CVE-2026-55581 in mcp-shell
Summary
by MITRE • 08/25/2026
mcp-shell is an MCP server for running shell commands securely, auditably, and on demand. Prior to 0.6.0, the default Docker security.yaml includes /bin/bash in allowed_executables, while security.go validates only the first token and checkBlockedPatternsAndCommands does not reject the shell command-mode flag -c. A caller of the shell_exec MCP tool can provide the command argument `/bin/bash -c <arbitrary-command>`, which passes validation and reaches executor.go, where parseCommand and exec.CommandContext execute the arbitrary command as mcpuser outside the intended allowlist. This issue is fixed in version 0.6.0.
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Analysis
by VulDB Data Team • 08/25/2026
The vulnerability identified in MCP-shell prior to version 0.6.0 represents a critical security flaw rooted in insufficient input validation and flawed access control logic within the shell execution mechanism. MCP-shell is designed as a Model Context Protocol server intended to execute shell commands securely, auditably, and on demand. However, the implementation contained significant weaknesses that allowed an attacker or malicious actor to bypass intended restrictions and achieve arbitrary command execution under the context of the mcpuser account. The core issue stems from two distinct but related failures in the security validation pipeline: a permissive default configuration and inadequate parsing logic for shell-specific arguments.
The first component of this vulnerability lies in the default Docker security.yaml configuration file, which included /bin/bash within the list of allowed_executables. While allowing specific shells is not inherently malicious if properly constrained, it becomes dangerous when combined with weak argument validation. The second, and more critical, failure occurred in the Go source code responsible for validating incoming commands. Specifically, the function checkBlockedPatternsAndCommands only validated the first token of the provided command string. This means that while the initial binary path might have been checked against an allowlist or blocklist, any subsequent arguments passed to that binary were largely ignored during the validation phase. Furthermore, this validation logic failed to reject shell commands utilizing the -c flag, which is standard for bash and sh shells to execute a command string provided as an argument rather than reading from stdin or a script file.
When a caller invokes the shell_exec MCP tool with a command argument such as /bin/bash -c <arbitrary-command>, the system's security checks are effectively bypassed. The validation logic sees /bin bash, which may be permitted depending on configuration, and stops checking further because it only inspects the first token. Consequently, the string -c <arbitrary-command> passes through unchecked to executor.go. Within this module, parseCommand processes the input without applying additional restrictions on shell-specific flags that enable arbitrary execution contexts. The final step involves exec.CommandContext, which executes the command as mcpuser outside of any intended allowlist constraints for the actual payload being run. This results in a classic Command Injection vulnerability where an attacker can execute arbitrary system commands with the privileges of the service account running the MCP server.
From a classification perspective, this flaw aligns closely with CWE-78 Improper Neutralization of Special Elements used in an OS Command commonly known as OS Command Injection. The failure to properly sanitize or validate user-supplied input before constructing operating system commands allows for unauthorized execution. Additionally, from a tactical standpoint within the MITRE ATT&CK framework, this vulnerability facilitates lateral movement and privilege escalation if the mcpuser account has broader access than intended. It also relates to CWE-20 Improper Input Validation, as the application failed to ensure that the input data met expected specifications for safe execution. The use of shell interpreters with -c flags is a common vector for such attacks because it allows complex command chaining and environment manipulation that simple binary executions do not permit.
The operational impact of this vulnerability is severe depending on the permissions granted to mcpuser. If this user has access to sensitive files, network resources, or administrative tools, an attacker could exfiltrate data, install malware, pivot to other systems, or disrupt services. Even if restricted, the ability to run arbitrary commands undermines the fundamental security model of MCP-shell, which is supposed to provide a controlled and auditable interface for shell interactions. The lack of auditability in this bypass scenario means that malicious activities could occur without proper logging or detection by standard monitoring tools designed to watch for specific binary executions rather than complex command strings passed through interpreters.
This issue was addressed in version 0.6.0, which likely involved tightening the validation logic to inspect all tokens within a command string, not just the first one. It also presumably included stricter handling of shell-specific flags like -c to prevent indirect execution paths that bypass allowlists. To mitigate similar risks in other systems or during migration to patched versions, administrators should ensure that default configurations do not include interpreters unless absolutely necessary and strictly constrained. Input validation must be comprehensive, checking all arguments passed to executables against a strict whitelist of allowed patterns. Additionally, employing principle of least privilege for service accounts like mcpuser is essential to limit the blast radius in case of future vulnerabilities. Regular security audits of configuration files and source code logic are recommended to identify such bypasses before they can be exploited in production environments.