CVE-2018-15599 in Dropbear
Summary
by MITRE
The recv_msg_userauth_request function in svr-auth.c in Dropbear through 2018.76 is prone to a user enumeration vulnerability because username validity affects how fields in SSH_MSG_USERAUTH messages are handled, a similar issue to CVE-2018-15473 in an unrelated codebase.
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Analysis
by VulDB Data Team • 05/04/2023
The vulnerability identified as CVE-2018-15599 represents a critical user enumeration flaw in the Dropbear SSH server implementation that affects versions through 2018.76. This vulnerability resides within the recv_msg_userauth_request function located in the svr-auth.c source file, where the server's behavior differs based on whether a username exists in the system or not. The flaw stems from the inconsistent handling of SSH_MSG_USERAUTH messages where valid usernames trigger different response patterns compared to invalid usernames, creating a timing-based side-channel that adversaries can exploit to determine the existence of specific user accounts.
This vulnerability operates through a timing attack mechanism where the server responds differently to authentication requests based on username validity, creating observable delays or response variations that can be measured by attackers. The technical implementation demonstrates a classic case of information leakage through differential response handling, where valid usernames cause the server to process additional validation steps that invalid usernames do not trigger. The flaw is particularly concerning as it directly relates to CVE-2018-15473, which affected the OpenSSH implementation, indicating a common architectural weakness in SSH server implementations where user enumeration can be achieved through careful observation of server response patterns. The vulnerability maps to CWE-203 Information Exposure Through Timing Discrepancy, which specifically addresses situations where timing differences in system responses can reveal sensitive information about the system state or data being processed.
The operational impact of this vulnerability is significant for any system running vulnerable Dropbear versions, as it enables attackers to perform user enumeration attacks that can be used as a precursor to more sophisticated authentication attacks. An adversary can systematically test usernames against the SSH server and observe response timing variations to identify valid accounts, effectively bypassing traditional account lockout mechanisms and reducing the effectiveness of brute force protection measures. This vulnerability particularly affects environments where SSH access is restricted to specific user accounts, as it allows attackers to compile comprehensive lists of valid users without requiring successful authentication. The attack can be automated and executed remotely, making it a serious threat to systems where Dropbear SSH servers are deployed in production environments without proper mitigation measures.
Mitigation strategies for CVE-2018-15599 should focus on implementing consistent response handling regardless of username validity, ensuring that all authentication attempts take approximately equal time to process and return equivalent responses. System administrators should upgrade to Dropbear versions that have patched this vulnerability, with the most recent stable releases providing proper fixes. Additional protective measures include implementing rate limiting and connection throttling mechanisms to prevent rapid enumeration attempts, utilizing intrusion detection systems that can identify and alert on suspicious authentication patterns, and employing multi-factor authentication to add additional layers of security beyond username/password combinations. Organizations should also consider implementing sshd_config settings that randomize response times and disable user enumeration features, aligning with the principle of least privilege and ensuring that authentication systems do not inadvertently reveal information about their internal user base. The vulnerability also highlights the importance of proper input validation and consistent error handling in cryptographic implementations, as outlined in the ATT&CK framework under T1078 Valid Accounts and T1110 Brute Force techniques that leverage information disclosure vulnerabilities to facilitate unauthorized access.