proc_jail

A fail-closed process execution guard for untrusted input, automation, and agentic systems.

proc_jail provides a policy-enforced wrapper around process spawning for Rust and Python. It applies deterministic bounds to commands derived from untrusted input, helping prevent command injection, unauthorized binary execution, and resource abuse.

Features

• No shell interpretation: Commands use argv-style execution, not shell strings
• Allowlist-only: Explicit enumeration of permitted binaries and flags
• Fail closed: Any error or ambiguity results in denial
• Resource limits: Timeout, stdout/stderr byte limits
• Double-dash injection: Automatic -- insertion to prevent flag injection
• Python and Rust APIs: Native bindings for both languages

Quick Start (Python)

pip install proc_jail

from proc_jail import ProcPolicyBuilder, ProcRequest, ArgRules

# Define a policy
policy = (
    ProcPolicyBuilder()
    .allow_bin("/usr/bin/grep")
    .arg_rules("/usr/bin/grep", 
        ArgRules()
        .allowed_flags(["-n", "-i", "-l", "-c"])
        .max_flags(4)
        .max_positionals(10)
        .inject_double_dash())
    .timeout(30)
    .build()
)

# Create and execute a request
request = ProcRequest("/usr/bin/grep", ["-n", "pattern", "file.txt"])
output = policy.prepare(request).spawn_sync()

print(output.stdout_string())

Quick Start (Rust)

[dependencies]
proc_jail = "0.2"
tokio = { version = "1", features = ["rt-multi-thread", "macros"] }

use proc_jail::{ProcPolicy, ProcRequest, ArgRules, InjectDoubleDash};
use std::time::Duration;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let policy = ProcPolicy::builder()
        .allow_bin("/usr/bin/grep")
        .arg_rules("/usr/bin/grep", ArgRules::new()
            .allowed_flags(&["-n", "-i", "-l", "-c"])
            .max_flags(4)
            .max_positionals(10)
            .inject_double_dash(InjectDoubleDash::AfterFlags))
        .timeout(Duration::from_secs(30))
        .build()?;

    let request = ProcRequest::new(
        "/usr/bin/grep",
        vec!["-n".into(), "pattern".into(), "file.txt".into()],
    );

    let output = policy.prepare(request)?.spawn().await?;
    println!("{}", output.stdout_string());
    Ok(())
}

Why proc_jail?

Traditional process spawning is dangerous in agentic systems:

# VULNERABLE: Shell injection
subprocess.run(f"grep '{query}' file.txt", shell=True)

# Attacker sets: query = "x'; rm -rf / #"
# Executes: grep 'x'; rm -rf / #' file.txt

With proc_jail, the same attack becomes harmless:

request = ProcRequest("/usr/bin/grep", [query, "file.txt"])
output = policy.prepare(request).spawn_sync()

# If query = "x'; rm -rf / #"
# Executes: grep "x'; rm -rf / #" file.txt
# The injection is just a literal string, not interpreted

Policies

Binary Allowlist

Only explicitly allowed binaries can be executed:

ProcPolicyBuilder()
    .allow_bin("/usr/bin/grep")
    .allow_bin("/usr/bin/jq")
    # ...

Argument Rules

Every binary requires explicit argument rules:

.arg_rules("/usr/bin/grep", 
    ArgRules()
    .allowed_flags(["-n", "-i", "-l"])
    .max_flags(3)
    .max_positionals(10)
    .inject_double_dash())

Subcommand Pinning

Pin allowed subcommands for tools like git:

.arg_rules("/usr/bin/git",
    ArgRules()
    .subcommand("status")
    .allowed_flags(["--porcelain", "-s"])
    .max_flags(2)
    .max_positionals(0))

Risky Binary Detection

Shells, interpreters, and privilege escalation tools are blocked by default:

# Even if allowed, bash is denied by default
policy.prepare(ProcRequest("/bin/bash", []))  # Error: BinRiskyDenied

# Opt-in with explicit acknowledgment
ProcPolicyBuilder()
    .allow_risky_binaries()
    # ...

Environment Control

By default, no environment variables are passed. Dangerous variables (LD_PRELOAD, PYTHONPATH, etc.) are always stripped.

Resource Limits

ProcPolicyBuilder()
    .timeout(30)           # seconds
    .max_stdout(10485760)  # 10 MB
    .max_stderr(1048576)   # 1 MB

The command runs in its own process group. When a limit is hit, the command exits, or the returned future is cancelled, the whole group is killed with SIGKILL, so grandchildren (for example npm run build → node) do not outlive the call.

Launching Through a Wrapper

To run a validated command inside an OS sandbox, launch it through the sandbox's launcher. Validation still applies to the original binary and arguments; the wrapper and launch env are trusted caller input.

let prepared = policy
    .prepare(request)?
    .wrap_with("/usr/bin/sandbox-exec", ["-p", &profile])  // sandbox-exec -p <profile> <bin> <argv...>
    .with_launch_env("TMPDIR", sandbox_tmp);                // added after the env policy
let output = prepared.spawn().await?;

bin()/argv() still return the validated command; launch_program()/launch_args() return what is executed. Python exposes the same methods on PreparedCommand.

Platform Support

Unix only (Linux, macOS). Windows is not supported because CreateProcess passes arguments as a single string that each program parses differently, making injection prevention impossible to guarantee. See docs/windows.md for details.

Related Projects

Project	Description	PyPI/Crates.io
path_jail	Path traversal prevention
url_jail	SSRF-safe URL validation
safe_unzip	Zip Slip and zip bomb prevention
tenuo	Capability-based authorization for AI agents

Integration with Tenuo

proc_jail provides the execution layer, while Tenuo provides cryptographic authorization. Together they offer defense in depth:

from proc_jail import ProcPolicyBuilder, ProcRequest, ArgRules

# proc_jail handles the safe execution
# Tenuo handles the authorization (who can call which tools)

def execute_grep(user_query: str, target_file: str) -> str:
    """Execute grep with proc_jail protection."""
    policy = (
        ProcPolicyBuilder()
        .allow_bin("/usr/bin/grep")
        .arg_rules("/usr/bin/grep", 
            ArgRules()
            .allowed_flags(["-n", "-i"])
            .max_positionals(10)
            .inject_double_dash())
        .timeout(30)
        .build()
    )
    
    # Even if user_query = "'; rm -rf / #", it's treated as a literal string
    request = ProcRequest("/usr/bin/grep", ["-n", user_query, target_file])
    output = policy.prepare(request).spawn_sync()
    return output.stdout_string()

Why both?

• Tenuo: Cryptographic proof the agent is authorized for this tool
• proc_jail: Prevents command injection even if the agent is compromised

Documentation

• SECURITY.md - Security properties, limitations, threat model
• CHANGELOG.md - Version history
• docs/windows.md - Why Windows is not supported
• docs/risky-binaries.md - Blocked binary categories
• docs/design-decisions.md - Rationale for key decisions

Repository Structure

proc_jail/
├── src/           # Rust library source
├── tests/         # Rust integration tests
├── docs/          # Documentation
├── python/        # Python bindings (PyO3)
│   ├── src/       # Rust binding code
│   └── proc_jail/ # Python package
└── ...

Development

# Build Rust library
cargo build

# Run tests
cargo test

# Build Python bindings
cd python
pip install maturin
maturin develop

License

MIT OR Apache-2.0