CVE-2017-6542 in PuTTY
Summary
by MITRE
The ssh_agent_channel_data function in PuTTY before 0.68 allows remote attackers to have unspecified impact via a large length value in an agent protocol message and leveraging the ability to connect to the Unix-domain socket representing the forwarded agent connection, which trigger a buffer overflow.
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Analysis
by VulDB Data Team • 12/22/2024
The vulnerability identified as CVE-2017-6542 represents a critical buffer overflow flaw within the PuTTY SSH client software family, specifically affecting versions prior to 0.68. This vulnerability resides within the ssh_agent_channel_data function which handles agent protocol messages during SSH connections. The flaw manifests when remote attackers manipulate the length field within agent protocol messages to unusually large values, exploiting the lack of proper input validation in the agent forwarding mechanism. The vulnerability is particularly concerning because it leverages the Unix-domain socket interface that represents forwarded agent connections, allowing attackers to execute code on the target system where PuTTY is running.
The technical implementation of this vulnerability follows a classic buffer overflow pattern where insufficient bounds checking allows an attacker to write beyond allocated memory boundaries. When the ssh_agent_channel_data function processes an agent protocol message with an oversized length value, it fails to validate that the specified length falls within acceptable parameters for the buffer allocation. This allows attackers to craft malicious protocol messages that cause memory corruption, potentially leading to arbitrary code execution or denial of service conditions. The vulnerability specifically impacts the agent forwarding feature which enables SSH clients to forward authentication requests to a local SSH agent, creating a potential attack vector through the Unix-domain socket interface.
From an operational perspective, this vulnerability creates significant risk for systems using PuTTY for SSH connections, particularly in enterprise environments where agent forwarding is commonly employed. Attackers can exploit this flaw remotely without requiring authentication to the target system, provided they can establish a connection to the Unix-domain socket representing the forwarded agent. The impact extends beyond simple code execution to include potential privilege escalation scenarios, as the compromised system may be running with elevated privileges. This vulnerability aligns with CWE-121, which describes heap-based buffer overflow conditions, and represents a direct threat to the integrity of SSH authentication processes. The attack surface is particularly wide given that PuTTY is widely used across various operating systems including Windows, and the agent forwarding feature is commonly enabled in production environments.
The exploitation of CVE-2017-6542 demonstrates characteristics consistent with techniques found in the ATT&CK framework under the T1059 category for execution through command and scripting interpreter, where the buffer overflow creates opportunities for code injection. Organizations using PuTTY should implement immediate mitigations including updating to version 0.68 or later, disabling agent forwarding when not required, and monitoring for suspicious connections to Unix-domain sockets. The vulnerability highlights the importance of input validation in network protocol implementations and serves as a reminder of the critical security considerations when implementing agent forwarding features. Additionally, system administrators should consider implementing network segmentation to limit access to Unix-domain sockets and deploy intrusion detection systems that can identify abnormal protocol message patterns that may indicate exploitation attempts. The flaw underscores the necessity of maintaining up-to-date security patches and implementing defense-in-depth strategies to protect against such memory corruption vulnerabilities that can compromise entire authentication infrastructures.