GitHub - navikt/cplt: Drop in replacement sandbox for GitHub Copilot CLI for macOS

Sandbox wrapper for AI coding agents. Runs GitHub Copilot CLI, OpenCode, or a plain shell inside a kernel-level sandbox so the agent can work on your project but cannot access your secrets.

• macOS: Apple Seatbelt/SBPL via sandbox-exec
• Linux: Landlock LSM + seccomp-BPF (kernel 5.13+; full network filtering on 6.7+)

Table of contents

• Quick start
• Install
  • Shell setup
• What it does
• Usage
• Configuration file
• Architecture
• Security
• Domain filtering
• Proxy operations
• Known impacts
• Limitations
• Contributing
• References

Quick start

# Install
brew install navikt/tap/cplt

# Make 'copilot' run the sandboxed version (persistent)
cplt --shell-install

# Check your environment
cplt --doctor

# Run Copilot in sandbox
cplt -- -p "fix the tests"

# Or run OpenCode in sandbox (with Copilot subscription — no API key needed)
cplt --agent opencode

# Or with a third-party provider
cplt --agent opencode --pass-env ANTHROPIC_API_KEY

# Or just a sandboxed shell (no AI agent, same security restrictions)
cplt --agent shell

Primary control: filesystem isolation. The sandbox blocks access to credentials and secrets at the kernel level. All restrictions apply to the agent and every process it spawns.

This table is a summary. The sandbox also allows access to system files (SSL certs, /etc/hosts), temp directories (read/write but no exec), and system tool paths (/usr/bin, /opt/homebrew). Run cplt --print-profile to see the complete SBPL rules.

For the full security model, threat analysis, and test strategy, see SECURITY.md.

Install

Homebrew (recommended)

brew install navikt/tap/cplt

curl | bash

curl -fsSL https://raw.githubusercontent.com/navikt/cplt/main/install.sh | bash

Options:

# Install a specific version
curl -fsSL ... | bash -s -- --version 2026.05.05-174753-75bae5b

# Install to a custom directory
curl -fsSL ... | bash -s -- --dir ~/.local/bin

# Skip Homebrew (force direct download)
curl -fsSL ... | bash -s -- --no-brew

Download from releases

Download the latest release for your platform from GitHub Releases:

# macOS — Apple Silicon (M1/M2/M3/M4)
curl -fsSL https://github.com/navikt/cplt/releases/latest/download/cplt-aarch64-apple-darwin.tar.gz | tar xz
sudo mv cplt /usr/local/bin/

# macOS — Intel
curl -fsSL https://github.com/navikt/cplt/releases/latest/download/cplt-x86_64-apple-darwin.tar.gz | tar xz
sudo mv cplt /usr/local/bin/

# Linux — x86_64
curl -fsSL https://github.com/navikt/cplt/releases/latest/download/cplt-x86_64-unknown-linux-gnu.tar.gz | tar xz
sudo mv cplt /usr/local/bin/

# Linux — ARM64
curl -fsSL https://github.com/navikt/cplt/releases/latest/download/cplt-aarch64-unknown-linux-gnu.tar.gz | tar xz
sudo mv cplt /usr/local/bin/

Every release binary has build provenance attestation — verify it with:

gh attestation verify cplt -o navikt

Build from source

git clone https://github.com/navikt/cplt.git && cd cplt
cargo build --release
sudo cp target/release/cplt /usr/local/bin/

Or with mise:

Shell setup (recommended)

By default, you run the sandboxed version with cplt. To make copilot run the sandboxed version too, use the one-command installer:

This detects your shell, appends the alias to your rc file, and prints what it did. Safe to run multiple times — it won't add duplicates.

After installing, restart your shell or source the file to activate.

# zsh / bash
eval "$(cplt --shell-setup)"

# fish
alias copilot cplt

Why an alias instead of a symlink? Both cplt and Copilot CLI install into the same Homebrew bin directory (/opt/homebrew/bin/). A symlink would conflict — only one file named copilot can exist there. A shell alias avoids this entirely: the real copilot binary stays in PATH (so cplt can find and wrap it), and the alias transparently redirects your command.

