CVE-2016-7407 in Dropbear SSH
Summary
by MITRE
The dropbearconvert command in Dropbear SSH before 2016.74 allows attackers to execute arbitrary code via a crafted OpenSSH key file.
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Analysis
by VulDB Data Team • 09/03/2020
The vulnerability identified as CVE-2016-7407 resides within the dropbearconvert utility of the Dropbear SSH implementation, a lightweight SSH server and client commonly used in embedded systems and resource-constrained environments. This flaw represents a critical code execution vulnerability that affects versions of Dropbear prior to 2016.74, specifically targeting the command-line utility responsible for converting SSH key formats between different implementations. The vulnerability stems from inadequate input validation and sanitization within the key conversion process, creating a pathway for malicious actors to inject and execute arbitrary code on systems running vulnerable versions of Dropbear SSH.
The technical exploitation of this vulnerability occurs through the manipulation of OpenSSH key files that are processed by the dropbearconvert command. When an attacker crafts a specially formatted OpenSSH key file, the dropbearconvert utility fails to properly validate the input structure, allowing malicious code to be embedded within the key data. This flaw falls under the category of improper input validation, which is classified as CWE-20 by the CWE standard, specifically relating to the injection of untrusted data into a command or shell. The vulnerability enables attackers to leverage the key conversion process as a vector for code execution, bypassing normal authentication and authorization mechanisms that would typically protect the system from unauthorized access.
The operational impact of CVE-2016-7407 extends beyond simple privilege escalation, as it can be exploited to gain full system control over devices running vulnerable Dropbear implementations. This is particularly concerning in embedded environments where Dropbear SSH is commonly deployed, such as network routers, IoT devices, and other embedded systems where security updates may be infrequent or impossible to apply. The vulnerability can be exploited remotely through the key conversion process, making it especially dangerous for systems that accept key files from untrusted sources or for administrators who might inadvertently process malicious key files during routine maintenance operations. According to ATT&CK framework, this vulnerability maps to T1059.001 (Command and Scripting Interpreter: PowerShell) and T1068 (Exploitation for Privilege Escalation) techniques, as the arbitrary code execution enables attackers to escalate privileges and establish persistent access to compromised systems.
Mitigation strategies for CVE-2016-7407 primarily focus on immediate remediation through version updates to Dropbear 2016.74 or later, which includes proper input validation and sanitization within the dropbearconvert utility. Organizations should also implement strict access controls to prevent unauthorized users from processing key files, particularly those received from external sources. Network segmentation and monitoring of SSH key conversion activities can help detect potential exploitation attempts, while regular security audits should verify that no vulnerable versions remain operational within the environment. The vulnerability demonstrates the critical importance of validating all external inputs, especially in security-critical components like key management utilities, as highlighted by the CWE-20 classification and the ATT&CK framework's emphasis on input validation and command execution techniques that attackers leverage to establish persistent access to compromised systems.