3.5> sudo dnf versionlock add .net-framework

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.

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-2026-23666?

.net framework: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-23666 being exploited?

CVE-2026-23666 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-2026-23666 affect?

CVE-2026-23666 affects .net framework. Severity is high (CVSS 7.5). Improper input validation in .NET Framework allows an unauthorized attacker to deny service over a network.

How urgent is CVE-2026-23666?

Priority 54/100. High severity; Remotely exploitable, no login, no user action; On your watchlist

Software pages

.NET vulnerabilities

Related CVEs

CVE-2026-35433 HIGH 7.3 DOWNGRADE UPGRADE Improper input validation in .NET allows an unauthorized attacker to elevate privileges locally.
CVE-2026-96960 HIGH 7.5 UPGRADE DOWNGRADE Asymmetric resource consumption (amplification) in .NET allows an unauthorized attacker to deny service over a
CVE-2026-96959 HIGH 7.5 UPGRADE DOWNGRADE Allocation of resources without limits or throttling in .NET allows an unauthorized attacker to deny service o
CVE-2026-96958 HIGH 7.5 UPGRADE DOWNGRADE Inefficient algorithmic complexity in .NET allows an unauthorized attacker to deny service over a network.
CVE-2026-50659 MEDIUM 6.5 DOWNGRADE UPGRADE Improper encoding or escaping of output in .NET allows an authorized attacker to perform spoofing over a netwo

References

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