Note: cplt has recursion prevention built in. If it detects it's already running inside a sandbox (via the __CPLT_WRAPPED environment variable), it will refuse to launch again. Read-only subcommands like --print-profile and --doctor still work inside an existing sandbox.

What it does

Usage

cplt [OPTIONS] [-- <AGENT_ARGS>...]

Everything after -- is passed directly to the agent process (copilot, opencode, or shell).

File access

The project directory is the primary writable workspace, plus a narrow allowlist required for auth, runtime, and tooling (see capability table above). Everything else (SSH keys, cloud credentials, etc.) is blocked by the kernel.

Environment variables

By default, cplt sanitizes the child environment — only safe variables pass through. Cloud credentials, database URLs, and package tokens are stripped. Additionally, security hardening variables are injected to block npm/yarn/pnpm lifecycle scripts (postinstall hooks) — the #1 supply chain attack vector — and disable git commit/tag signing (since ~/.ssh and ~/.gnupg are inaccessible inside the sandbox).

What passes through:

Prefix allowlist with secret-suffix protection: Variables matching allowed prefixes (e.g. COPILOT_*, YARN_*) are passed through unless they end with a secret-bearing suffix: _TOKEN, _AUTH, _SECRET, _SECRET_KEY, _KEY, _PASSWORD, or _CREDENTIALS. For example, COPILOT_DEBUG passes through but COPILOT_API_KEY is blocked.

Always blocked: AWS_*, AZURE_*, NPM_TOKEN, DATABASE_URL, VAULT_TOKEN, SSH_AUTH_SOCK, Docker vars, CI tokens, and anything not in the allowlist.

Supported runtimes

cplt auto-discovers installed tools and configures sandbox rules accordingly. Only directories that exist on disk get rules (no phantom paths).

To see which tools cplt detected, run cplt --doctor.

Proxy

The proxy is enabled by default — all outbound traffic (Copilot CLI, gh, curl) is routed through a localhost CONNECT proxy via HTTP_PROXY/HTTPS_PROXY and NODE_USE_ENV_PROXY=1. The proxy listens on an OS-assigned ephemeral port, so there are no port conflicts.

What the proxy gives you:

• Connection logging — see every domain Copilot connects to in real time
• Domain blocking — block known exfiltration infrastructure (paste sites, webhook services, etc.)
• Domain allowlisting — restrict connections to only known-safe domains
• Audit log — persistent file log of all connections for post-session review
• Port enforcement — the proxy enforces the same port restrictions as the sandbox (443 + --allow-port)

Disable for a single run:

cplt --no-proxy -- -p "fix the tests"

Disable permanently (in ~/.config/cplt/config.toml):

Add connection filtering (recommended):

Then edit ~/.config/cplt/config.toml:

[proxy]
# blocked_domains = "~/.config/cplt/blocked-domains.txt"  # block known-bad domains
# allowed_domains = "~/.config/cplt/allowed-domains.txt"  # restrict to known-safe domains
# log_file = "~/.config/cplt/proxy.log"                   # persistent audit log
# log_level = "none"                                      # stderr: none|error|blocked|all

Domain matching: both blocklist and allowlist use the same rules — example.com matches the exact domain and all subdomains (sub.example.com, deep.sub.example.com). Matching is case-insensitive. Trailing dots are stripped.

Localhost traffic (MCP servers, dev servers) bypasses the proxy via NO_PROXY and will not appear in the audit log.

Quiet mode (-q / sandbox.quiet = true) suppresses the startup banner. Proxy stderr output is controlled separately by --proxy-log-level (default: none — silent). Use --proxy-log to capture all connections to a file.

Debugging

Copilot session flags

These flags are forwarded directly to the copilot process for convenience — no -- separator needed.

You can combine these with cplt sandbox flags and -- pass-through args:

cplt --resume=my-task                          # resume by name
cplt --remote --name my-task -- -p "fix tests" # remote + named + prompt

OpenCode support

cplt can sandbox OpenCode in addition to GitHub Copilot CLI. OpenCode is officially supported as a GitHub Copilot client — use your existing Copilot subscription with /connect inside OpenCode.

