CVE-2017-3204 in Go SSH Library
Summary
by MITRE
The Go SSH library (x/crypto/ssh) by default does not verify host keys, facilitating man-in-the-middle attacks. Default behavior changed in commit e4e2799 to require explicitly registering a hostkey verification mechanism.
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Analysis
by VulDB Data Team • 11/27/2022
The vulnerability identified as CVE-2017-3204 resides within the Go programming language's standard library implementation of the Secure Shell protocol through the x/crypto/ssh package. This flaw represents a critical security oversight that fundamentally compromised the integrity of SSH connections by failing to validate server host keys during the connection establishment process. The vulnerability stems from the library's default configuration that permitted connections to proceed without any verification of the remote server's identity, creating an environment where malicious actors could seamlessly intercept and manipulate communications between clients and servers.
The technical root cause of this vulnerability lies in the implementation design where the SSH client library defaulted to accepting any host key presented by a remote server without performing cryptographic validation. This behavior directly violates fundamental security principles of the SSH protocol, which relies on host key verification to prevent man-in-the-middle attacks. The flaw manifested because the library's default configuration did not include any host key checking mechanism, leaving applications using the library vulnerable to attacks where an attacker could present a forged host key to establish a false connection. This vulnerability aligns with CWE-295 which specifically addresses "Improper Certificate Validation" and represents a classic case of inadequate cryptographic validation in network security protocols.
The operational impact of CVE-2017-3204 was severe and far-reaching across numerous applications and systems that relied on the Go SSH library for secure communications. Any application using the default SSH client configuration without explicit host key verification mechanisms became susceptible to active attack scenarios where adversaries could intercept sensitive data, modify communications, or establish unauthorized access points. The vulnerability particularly affected automated systems, deployment tools, and any software that programmatically established SSH connections without proper security configuration. Security researchers and system administrators discovered that this flaw was present in many widely-used Go applications and libraries that handled SSH connections, including container orchestration tools, deployment frameworks, and network automation systems. The implications extended beyond simple data interception to potentially enabling complete system compromise through credential theft and privilege escalation attacks.
The remediation for CVE-2017-3204 required developers to explicitly implement host key verification mechanisms within their applications using the Go SSH library. The Go team addressed this issue by changing the default behavior in commit e4e2799 to require developers to explicitly register a host key verification mechanism before establishing secure connections. This change aligned with the principle of least privilege and security by default, ensuring that applications would not inadvertently create security vulnerabilities through default configurations. Organizations needed to update their codebases to implement proper host key checking, typically through the use of the HostKeyCallback function which allows developers to specify their own verification logic. The fix required comprehensive code review and modification across all systems using the affected library, emphasizing the importance of security-conscious development practices and the necessity of explicit security controls rather than relying on implicit defaults.
This vulnerability demonstrates the critical importance of cryptographic library design and the need for security-by-default implementations in network protocols. The issue highlights how seemingly innocuous default behaviors can create significant security risks, particularly in cryptographic implementations where trust relationships are fundamental. The remediation approach taken by the Go team reflects best practices in vulnerability management and security engineering, emphasizing the need for explicit security configuration rather than implicit trust assumptions. The vulnerability also underscores the importance of regular security audits of cryptographic libraries and the necessity of maintaining up-to-date security practices in software development, particularly when dealing with network protocols that form the foundation of secure communications infrastructure.