CVE-2017-9079 in Dropbear
Summary
by MITRE
Dropbear before 2017.75 might allow local users to read certain files as root, if the file has the authorized_keys file format with a command= option. This occurs because ~/.ssh/authorized_keys is read with root privileges and symlinks are followed.
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Analysis
by VulDB Data Team • 09/30/2020
The vulnerability identified as CVE-2017-9079 affects the Dropbear SSH server implementation prior to version 2017.75, representing a significant local privilege escalation risk that leverages improper file access controls and symlink handling mechanisms. This flaw specifically targets the authorized_keys file processing functionality within Dropbear's authentication system, creating a pathway for local attackers to potentially access sensitive root-privileged files through crafted symlink manipulation.
The technical root cause of this vulnerability stems from Dropbear's handling of the ~/.ssh/authorized_keys file during authentication processes. When Dropbear processes SSH authentication requests, it reads the authorized_keys file with root privileges, which is a necessary function for proper authentication. However, the implementation fails to properly validate or sanitize symbolic links present within the file structure, allowing attackers to manipulate the file system through symlinks that point to sensitive root-owned files. This behavior directly violates secure coding principles and creates an attack surface where local users can leverage the elevated privileges used during authorized_keys processing to read files they should not normally have access to.
The operational impact of this vulnerability extends beyond simple information disclosure, as it enables local privilege escalation attacks that can potentially allow attackers to read any file accessible to the root user. The attack vector specifically exploits the command= option within authorized_keys files, which is typically used to restrict commands that can be executed via SSH connections. When combined with symlink manipulation, this creates a scenario where an attacker can construct a malicious authorized_keys entry that, when processed by Dropbear with root privileges, results in unauthorized file access. This vulnerability is particularly dangerous because it operates at the authentication layer where privileged file access is expected and normal, making detection more challenging.
Security researchers have classified this vulnerability under CWE-22, which describes improper limitation of a pathname to a restricted directory, commonly known as path traversal. The issue also aligns with ATT&CK technique T1068, which covers "Exploitation for Privilege Escalation" through local system manipulation. The vulnerability demonstrates how seemingly benign file processing functions can create dangerous security implications when proper access control and input validation are not implemented. Organizations running Dropbear SSH servers in environments where local users might have limited access but could potentially escalate privileges through this mechanism face significant risk exposure. The vulnerability's impact is particularly severe in multi-user environments where users might have shell access but should not be able to read system-critical files.
Mitigation strategies for CVE-2017-9079 primarily focus on updating to Dropbear version 2017.75 or later, which includes proper symlink handling and access control measures. System administrators should also implement strict file permissions on SSH-related directories and files, ensuring that authorized_keys files are not writable by non-root users. Additional protective measures include monitoring for unauthorized file modifications, implementing proper file system access controls, and conducting regular security audits of SSH configurations. The vulnerability highlights the importance of secure file handling practices and demonstrates how privilege escalation attacks can exploit seemingly routine system functions when proper security controls are absent. Organizations should also consider implementing additional monitoring and logging of SSH authentication processes to detect potential exploitation attempts.