CVE-2026-20849
Microsoft Windows Server 2008 r2
Raw vector
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:HCVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.
Summary
CVE-2026-20849 is a high-severity Reliance on Untrusted Inputs in a Security Decision (CWE-807) vulnerability in Microsoft Windows Server 2008. Its CVSS base score is 7.5 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked in the top 39% of CVEs by exploit likelihood; it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to AC-24 (Access Control Decisions) and SI-10 (Information Input Validation) — see the control section below for these in your framework.
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-2026-20849 is a vulnerability in Windows Kerberos stemming from reliance on untrusted inputs in a security decision, classified under CWE-807. It affects the Windows Kerberos authentication component, enabling an authorized attacker to potentially elevate privileges. The vulnerability received a CVSS v3.1 base score of 7.5, reflecting network accessibility (AV:N), high attack complexity (AC:H), low privileges required (PR:L), no user interaction (UI:N), unchanged scope (S:U), and high impact on confidentiality, integrity, and availability (C:H/I:H/A:H). It was published on January 13, 2026.
An attacker with low-level authorized access, such as a domain user, can exploit this flaw over the network by manipulating untrusted inputs during Kerberos security decisions. Successful exploitation allows privilege escalation, potentially granting higher-level access within the Windows environment without requiring local presence or user interaction.
Microsoft's Security Response Center provides an update guide at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20849, which details recommended mitigations and patches for addressing the vulnerability.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-2160
Vulnerability Data
Reliance on untrusted inputs in a security decision in Windows Kerberos allows an authorized attacker to elevate privileges over a network.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
—
—
- 1 hardening rule · 1 OS baseline
—
Mitigating Controls (NIST 800-53 r5) AI
Explicitly requires that every access decision be based on authoritative, protected decision data rather than caller-supplied inputs.
Input validation can reject malformed or attacker-controlled values before they reach a security decision point.
Access enforcement requires decisions to be made from trusted policy data rather than modifiable client-supplied inputs.
Information-flow enforcement applies rules to validated, internal attributes instead of untrusted external values.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Strong authentication mechanisms directly prevent security decisions from depending on modifiable, untrusted inputs.
Verification and protection of identity assertions stops reliance on attacker-controlled values for authorization decisions.
Proper management of identities/credentials reduces the chance that security decisions will be driven by untrusted inputs.
Policy-driven, least-privilege authorization reduces opportunities to bypass controls via tampered inputs.
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.
Secure coding standards directly prohibit using untrusted inputs for security-critical decisions.
Security testing in development catches input-validation flaws before deployment.
Monitoring can detect exploitation but does not stop the underlying weakness.
Application security requirements explicitly call for validation of inputs used in security decisions.
Secure architecture principles mandate treating all external inputs as untrusted.
Access-control rules can be bypassed if they rely on untrusted inputs.