CVE-2016-3116 in SSH
Summary
by MITRE
CRLF injection vulnerability in Dropbear SSH before 2016.72 allows remote authenticated users to bypass intended shell-command restrictions via crafted X11 forwarding data.
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Analysis
by VulDB Data Team • 10/09/2024
The CVE-2016-3116 vulnerability represents a critical CRLF injection flaw discovered in the Dropbear SSH implementation prior to version 2016.72. This vulnerability specifically affects the X11 forwarding functionality within the SSH protocol, creating a pathway for authenticated remote attackers to circumvent shell command restrictions that administrators have implemented for security purposes. The flaw resides in how Dropbear handles X11 forwarding data, where maliciously crafted input can introduce carriage return and line feed characters that manipulate the underlying command execution flow.
The technical nature of this vulnerability stems from insufficient input validation within the X11 forwarding mechanism of Dropbear SSH. When users establish SSH connections with X11 forwarding enabled, the system processes the forwarded data through a series of parsing operations that fail to properly sanitize the input. This allows an authenticated user to inject CRLF sequences that can alter the behavior of the shell command execution environment. The vulnerability operates at the application layer and requires authentication to exploit, making it particularly dangerous in environments where privileged users maintain SSH access. According to CWE classification, this maps to CWE-1107, which specifically addresses CRLF injection vulnerabilities in web applications and network protocols, though the implementation in Dropbear demonstrates how such flaws can manifest in SSH implementations.
The operational impact of CVE-2016-3116 extends beyond simple command bypass, as it can enable attackers to execute arbitrary commands within the constrained shell environment. This vulnerability can be leveraged to escalate privileges, access restricted files, or perform unauthorized operations on systems where SSH is used for administrative access. The attack vector requires that the attacker already possess valid authentication credentials, which makes this vulnerability particularly concerning in environments where credential compromise is possible through other attack vectors. In enterprise settings, this could allow attackers to move laterally through systems or maintain persistent access while evading typical command restriction policies. The vulnerability aligns with ATT&CK technique T1059.001, which covers command and script interpreter usage, and specifically demonstrates how protocol-level weaknesses can be exploited to bypass security controls.
Mitigation strategies for CVE-2016-3116 primarily focus on updating to Dropbear version 2016.72 or later, which includes patches specifically addressing the CRLF injection vulnerability in X11 forwarding. Organizations should implement comprehensive patch management processes to ensure all SSH implementations are updated promptly, particularly in environments where privileged access is granted through SSH connections. Network segmentation and monitoring of SSH traffic can provide additional layers of defense, while disabling X11 forwarding when not required can reduce the attack surface. Security teams should also consider implementing strict access controls and monitoring for unusual command execution patterns that might indicate exploitation attempts. The vulnerability serves as a reminder of the importance of input validation in network protocols and demonstrates how seemingly minor implementation flaws can create significant security risks in authentication systems.