Security Resilience

CVE-2023-21818

Microsoft Windows Server 2008 r2

Published
14 February 2023
Modified
19 August 2026
Patch / advisory
CVSS Score v3.1 7.5
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS Score 0.43 99th percentile
Risk Priority 76 floored blend · peak EPSS

CVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.

Summary

CVE-2023-21818 is a high-severity Improper Input Validation (CWE-20) vulnerability in Microsoft Windows Server 2008. Its CVSS base score is 7.5 (High).

Operationally, ranked in the top 1% of CVEs by exploit likelihood; it is not currently listed in the CISA KEV catalog.

Deeper analysis AI-assisted summary

Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.

CVE-2023-21818 is a denial-of-service vulnerability in Windows Secure Channel, the component responsible for implementing TLS and other cryptographic protocols on Windows systems. It carries a CVSS 3.1 base score of 7.5 with a vector indicating network attackability, low complexity, no required privileges or user interaction, and high impact on availability.

An unauthenticated remote attacker can send specially crafted network traffic to a vulnerable Windows host and trigger the flaw, causing the Secure Channel service to stop responding and thereby disrupting any applications or services that rely on it for secure communications.

Microsoft has published guidance for the issue in its Security Response Center update guide, which includes details on available patches and recommended actions for affected Windows versions.

The associated EPSS score reached a peak of 0.4151, indicating a period of elevated exploitation interest after disclosure.

OWASP Top 10 for Web (2025)

EU & UK References

Vulnerability Data

Windows Secure Channel Denial of Service Vulnerability

CWE(s)

Related Threats

CVEs Like This One

CVE-2023-21816Same product: Microsoft Windows 10
CVE-2023-36697Same product: Microsoft Windows 10
CVE-2023-28302Same product: Microsoft Windows 10 1607
CVE-2023-21554Same product: Microsoft Windows 10 1607
CVE-2023-24856Same product: Microsoft Windows 10
CVE-2023-36912Same product: Microsoft Windows 10
CVE-2023-21685Same product: Microsoft Windows 10 1607
CVE-2023-35367Same product: Microsoft Windows 10 1607
CVE-2023-32057Same product: Microsoft Windows 10 1607
CVE-2023-35377Same product: Microsoft Windows 10 1607

Affected Assets

microsoft
windows 10
≤ 10.0.10240.19747
microsoft
windows 10 1607
≤ 10.0.14393.5717
microsoft
windows 10 1809
≤ 10.0.17763.4010
microsoft
windows 10 20h2
≤ 10.0.19042.2604
microsoft
windows 10 21h2
≤ 10.0.19044.2604
microsoft
windows 10 22h2
≤ 10.0.19045.2604
microsoft
windows 11 21h2
≤ 10.0.22000.1574
microsoft
windows server 2008
all versions, r2
microsoft
windows server 2012
all versions, r2
microsoft
windows server 2016
all versions
+2 more product configuration(s) — see NVD for full list

Mitigating Controls

Likely Mitigating Controls AI

Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.

addresses: CWE-20

Security testing and developer training directly verify and enforce proper input validation, reducing exploitability of injection and malformed-data weaknesses.

addresses: CWE-20

Security testing and evaluation at multiple SDLC stages directly detects missing or flawed input validation, with the required remediation process ensuring fixes are applied.

addresses: CWE-20

Directly implements checks on information inputs to reject invalid data before processing.

addresses: CWE-20

Spam protection mechanisms perform filtering and detection on inbound/outbound messages, directly compensating for missing or weak input validation of unsolicited content.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

blocks

A.8.25 mandates security requirements, secure coding guidelines, checkpoints, testing (incl. code scans) and developer training in the SDLC, which reaches a slice of input validation defects but leaves the majority (incomplete enforcement, second-order inputs, parser differentials, post-validation canonicalization) unaddressed as in the SA-3 and A.8.28 anchors.

blocks

Mandating input controls that include integrity checks and input validation ensures that untrusted data is examined before use, blocking the root cause of many injection and malformed-data weaknesses.

blocks

Security-by-design principles explicitly call for data validation and sanitization at every layer, reducing the chance that malformed or malicious input will be processed without scrutiny.

prevents

Requiring language-specific secure coding standards, peer review, SAST and documented mitigation of common programming errors forces validation of all inputs before they are trusted.

finds

Testing against a defined set of requirements and using code review plus vulnerability scanning forces validation of inputs and handling of unanticipated conditions, reducing the chance that malformed data will be accepted.

Hardening callouts derived

DISA STIG baselines are authored against specific weaknesses. Where this CVE cites a broad weakness class, the mapping is not asserted at that abstraction (owner ruling, 2026-09-25: grade only at Base and below), so no rule is shown here rather than a rule shown against the wrong grain.

  • No fine-grained STIG row for CWE-20, a class-level weakness, and none of its children carries one either.

References