CVE-2026-83646 in Windows
Summary
by MITRE • 10/14/2026
Url redirection to untrusted site ('open redirect') in Active Directory Federation Services (AD FS) allows an authorized attacker to perform spoofing over a network.
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Analysis
by VulDB Data Team • 10/15/2026
Active Directory Federation Services, commonly referred to as AD FS, serves as a critical component in modern enterprise identity management architectures by facilitating single sign-on capabilities across organizational boundaries using standard protocols such as SAML and WS-Federation. The vulnerability described constitutes an open redirect flaw within the authentication or token issuance workflow of this service. In typical operation, AD FS handles various query parameters that dictate post-authentication behavior, including where to send the user after successful credential verification. When these parameters are not rigorously validated against a whitelist of trusted domains, they can be manipulated by an attacker to point toward external, untrusted destinations. This architectural oversight allows for the interception and redirection of legitimate users or service accounts during the federation process.
The technical mechanism behind this flaw relies on the improper sanitization of URL inputs passed through AD FS endpoints. An authorized attacker, possessing valid credentials or exploiting a scenario where authentication is bypassed via social engineering, can craft a malicious request containing a specially constructed redirect parameter. Upon processing this request, the service fails to verify that the target domain belongs to an approved list before issuing the HTTP 302 response code associated with redirections. Consequently, the user's browser or client application follows the link provided in the header, leading them away from the intended corporate identity provider and toward a server controlled by the adversary. This behavior is particularly dangerous because it occurs within the context of an authenticated session, lending apparent legitimacy to the malicious destination.
From an operational perspective, this vulnerability enables sophisticated phishing campaigns that are significantly more effective than standard social engineering attacks due to their technical authenticity. Attackers can host fake login pages on domains that closely mimic legitimate services or use visually similar URLs to deceive users into believing they are still interacting with a trusted corporate environment. Once redirected, the attacker’s site can harvest sensitive credentials, session cookies, or multi-factor authentication tokens under the guise of a standard identity verification step. This compromises not only individual user accounts but also potentially grants access to downstream applications and resources that rely on AD FS for trust validation. The impact extends beyond immediate credential theft, as it undermines the integrity of the entire federation chain, allowing attackers to establish persistent footholds within internal networks by compromising high-privilege administrative accounts through deceptive redirections.
This vulnerability aligns with Common Weakness Enumeration identifier CWE-601, which defines URL Redirection to Untrusted Site ('Open Redirect'). It also maps directly to MITRE ATT&CK technique T1566.002, known as Spearphishing Link, where attackers use links in spear-phishing emails or messages to direct victims to sites controlled by the attacker for credential harvesting. The exploitation of this flaw represents a critical failure in input validation and trust boundary enforcement within identity management systems.
Mitigation strategies must focus on strict input validation and secure configuration practices. Administrators should implement allow-listing mechanisms that restrict redirect targets exclusively to known, trusted internal domains and approved external partners. Any URL parameter involved in post-authentication flows must be validated against this whitelist before the redirection logic is executed. Additionally, enabling security headers such as Content-Security-Policy with appropriate frame-ancestors directives can help mitigate some downstream effects of phishing pages hosted on attacker-controlled infrastructure. Regular auditing of AD FS configuration settings and keeping the service updated to the latest cumulative updates released by Microsoft are essential steps to ensure that known patches for open redirect vulnerabilities are applied promptly. Security awareness training for users remains a complementary defense, emphasizing caution when redirected from identity providers even if the initial link appears legitimate.