# Run OpenCode with Copilot subscription (no API key needed)
cplt --agent opencode

# Auto-detect: if only opencode is in PATH, cplt uses it automatically
cplt

# Or use a third-party provider
cplt --agent opencode --pass-env ANTHROPIC_API_KEY

Security notes for OpenCode:

• Copilot provider: auth is handled via /connect device flow, stored in ~/.local/share/opencode/auth.json — no env vars needed
• Third-party providers: API keys (ANTHROPIC_API_KEY, OPENAI_API_KEY, etc.) are not passed through by default — use --pass-env:

cplt --agent opencode --pass-env ANTHROPIC_API_KEY
cplt --agent opencode --pass-env OPENAI_API_KEY --pass-env OPENAI_ORG_ID

• OpenCode config (~/.config/opencode/) is read-only in the sandbox
• OpenCode data (~/.local/share/opencode/) is writable but execution is denied (write+exec prevention)
• Keychain access is disabled (OpenCode uses its own auth file, not macOS Keychain)
• --resume, --continue, --remote, --name flags are Copilot-specific and ignored for OpenCode

Shell mode

Run a plain sandboxed shell — no AI agent, same security restrictions. Useful for testing build tools, debugging sandbox issues, or working in a secure environment manually.

# Interactive sandboxed shell (uses $SHELL — fish, zsh, bash)
cplt --agent shell

# Run a single command inside the sandbox
cplt --agent shell -- -c 'go test ./...'

# Inspect what's allowed without entering the shell
cplt --agent shell --print-profile

The sandbox applies the same deny-by-default rules — filesystem isolation, network restrictions, env sanitization. Shell config directories (fish variables/history, zsh history) are writable.

Examples

# Most common: run Copilot in sandbox
cplt -- -p "fix the tests"

# Resume a previous session
cplt --resume

# Resume a named session
cplt --resume=my-refactor

# Resume the most recent session in this directory
cplt --continue

# Start a named remote session
cplt --remote --name my-task -- -p "fix the tests"

# Check environment before first run
cplt --doctor

# Disable proxy for a single run (proxy is on by default)
cplt --no-proxy -- -p "fix the tests"

# Let Copilot read a shared library directory
cplt --allow-read ~/shared-libs -- -p "use shared-libs"

# Allow outbound on extra ports (e.g., external API)
cplt --allow-port 8443 -- -p "test the API"

# Allow localhost for MCP servers or dev servers
cplt --allow-localhost 3000 --allow-localhost 8080 -- -p "use the MCP server"

# Allow all localhost (needed for Next.js/Turbopack, Vite builds)
cplt --allow-localhost-any -- -p "fix the build"

# Non-interactive / CI usage (skip confirmation prompt)
cplt --yes -- -p "fix the tests"

# Block a path you don't want Copilot to see
cplt --deny-path ~/.config/gh -- -p "refactor auth"

# Pass a specific env var through (e.g. custom tool config)
cplt --pass-env MY_CUSTOM_VAR --pass-env ANOTHER_VAR -- -p "run with custom config"

# Inherit full environment (dangerous — only for debugging)
cplt --inherit-env -- -p "debug the build"

# Block paste sites
cplt --blocked-domains ./blocked-domains.txt -- -p "refactor"

# Use an internal MCP server on the corporate network
cplt --allow-private-domain intern.nav.no -- -p "use mcp-onboarding"

# Inspect the generated sandbox profile
cplt --print-profile

# Debug: see what the sandbox blocks in real time
cplt --show-denials -- -p "fix the tests"

# Run OpenCode with Copilot subscription (no API key needed)
cplt --agent opencode

# Run OpenCode with a third-party provider
cplt --agent opencode --pass-env ANTHROPIC_API_KEY

# Run OpenCode with auto-detection (if copilot not in PATH)
cplt --pass-env ANTHROPIC_API_KEY

# Run a sandboxed shell (no AI agent)
cplt --agent shell

# Run a one-off command in the sandbox
cplt --agent shell -- -c 'npm test'

Configuration file

Save your preferred defaults to ~/.config/cplt/config.toml so you don't need to pass flags every time.

