CVE-2026-97788 in Active Directory Federation Services
Summary
by MITRE • 10/14/2026
Observable discrepancy in Active Directory Federation Services (AD FS) allows an unauthorized attacker to disclose information over a network.
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Analysis
by VulDB Data Team • 10/14/2026
Active Directory Federation Services contains an observable discrepancy that enables an unauthorized actor to perform information disclosure attacks against the federation infrastructure. This vulnerability stems from inconsistent error handling or response formatting within the AD FS service, which inadvertently leaks sensitive internal state data during authentication and token issuance processes. When a client initiates a federated login request, particularly through protocols such as SAML 2.0 or WS-Federation, the server may return distinct responses depending on whether specific parameters are malformed, missing, or invalid in ways that expose underlying system configurations or user existence states. This behavior creates a side-channel attack vector where an attacker can iteratively probe the service to gather intelligence about the network topology, valid usernames, and federation trust relationships without requiring prior authentication credentials.
The technical flaw is rooted in how AD FS processes incoming requests and generates error messages for invalid inputs. Instead of returning uniform generic errors that obscure internal details, the service exhibits differential behavior based on specific input conditions. For instance, providing an incorrect username might trigger a different response time or payload structure compared to providing a valid username with an incorrect password. This discrepancy allows attackers to enumerate valid user accounts by observing these subtle differences in server responses. Furthermore, the vulnerability may expose metadata about connected identity providers and relying parties, effectively mapping out the federation trust landscape for potential future exploitation.
From an operational perspective, this information disclosure significantly lowers the barrier of entry for subsequent attacks such as credential stuffing or targeted phishing campaigns. By confirming which user accounts exist within the organization, attackers can focus their efforts on high-value targets rather than guessing randomly. The exposure of internal network details and federation configurations also aids in lateral movement planning, allowing adversaries to identify critical authentication gateways that serve as choke points for accessing protected resources. This undermines the principle of least privilege by revealing more system information than necessary for normal operation, thereby expanding the attack surface beyond what is intended by security architects.
Mitigation strategies should focus on hardening AD FS configurations and monitoring for anomalous behavior. Administrators must ensure that all patches related to Active Directory Federation Services are applied promptly, as Microsoft frequently releases updates addressing such enumeration vulnerabilities. Configuring custom error pages or generic response messages can help mask internal details from external requests, although this requires careful testing to avoid breaking legitimate federation flows. Additionally, implementing rate limiting on authentication endpoints and monitoring for high volumes of failed login attempts with varying usernames can detect active enumeration attacks in real time. Network segmentation should be enforced to restrict direct internet access to AD FS servers, ensuring they are only reachable through secure proxies or firewalls that inspect and filter inbound traffic. Regular audits of federation trust configurations help ensure that unnecessary information is not exposed during token issuance processes.