CVE-2016-3115 in OpenSSH
Summary
by MITRE
Multiple CRLF injection vulnerabilities in session.c in sshd in OpenSSH before 7.2p2 allow remote authenticated users to bypass intended shell-command restrictions via crafted X11 forwarding data, related to the (1) do_authenticated1 and (2) session_x11_req functions.
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Analysis
by VulDB Data Team • 05/29/2026
The vulnerability identified as CVE-2016-3115 represents a critical CRLF injection flaw within the OpenSSH implementation that affects versions prior to 7.2p2. This security weakness resides in the session.c file of the sshd daemon and specifically targets the X11 forwarding functionality. The vulnerability allows authenticated remote attackers to manipulate session handling mechanisms by injecting carriage return line feed sequences into session data, thereby bypassing intended shell command restrictions that are typically enforced during SSH sessions.
The technical exploitation of this vulnerability occurs through the manipulation of X11 forwarding data, which is processed by two distinct functions within the sshd codebase: do_authenticated1 and session_x11_req. These functions handle authentication and X11 request processing respectively, and they fail to properly sanitize input data that flows through the session management pipeline. When crafted malicious data is transmitted during X11 forwarding operations, the CRLF injection allows attackers to inject additional commands or manipulate session state information that should remain restricted. This flaw operates at the protocol level where session data is processed, making it particularly dangerous as it can be leveraged to extend the scope of commands that authenticated users can execute beyond their intended privileges.
The operational impact of this vulnerability extends beyond simple command injection, as it fundamentally undermines the security model of SSH sessions. Attackers can potentially execute arbitrary commands on the target system with the privileges of the authenticated user, or at minimum, manipulate session behavior to gain unauthorized access to resources that should be protected. The vulnerability affects the integrity of session management by allowing attackers to modify session state information, potentially leading to privilege escalation scenarios or persistent access to systems. The fact that this requires only authenticated access means that the attack surface is limited to users who already have legitimate SSH access, but the implications are severe as it can be used to circumvent security controls that are fundamental to SSH operations.
From a cybersecurity perspective, this vulnerability aligns with CWE-113, which describes improper neutralization of CRLF sequences in HTTP headers, and relates to the broader category of injection flaws that compromise system integrity. The ATT&CK framework would classify this under T1059 for command and scripting interpreter, and potentially T1078 for valid accounts, as it leverages existing authenticated sessions to extend privileges. The vulnerability demonstrates how session management flaws can be exploited to undermine authentication mechanisms, making it particularly concerning for enterprise environments where SSH is used for remote administration and access control. Organizations implementing SSH-based security models must consider this vulnerability as a potential vector for attackers to escalate privileges or bypass security controls that rely on proper session handling and command restriction enforcement.
Mitigation strategies for this vulnerability require immediate patching of OpenSSH installations to version 7.2p2 or later, which contains the necessary fixes to properly sanitize X11 forwarding data and prevent CRLF injection in session processing functions. System administrators should also implement network monitoring to detect anomalous X11 forwarding patterns that might indicate exploitation attempts. Additional protective measures include implementing strict access controls for SSH access, monitoring session logs for unusual command execution patterns, and ensuring that X11 forwarding is disabled when not required for legitimate administrative tasks. The vulnerability serves as a reminder of the importance of proper input validation in protocol implementations and the need for comprehensive security testing of session management components in network services.