Create the default config:

This creates a commented template at ~/.config/cplt/config.toml:

[proxy]
# enabled = true             # Default: true — disable with --no-proxy or set false
# port = 0                   # Default: 0 (OS-assigned ephemeral port)
# blocked_domains = "~/.config/cplt/blocked-domains.txt"
# allowed_domains = "~/.config/cplt/allowed-domains.txt"
# log_file = "~/.config/cplt/proxy.log"
# log_level = "none"             # Stderr verbosity: none, error, blocked, all
# allow_private_domains = ["intern.nav.no"]  # Allow internal/intranet domains to resolve to private IPs

[sandbox]
# validate = true
# allow_env_files = false
# allow_lifecycle_scripts = false
# allow_gpg_signing = false    # Allow GPG commit signing (see SECURITY.md)
# allow_jvm_attach = false     # Allow JVM Attach API unix sockets (MockK, Mockito)
# allow_localhost_any = false
# scratch_dir = true           # On by default; set false to disable
# allow_tmp_exec = false       # Dangerous — prefer scratch_dir
# allow_cache_exec = []        # Allow exec from specific ~/Library/Caches subdirs, e.g. ["ms-playwright", "pnpm/dlx"]
# allow_cache_exec_any = false # Dangerous — allow exec from all of ~/Library/Caches
# inherit_env = false          # Dangerous — exposes all env vars
# pass_env = ["MY_CUSTOM_VAR"]

[allow]
# read = ["~/some/path"]
# write = ["~/another/path"]
# ports = [8080]
# localhost = [3000, 8080]

[deny]
# paths = ["~/extra/secret"]

Precedence (highest to lowest):

1. CLI flags (--with-proxy, --no-proxy, --proxy-port, etc.)
2. Config file (~/.config/cplt/config.toml)
3. Built-in defaults

CLI flags always override the config file. Use --no-proxy to disable the proxy for a single run.

Environment variable override:

Set CPLT_CONFIG to use a config file at a custom location:

CPLT_CONFIG=/path/to/custom.toml cplt -- --version

Path expansion: Paths in [allow] and [deny] support ~/ expansion and are resolved relative to the config file directory. proxy.blocked_domains supports ~/ expansion only.

Managing config from the CLI

Instead of editing TOML by hand, use cplt config:

cplt config show                          # show effective config (file + defaults)
cplt config get sandbox.quiet             # get a single value
cplt config explain                       # list all keys with descriptions
cplt config explain sandbox.pass_env      # explain a specific key
cplt config validate                      # check for syntax errors and unknown keys

Setting values:

# Scalar keys — set replaces the value
cplt config set sandbox.quiet true
cplt config set proxy.port 9090

# Array keys — set appends (idempotent, no duplicates)
cplt config set allow.read ~/Desktop
cplt config set allow.read ~/Documents    # adds a second entry
cplt config set allow.read ~/Desktop      # no-op, already present
cplt config set allow.ports 8080

Removing values:

# Remove a single element from an array
cplt config set allow.read ~/Desktop --unset

# Remove an entire key (reverts to default)
cplt config set allow.read --unset
cplt config set sandbox.quiet --unset

Architecture

┌──────────────────────────────────┐
│  cplt (Rust binary)              │
│  ┌───────────┐  ┌─────────────┐  │
│  │ Policy    │  │ CONNECT     │  │
│  │ Generator │  │ Proxy       │  │
│  └─────┬─────┘  │ (optional)  │  │
│        │        └─────────────┘  │
│        ▼                         │
│  ┌─────────────┬────────────┐    │
│  │   macOS     │   Linux    │    │
│  │  Seatbelt   │  Landlock  │    │
│  │  sandbox-   │  + seccomp │    │
│  │  exec       │  pre_exec  │    │
│  └─────────────┴────────────┘    │
│        │                         │
│        ▼                         │
│  copilot (sandboxed)             │
│  ├── All child processes         │
│  ├── Cannot read ~/.ssh          │
│  ├── Network port-restricted     │
│  ├── SSH agent blocked           │
│  └── Filesystem = primary ctrl   │
└──────────────────────────────────┘

Security model: deny-by-default filesystem with kernel enforcement. On macOS and Linux with kernel 6.7+ (Landlock ABI v4), network is restricted to port 443 (HTTPS) by default (use --allow-port for extras). On older Linux kernels, network restriction is provided by the CONNECT proxy (enabled by default). SSH agent access and localhost outbound are blocked at the kernel level (macOS) or via the proxy (Linux). The profile generator auto-discovers your environment (--doctor) and only includes tool directories that actually exist on disk — fewer rules means a tighter sandbox.

