CVE-2017-8495 in Windows
Summary
by MITRE
Microsoft Windows 7 SP1, Windows Server 2008 SP2 and R2 SP1, Windows 8.1 and Windows RT 8.1, Windows Server 2012 and R2, Windows 10 Gold, 1511, 1607, and 1703, and Windows Server 2016 allows an attacker to bypass Extended Protection for Authentication when Kerberos fails to prevent tampering with the SNAME field during ticket exchange, aka "Kerberos SNAME Security Feature Bypass Vulnerability".
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Analysis
by VulDB Data Team • 12/31/2020
The vulnerability identified as CVE-2017-8495 represents a critical security flaw in Microsoft Windows authentication systems that specifically targets the Kerberos authentication protocol implementation. This weakness affects a broad range of Windows operating systems including legacy versions like Windows 7 SP1 and Windows Server 2008 SP2, alongside newer releases such as Windows 10 versions 1511, 1607, and 1703, along with Windows Server 2012, 2012 R2, and Windows Server 2016. The vulnerability stems from an insufficient validation mechanism within the Kerberos authentication process that fails to properly protect the SNAME field during ticket exchanges, creating a pathway for malicious actors to manipulate authentication tokens.
The technical root cause of this vulnerability lies in the Extended Protection for Authentication feature's inability to effectively prevent tampering with the SNAME (Service Name) field within Kerberos tickets. The SNAME field contains critical information about the target service that the client intends to access, and when this field becomes compromised during the authentication process, it allows attackers to bypass the normal security protections that should prevent unauthorized access. This flaw specifically manifests when Kerberos fails to properly validate that the service name in the ticket matches the expected service name, enabling attackers to forge or manipulate authentication tokens that would otherwise be rejected by proper validation mechanisms.
The operational impact of CVE-2017-8495 is significant as it provides attackers with a method to bypass critical authentication security measures without requiring elevated privileges or complex exploitation techniques. An attacker who successfully exploits this vulnerability can potentially impersonate legitimate users or services within the network, gaining unauthorized access to protected resources and systems. This vulnerability particularly affects environments where Kerberos authentication is heavily relied upon for network security, including domain environments where service account authentication and single sign-on functionality are critical components of the security infrastructure. The bypass capability means that even if other authentication protections are in place, the compromised SNAME field allows attackers to circumvent these defenses effectively.
Organizations affected by this vulnerability should implement immediate mitigations including applying the relevant Microsoft security updates that address the Kerberos SNAME validation issue. The vulnerability aligns with CWE-284, which describes improper access control, and represents a specific instance where the authentication system fails to properly enforce access restrictions. From an ATT&CK framework perspective, this vulnerability maps to techniques involving credential access and defense evasion, as attackers can leverage this flaw to obtain valid credentials and then use those credentials to maintain persistent access to systems while avoiding detection mechanisms. Network administrators should also consider implementing additional monitoring for unusual authentication patterns and service ticket requests that might indicate exploitation attempts, as well as reviewing and strengthening Kerberos configuration settings to minimize the potential impact of this vulnerability across their infrastructure.