CVE-2017-9078 in Dropbear
Summary
by MITRE
The server in Dropbear before 2017.75 might allow post-authentication root remote code execution because of a double free in cleanup of TCP listeners when the -a option is enabled.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 09/30/2020
The vulnerability identified as CVE-2017-9078 affects the Dropbear SSH server implementation prior to version 2017.75 and represents a critical post-authentication remote code execution flaw. This vulnerability specifically manifests when the server is configured with the -a option enabled, which allows the server to act as an SSH proxy or relay. The flaw stems from improper memory management during the cleanup process of TCP listeners, creating a double free condition that can be exploited by remote attackers who have already established an authenticated SSH session. This represents a significant escalation of privilege vulnerability as it allows an attacker who has successfully authenticated to the SSH server to execute arbitrary code with root privileges on the target system.
The technical root cause of this vulnerability aligns with CWE-415, which describes improper cleanup of memory resources leading to double free conditions in memory management. The double free occurs during the server's cleanup routine when handling TCP listeners, specifically when the -a option is active. When the server processes connections and subsequently cleans up resources, the same memory block gets freed twice, causing undefined behavior that can be leveraged by attackers to manipulate the program's execution flow. This memory corruption vulnerability allows for arbitrary code execution because the double free creates opportunities for heap manipulation and can potentially lead to control flow hijacking through return-oriented programming techniques or direct code injection.
The operational impact of CVE-2017-9078 is severe as it transforms a legitimate authenticated session into a privilege escalation vector. An attacker who gains initial access through legitimate authentication can exploit this vulnerability to execute arbitrary commands with root privileges, effectively compromising the entire system. This vulnerability directly maps to the ATT&CK technique T1068, which covers 'Exploitation for Privilege Escalation', and can be classified under T1190 for 'Exploit Public-Facing Application'. The vulnerability affects systems where Dropbear SSH server is deployed as a proxy server or when the -a option is enabled for relay functionality, making it particularly dangerous in environments where SSH proxying is utilized for network access control or remote system management.
Mitigation strategies for CVE-2017-9078 primarily focus on immediate patching of the Dropbear SSH server to version 2017.75 or later, which contains the fix for the double free condition in TCP listener cleanup. Organizations should also consider disabling the -a option if proxy functionality is not required, as this eliminates the attack surface for this specific vulnerability. Additionally, implementing network segmentation and access controls to limit exposure of Dropbear servers to untrusted networks can reduce the risk of exploitation. Security monitoring should include detection of unusual authentication patterns and system behavior that might indicate exploitation attempts. The vulnerability demonstrates the importance of proper memory management practices in server applications and highlights the need for regular security updates and vulnerability assessments, particularly for critical infrastructure components like SSH servers that handle authentication and remote access.