CVE-2023-43809 in Soft Serve
Summary
by MITRE • 10/25/2023
Soft Serve is a self-hostable Git server for the command line. Prior to version 0.6.2, a security vulnerability in Soft Serve could allow an unauthenticated, remote attacker to bypass public key authentication when keyboard-interactive SSH authentication is active, through the `allow-keyless` setting, and the public key requires additional client-side verification for example using FIDO2 or GPG. This is due to insufficient validation procedures of the public key step during SSH request handshake, granting unauthorized access if the keyboard-interaction mode is utilized. An attacker could exploit this vulnerability by presenting manipulated SSH requests using keyboard-interactive authentication mode. This could potentially result in unauthorized access to the Soft Serve. Users should upgrade to the latest Soft Serve version `v0.6.2` to receive the patch for this issue. To workaround this vulnerability without upgrading, users can temporarily disable Keyboard-Interactive SSH Authentication using the `allow-keyless` setting.
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Analysis
by VulDB Data Team • 10/25/2023
The vulnerability identified as CVE-2023-43809 affects Soft Serve, a command-line self-hostable Git server that relies on SSH for authentication and access control. This security flaw exists in versions prior to 0.6.2 and represents a critical weakness in the SSH authentication handshake process that could permit unauthorized access to Git repositories hosted on the server. The vulnerability specifically targets the interaction between public key authentication and keyboard-interactive authentication modes, creating a potential attack vector that bypasses normal security controls.
The technical flaw stems from insufficient validation procedures during the SSH request handshake process, particularly when the `allow-keyless` setting is enabled. During the authentication process, when keyboard-interactive authentication is active, the system fails to properly validate public key credentials that require additional client-side verification such as FIDO2 or GPG signatures. This validation gap occurs because the system does not adequately verify that the presented public key matches the expected cryptographic parameters or that the key has undergone proper authentication verification before granting access. The vulnerability essentially allows an attacker to manipulate SSH requests in a way that circumvents the normal public key verification process, particularly when the system is configured to accept keyboard-interactive authentication methods.
The operational impact of this vulnerability is significant for organizations relying on Soft Serve for their Git repository management. An unauthenticated remote attacker could potentially gain unauthorized access to repositories, leading to data breaches, code manipulation, and potential compromise of the entire Git infrastructure. The vulnerability is particularly dangerous because it can be exploited without requiring any prior authentication credentials, and the attack can be executed through standard SSH client tools that support keyboard-interactive authentication. This creates a scenario where legitimate users with valid public keys could be bypassed if the system is configured with keyboard-interactive authentication enabled, while attackers could exploit the weakness to gain access to sensitive code repositories and associated data.
The vulnerability aligns with CWE-287, which addresses improper authentication issues in software systems, and relates to ATT&CK technique T1566.001 for credential access through valid accounts. Organizations using Soft Serve should implement immediate mitigations by upgrading to version 0.6.2 which contains the necessary patch to address the validation gap in the SSH authentication handshake. Alternative workarounds include temporarily disabling keyboard-interactive SSH authentication through the `allow-keyless` setting, though this may reduce system functionality for legitimate users who require this authentication method. The patch addresses the core validation issue by implementing proper cryptographic verification procedures during the SSH handshake, ensuring that public key credentials undergo complete verification before access is granted, thereby preventing the bypass of authentication controls that occurred in vulnerable versions.
This vulnerability demonstrates the importance of proper authentication validation in SSH-based systems and highlights the risks associated with mixed authentication modes. The security implications extend beyond simple access control to include potential data integrity compromises, as unauthorized access could enable attackers to modify repository contents or inject malicious code. System administrators should conduct thorough security assessments of their Soft Serve installations to identify any instances of vulnerable versions and ensure proper configuration of authentication methods to prevent exploitation of this and similar vulnerabilities in the future.