Microsoft AD FS EUVD-2026-100389

Exploitation requires the AD FS sign-in/redirect endpoint to be reachable by the attacker (typically internet-facing in federation deployments) and, critically, requires a victim to click an attacker-crafted AD FS URL whose redirect parameter points to an untrusted site (UI:R). No authentication or privileges are needed by the attacker (PR:N), but the mandatory user interaction and the fact that the attacker cannot directly compromise the server (impact is spoofing/token-theft against the user) are the key limiting factors. Affected builds are specifically Windows Server 2016 (<10.0.14393.9612), 2019 (<10.0.17763.9376), 2022 (<10.0.20348.5761), 2025 (<10.0.26100.33650), and Windows 10 1607/1809 running the AD FS role; systems without the AD FS role deployed are not exploitable.

This is a moderate-severity open redirect (CWE-601) in Microsoft AD FS carrying a CVSS 3.1 base score of 6.5 with vector AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N. The network vector, low complexity, and no-privilege requirements favor the attacker, but UI:R means a victim must be induced to click a crafted link, which meaningfully lowers automatability and keeps this out of worm/mass-exploitation territory. Notably, the vendor assigned C:H (high confidentiality impact) despite classifying this as 'spoofing' - this is consistent with AD FS's role as a federation/token broker, where an open redirect can be abused to exfiltrate authentication artifacts (e.g. tokens/codes) during the sign-in flow rather than merely displaying a misleading URL; defenders should treat the primary impact as credential/token theft via phishing, not cosmetic redirection. No EPSS score, SSVC decision, KEV listing, or POC was supplied in the input: exploitation status is therefore 'no public exploit identified at time of analysis.' Net assessment: a real but not top-tier priority - patch in the normal expedited cycle. The dependence on user interaction and the absence of any exploitation signal argue against emergency treatment, but the exposed, pre-authentication nature of AD FS (typically internet-facing) and the high confidentiality impact justify prompt remediation over deferral.

An attacker crafts a malicious AD FS sign-in URL containing an attacker-controlled redirect parameter and delivers it to a target user via phishing email or a malicious link. The victim, trusting the legitimate AD FS domain, clicks and authenticates, after which they are redirected to the attacker's site - enabling token/credential interception or convincing credential-harvesting spoofing. No public exploit code was identified in the provided data; exploitation requires the victim to interact with the crafted link.