CVE-2015-5600 in Communications
Summary
by MITRE
The kbdint_next_device function in auth2-chall.c in sshd in OpenSSH through 6.9 does not properly restrict the processing of keyboard-interactive devices within a single connection, which makes it easier for remote attackers to conduct brute-force attacks or cause a denial of service (CPU consumption) via a long and duplicative list in the ssh -oKbdInteractiveDevices option, as demonstrated by a modified client that provides a different password for each pam element on this list.
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Analysis
by VulDB Data Team • 05/28/2026
The vulnerability identified as CVE-2015-5600 represents a critical flaw in the OpenSSH implementation that affects versions through 6.9. This issue resides within the kbdint_next_device function located in auth2-chall.c, which governs the processing of keyboard-interactive authentication devices during SSH connections. The flaw stems from insufficient validation and restriction of keyboard-interactive device processing, creating a pathway for malicious actors to exploit the authentication mechanism through carefully crafted input sequences. The vulnerability is particularly concerning because it operates at the core authentication layer of SSH protocols, potentially allowing unauthorized access or system disruption.
The technical implementation of this vulnerability exploits the lack of proper bounds checking and input validation within the keyboard-interactive authentication flow. When sshd processes authentication requests containing excessive or duplicate keyboard-interactive device specifications through the KbdInteractiveDevices option, the kbdint_next_device function fails to properly terminate or limit its processing of these device entries. This creates a scenario where an attacker can construct a malicious client that provides a unique password for each element in an extended list, causing the authentication daemon to repeatedly process these entries without proper termination conditions. The vulnerability manifests as an exponential increase in CPU consumption during authentication attempts, as the system processes each device entry sequentially without adequate safeguards against malformed or excessively long device lists.
From an operational perspective, this vulnerability enables two primary attack vectors that significantly impact system security and availability. The first vector allows for brute-force attack acceleration, where attackers can systematically test numerous password combinations against each device entry in the malicious list, effectively multiplying the effectiveness of credential guessing attacks. The second vector creates a denial-of-service condition through excessive CPU consumption, as the sshd process becomes overwhelmed processing the duplicative device entries, potentially leading to system resource exhaustion and service unavailability for legitimate users. This vulnerability directly impacts the authentication integrity and system availability of SSH services, making it particularly dangerous in environments where SSH access is critical for system administration and remote access.
The vulnerability aligns with CWE-770, which addresses the allocation of resources without proper limits or controls, and relates to ATT&CK technique T1110.004 for credential access through password spraying and brute force methods. Organizations implementing SSH services should consider this vulnerability as part of their broader authentication security posture, particularly in environments where SSH is exposed to external networks or where authentication services are critical to system operations. The flaw demonstrates how insufficient input validation at authentication layers can create cascading security issues that affect both access control and system availability.
Mitigation strategies for CVE-2015-5600 should focus on immediate software updates to OpenSSH versions that address this specific vulnerability, as well as implementing additional access controls and monitoring. System administrators should configure sshd to limit the number of authentication attempts and implement connection rate limiting to prevent exploitation of the CPU consumption aspect. Network-level controls such as firewall rules and intrusion detection systems can help detect and block suspicious authentication patterns that might indicate exploitation attempts. Additionally, organizations should consider implementing multi-factor authentication mechanisms to reduce the impact of credential-based attacks and establish monitoring procedures to detect unusual CPU consumption patterns during authentication processes, which could indicate exploitation of this vulnerability.