CVE-2018-15919 in OpenSSH
Summary
by MITRE
Remotely observable behaviour in auth-gss2.c in OpenSSH through 7.8 could be used by remote attackers to detect existence of users on a target system when GSS2 is in use. NOTE: the discoverer states 'We understand that the OpenSSH developers do not want to treat such a username enumeration (or "oracle") as a vulnerability.'
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 12/18/2025
The vulnerability identified as CVE-2018-15919 represents a subtle but significant information disclosure issue within the OpenSSH implementation that affects versions through 7.8. This flaw exists in the auth-gss2.c file and manifests as remotely observable behavior that can be exploited by attackers to determine the existence of specific user accounts on a target system. The vulnerability specifically occurs when the Generic Security Services Authentication Protocol version 2 (GSS2) is enabled, creating an oracle condition where the server's response behavior differs based on whether a username exists or not. This type of vulnerability falls under the category of username enumeration attacks that can provide attackers with valuable reconnaissance information for subsequent exploitation attempts.
The technical implementation of this vulnerability stems from how OpenSSH handles authentication requests when GSS2 is active. When a client attempts to authenticate using GSS2 with a non-existent username, the server responds in a manner that differs from when a valid username is provided. This differential response behavior creates an observable pattern that remote attackers can exploit to determine which user accounts exist on the system. The vulnerability operates at the protocol level within the authentication framework, specifically in the GSS2 authentication module where the server's handling of authentication requests does not maintain consistent timing or response characteristics regardless of the username provided. This behavior creates a timing oracle that can be leveraged to perform user enumeration attacks without requiring successful authentication.
The operational impact of this vulnerability extends beyond simple information disclosure, as it enables attackers to conduct reconnaissance activities that can significantly aid in planning more sophisticated attacks. While the vulnerability itself does not directly allow for authentication bypass or privilege escalation, it provides attackers with critical information about the target system's user base, which can be used to focus subsequent attack vectors such as password spraying, targeted credential theft, or social engineering campaigns. The vulnerability affects systems that have GSS2 authentication enabled, which is commonly used in enterprise environments where single sign-on and cross-platform authentication are required. This makes the impact more widespread in corporate and institutional settings where OpenSSH is deployed with GSS2 support.
Security practitioners should consider this vulnerability in the context of broader authentication security practices and the principles outlined in the MITRE ATT&CK framework under the credential access category, specifically the technique of credential dumping and reconnaissance activities. The vulnerability aligns with CWE-203, which describes "Observable Behavioral Changes" in security systems, and represents a classic example of how seemingly minor implementation details in cryptographic protocols can create information leakage vulnerabilities. Organizations should implement mitigations such as disabling GSS2 authentication when it is not strictly required, implementing rate limiting on authentication attempts, and monitoring for unusual authentication patterns that might indicate enumeration attempts. The recommended approach aligns with defense-in-depth strategies that reduce the attack surface and limit the information available to potential attackers, while also considering the broader security posture of the system.