Platform-specific details:

• macOS: Seatbelt/SBPL profile generated and passed to sandbox-exec
• Linux: Landlock LSM rules + seccomp-BPF filter applied via pre_exec (kernel 5.13+, TCP port filtering on 6.7+)

See SECURITY.md for the full threat model, defense layers, and honest gaps.

Security

~2500 lines of Rust. Four dependencies (clap, libc, serde, toml). No runtime services, no telemetry. Every security boundary is kernel-enforced and tested. Every design decision is documented with the threat it mitigates and the prior art it builds on.

Our priorities, in order:

1. Correct — every claim is tested, every edge case has a CVE or research reference
2. Transparent — read SECURITY.md, it hides nothing
3. Simple — single static binary, zero config required, sane defaults
4. Useful — get out of the way and let Copilot do its job, safely

For the full security model, threat analysis, and test strategy, see SECURITY.md.

~/.config/gh/hosts.yml is readable

Copilot spawns gh auth token to authenticate. This reads ~/.config/gh/hosts.yml which contains a GitHub OAuth token. We allow reading only hosts.yml and config.yml (not the entire .config/gh directory) because:

• Required for auth: Without gh auth, Copilot falls back to Keychain only. Many users rely on gh CLI for auth.
• Read-only: The sandbox cannot modify the token file.
• Minimal access: Only the two files gh actually reads — extensions, state, and other gh data are blocked.
• Same-destination token: The token is a GitHub token that Copilot already sends to GitHub's API. An attacker would need to exfiltrate it to a different server.
• Risk: A compromised Copilot could exfiltrate this token via port 443. Use --deny-path ~/.config/gh if this concerns you (Copilot will use Keychain auth instead).

Outbound network is port-restricted

SBPL (Seatbelt Profile Language) does not support wildcard port filtering by IP range. Copilot connects to multiple CDN-backed endpoints with changing IPs (api.business.githubcopilot.com, api.githubcopilot.com, proxy.business.githubcopilot.com). We cannot enumerate these IPs. Therefore:

• Only port 443 (HTTPS) is allowed — all other outbound TCP ports are blocked at the kernel level
• Localhost outbound is blocked — prevents access to local services (databases, dev servers, etc.)
• SSH agent is blocked — unix socket access is denied, preventing use of loaded SSH keys
• Filesystem isolation is the primary control — credentials are kernel-blocked regardless of network
• The proxy is on by default — logs and filters all outbound connections (Copilot, gh, curl)
• Use --allow-port to add extra ports when needed (e.g., --allow-port 8080 for a dev server)

See SECURITY.md for the full threat model and honest gaps.

Domain filtering

When the proxy is enabled, it supports both blocking (deny known-bad domains) and allowlisting (permit only known-good domains).

Blocklist

Block domains commonly used for data exfiltration. A default blocklist is included based on real attack infrastructure observed in 2025–2026 supply chain incidents:

cplt --blocked-domains blocked-domains.txt -- -p "fix tests"

The blocklist covers webhook capture services, paste sites, file sharing, tunneling services, and IP recon endpoints. See blocked-domains.txt for the full list with sources.

Allowlist

Restrict connections to only specific domains. When set, the proxy blocks everything not in the list:

cplt --allowed-domains allowed-domains.txt -- -p "fix tests"

Example allowed-domains.txt for Copilot-only access:

api.github.com
api.githubcopilot.com
api.business.githubcopilot.com
proxy.business.githubcopilot.com
telemetry.business.githubcopilot.com

Both blocklist and allowlist can be used together — allowlist is checked first, then blocklist.

Set either permanently in ~/.config/cplt/config.toml:

[proxy]
enabled = true
blocked_domains = "~/.config/cplt/blocked-domains.txt"
# allowed_domains = "~/.config/cplt/allowed-domains.txt"

Note: Both the allowlist and blocklist are re-read from disk every ~5 seconds (TTL-cached), so you can edit them live mid-session. Changes take effect within seconds without restarting cplt. If a file becomes unreadable at runtime, the last-known-good list is kept (fail-safe). At startup, an unreadable allowlist causes cplt to exit with an error (fail-closed).

