CVE-2019-3464 in rssh
Summary
by MITRE
Insufficient sanitization of environment variables passed to rsync can bypass the restrictions imposed by rssh, a restricted shell that should restrict users to perform only rsync operations, resulting in the execution of arbitrary shell commands.
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Analysis
by VulDB Data Team • 07/06/2023
The vulnerability identified as CVE-2019-3464 represents a critical security flaw in the rsync utility that directly impacts the effectiveness of restricted shell environments. This issue specifically affects rssh, a specialized shell designed to limit user capabilities to only rsync operations while preventing access to other system commands. The flaw stems from inadequate sanitization of environment variables that are passed to rsync during execution, creating a pathway for privilege escalation and arbitrary command execution. When users authenticate through rssh, the restricted shell should enforce strict boundaries on what operations they can perform, but this vulnerability allows malicious actors to circumvent these protections through carefully crafted environment variable manipulation.
The technical root cause of CVE-2019-3464 lies in how rsync processes environment variables, particularly those that may contain special shell characters or command sequences. When rssh passes environment variables to rsync, it fails to properly sanitize these inputs before they are interpreted by the underlying shell. This vulnerability maps directly to CWE-77 and CWE-78, which address insufficient input sanitization and command injection flaws respectively. Attackers can exploit this weakness by setting environment variables that contain shell metacharacters or command sequences that get executed when rsync processes them, effectively bypassing the intended restrictions of the rssh environment.
The operational impact of this vulnerability extends beyond simple command execution, as it fundamentally undermines the security model of restricted shell environments. Organizations relying on rssh for secure file transfer operations face significant risks including unauthorized access to system resources, potential data exfiltration, and privilege escalation to higher-level system accounts. The vulnerability is particularly dangerous in environments where multiple users share systems and administrative privileges are limited, as it allows unprivileged users to execute commands with elevated permissions. This flaw directly aligns with ATT&CK technique T1059.001, which covers command and scripting interpreter execution, and T1548.003, covering abuse of sudo privileges through environment variable manipulation.
Mitigation strategies for CVE-2019-3464 require immediate attention from system administrators and security teams. The primary solution involves updating to patched versions of rsync and rssh where environment variable sanitization has been properly implemented. Organizations should also implement additional security controls such as restricting environment variable passing through configuration files, implementing strict input validation for all user-supplied data, and monitoring for unusual environment variable usage patterns. Network segmentation and access control measures should be enhanced to limit the potential impact of successful exploitation, while regular security audits should verify that restricted shell configurations remain effective against similar vulnerabilities. System hardening practices should include disabling unnecessary environment variables, implementing strict command whitelisting, and ensuring that all users operating within restricted environments are properly authenticated and authorized through robust identity management systems.