CVE-2017-2659 in Dropbear
Summary
by MITRE
It was found that dropbear before version 2013.59 with GSSAPI leaks whether given username is valid or invalid. When an invalid username is given, the GSSAPI authentication failure was incorrectly counted towards the maximum allowed number of password attempts.
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Analysis
by VulDB Data Team • 08/03/2023
The vulnerability identified as CVE-2017-2659 affects the dropbear SSH server implementation prior to version 2013.59, specifically within its GSSAPI authentication mechanism. This represents a classic security flaw that undermines the principle of authentication robustness by creating a side-channel information leak. The issue manifests when an attacker attempts to authenticate using GSSAPI with a username that does not exist within the system's user database. The vulnerability stems from improper handling of authentication failure states within the GSSAPI subsystem, where the system incorrectly treats invalid username attempts as password attempt failures.
This authentication bypass vulnerability operates through a timing and state-based information disclosure mechanism that allows attackers to distinguish between valid and invalid usernames through the authentication process. The technical flaw lies in the improper accounting of authentication attempts where GSSAPI failures for non-existent usernames are counted against the configured maximum password attempt limits. This creates a differential response pattern that can be exploited by an attacker to enumerate valid users within the system. The vulnerability directly maps to CWE-203, Information Exposure Through Discrepancy, as it exposes information about the existence or non-existence of user accounts through inconsistent error handling.
The operational impact of this vulnerability extends beyond simple user enumeration, as it provides attackers with crucial reconnaissance data that can be leveraged in subsequent attack phases. When an attacker successfully identifies that a username exists in the system, they can then focus their efforts on password cracking or other authentication bypass techniques against that specific user account. This vulnerability aligns with ATT&CK technique T1078.004, Valid Accounts - Cloud Accounts, and T1590.001, reconnaissance - network scanning, as it enables attackers to map valid user accounts within the target environment. The leak of username validity information creates a pathway for credential stuffing attacks and can significantly reduce the time and resources required for successful unauthorized access.
The mitigation strategies for CVE-2017-2659 primarily involve upgrading to dropbear version 2013.59 or later, where the GSSAPI authentication handling has been corrected to properly distinguish between different types of authentication failures. Additionally, system administrators should implement rate limiting and account lockout mechanisms to prevent automated enumeration attempts, though these measures alone do not address the core vulnerability. Network-level protections such as fail2ban or similar intrusion prevention systems can help detect and block repeated authentication attempts that may indicate enumeration activity. The fix implemented in the updated version ensures that GSSAPI authentication failures for invalid usernames do not contribute to password attempt counters, thereby eliminating the information leakage that enabled the vulnerability. Security monitoring should include detection of unusual authentication patterns that may indicate user enumeration attempts, particularly those involving multiple failed GSSAPI authentication attempts against non-existent accounts.