CVE-2006-1206 in Dropbear SSH server
Summary
by MITRE
Matt Johnston Dropbear SSH server 0.47 and earlier, as used in embedded Linux devices and on general-purpose operating systems, allows remote attackers to cause a denial of service (connection slot exhaustion) via a large number of connection attempts that exceeds the MAX_UNAUTH_CLIENTS defined value of 30.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 03/16/2025
The CVE-2006-1206 vulnerability affects Matt Johnston Dropbear SSH server versions 0.47 and earlier, representing a significant denial of service weakness that impacts both embedded Linux devices and general-purpose operating systems. This vulnerability stems from inadequate connection handling mechanisms within the SSH server implementation, specifically targeting the unauthenticated client connection management system. The flaw manifests when remote attackers exploit the server's limited capacity to handle concurrent connection attempts, creating a scenario where legitimate users cannot establish SSH sessions due to resource exhaustion.
The technical root cause of this vulnerability lies in the server's hardcoded limitation of MAX_UNAUTH_CLIENTS set to 30, which represents a fundamental design flaw in connection queue management. When attackers flood the system with excessive connection attempts, they systematically consume all available unauthenticated connection slots, effectively preventing legitimate authentication attempts from succeeding. This mechanism operates at the protocol level where the SSH server maintains a finite pool of slots for handling incoming connections before authentication occurs. The vulnerability demonstrates poor resource management and lacks proper rate limiting or connection throttling mechanisms to prevent abuse.
From an operational perspective, this vulnerability presents a severe risk to systems relying on Dropbear SSH for remote access, particularly in embedded environments where system resources are constrained. The impact extends beyond simple service disruption to potentially compromise system availability for legitimate administrators who require remote access for maintenance or monitoring purposes. In embedded devices, this vulnerability can render critical infrastructure inaccessible, potentially leading to operational downtime and security posture degradation. The vulnerability aligns with CWE-400, which categorizes it as an Uncontrolled Resource Consumption weakness, and can be mapped to ATT&CK technique T1499.004 for network denial of service attacks.
Mitigation strategies for this vulnerability require immediate implementation of connection rate limiting mechanisms and proper resource management configuration. Administrators should upgrade to Dropbear SSH server versions 0.48 or later, which contain fixes addressing the connection slot exhaustion issue. Additionally, implementing firewall rules to limit the number of connection attempts from individual IP addresses provides effective protection against exploitation. Network administrators should also consider configuring connection timeouts and implementing proper monitoring to detect unusual connection patterns that may indicate exploitation attempts. The fix typically involves modifying the server's connection handling logic to properly manage resource allocation and prevent malicious users from exhausting connection slots through sustained attack patterns.