CVE-2016-6903 in lshell
Summary
by MITRE
lshell 0.9.16 allows remote authenticated users to break out of a limited shell and execute arbitrary commands.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 09/04/2025
The vulnerability identified as CVE-2016-6903 affects lshell version 0.9.16, a restricted shell designed to limit user access and prevent execution of dangerous commands. This authentication bypass flaw exists within the command parsing and execution mechanisms of the restricted shell environment. The vulnerability allows authenticated remote attackers to exploit a command injection vector that enables them to escape the confined shell environment and execute arbitrary system commands with elevated privileges.
The technical implementation of this vulnerability stems from inadequate input validation and command sanitization within lshell's execution pipeline. When users execute commands through the restricted shell, the system fails to properly escape or filter special characters that could be used to manipulate the command execution flow. This weakness creates a path for attackers to inject malicious command sequences that bypass the intended restrictions. The flaw specifically manifests during command processing when the shell fails to properly handle shell metacharacters and command separators, allowing attackers to chain commands or execute alternative shell invocations.
The operational impact of this vulnerability is severe as it fundamentally undermines the security model of lshell's restricted environment. An authenticated attacker can leverage this vulnerability to gain full system access, escalate privileges, and potentially compromise the entire system. The vulnerability affects any environment where lshell is used for user access control, including network devices, servers, and systems requiring restricted administrative access. The remote nature of the attack means that an authenticated user can exploit this flaw from any location without requiring physical access to the system, making it particularly dangerous in multi-user environments where different privilege levels are expected.
Mitigation strategies for CVE-2016-6903 should focus on immediate patching of lshell to version 0.9.17 or later, which contains the necessary fixes for command injection prevention. Organizations should implement additional monitoring and logging of shell command executions to detect anomalous behavior that might indicate exploitation attempts. Network segmentation and access controls should be reinforced to limit the potential impact of a successful attack. The vulnerability aligns with CWE-78, which addresses improper neutralization of special elements used in OS commands, and maps to ATT&CK technique T1059.001 for command and scripting interpreter. System administrators should also consider implementing additional security controls such as mandatory access controls, privilege separation, and regular security audits to prevent similar vulnerabilities in other restricted shell implementations.