The allow_private_domains list in config.toml is also re-read every ~5 seconds. Domains added via --allow-private-domain CLI flags are always preserved regardless of config changes.

Proxy operations

Connection log

Every connection attempt is printed to stderr in real time:

[proxy] 14:23:01 CONNECT api.githubcopilot.com:443 → CONNECTED
[proxy] 14:23:04 CONNECT pastebin.com:443 → BLOCKED
[proxy] 14:23:07 CONNECT mcp-onboarding.intern.nav.no:443 → BLOCKED-PRIVATE-RESOLVED

To write a persistent audit log:

cplt --proxy-log ~/.config/cplt/proxy.log -- -p "fix tests"

Log file format (one line per connection):

2025-01-15T14:23:01Z CONNECT api.githubcopilot.com:443 CONNECTED
2025-01-15T14:23:04Z CONNECT pastebin.com:443 BLOCKED

Status codes

Troubleshooting

Tool blocked with BLOCKED-PRIVATE-RESOLVED — a domain (typically corporate intranet) resolved to a private IP:

cplt --allow-private-domain mcp-onboarding.intern.nav.no -- -p "use the MCP server"
# Or match all subdomains:
cplt --allow-private-domain intern.nav.no -- -p "use the MCP server"

MCP server on localhost blocked — use --allow-localhost (not --allow-private-domain):

cplt --allow-localhost 3000 -- -p "use local MCP server"

Tool needs a non-443 port — add it explicitly:

cplt --allow-port 8443 -- -p "test the API"

Nothing connects — check if proxy is running:

cplt --print-profile | grep localhost   # shows the proxy port rule in the Seatbelt profile

Disable the proxy entirely for debugging:

cplt --no-proxy -- -p "fix tests"

Corporate proxy environments

cplt injects its own HTTP_PROXY/HTTPS_PROXY into the sandbox environment, replacing any corporate proxy you may have set. The sandbox environment is cleared by default (sensitive env vars stripped), so your external HTTP_PROXY does not flow in.

If you need to chain through a corporate proxy instead of using cplt's built-in proxy:

cplt --no-proxy --pass-env HTTP_PROXY --pass-env HTTPS_PROXY -- -p "fix tests"

Note that --no-proxy disables domain filtering, connection logging, and port enforcement. Use --allowed-domains or --blocked-domains as compensating controls when possible.

Copilot CLI network endpoints

Copilot CLI 1.0.21 connects directly to these endpoints (empirically verified):

Known impacts

The sandbox is kernel-enforced — all restrictions apply to every process spawned inside it, including dev servers, test runners, build tools, and package managers. This is by design (a sandboxed agent could otherwise escape by spawning a child process), but it affects some workflows:

.env file blocking

.env*, .pem, .key, .p12, .pfx, .jks files in the project directory are blocked from reading by default. This prevents a rogue agent from exfiltrating secrets, but has side effects:

Fix: Use --allow-env-files when working on projects that need env file loading:

cplt --allow-env-files -- -p "start the dev server and fix the failing test"

Or set it permanently in config:

[sandbox]
allow_env_files = true

Lifecycle scripts (postinstall hooks)

npm/yarn/pnpm lifecycle scripts are blocked by default via npm_config_ignore_scripts=true and YARN_ENABLE_SCRIPTS=false. This prevents supply chain attacks through postinstall hooks, but may break packages that require post-install steps:

Fix: Use --allow-lifecycle-scripts when the project needs postinstall hooks:

cplt --allow-lifecycle-scripts -- -p "install dependencies and build the project"

Or set it permanently in config:

[sandbox]
allow_lifecycle_scripts = true

Temp dir execution (go test, mise, node-gyp)

Tools that compile-then-execute from $TMPDIR are blocked by default because the sandbox denies process-exec and file-map-executable from /private/tmp and /private/var/folders. This affects:

Fix: The scratch dir is now on by default — cplt creates ~/Library/Caches/cplt/tmp/{session-id}/ with rwx permissions, redirects TMPDIR, TMP, TEMP, and GOTMPDIR there, and cleans up on exit. Stale directories older than 24 hours are garbage-collected on startup.

JVM note: On macOS, the JVM ignores TMPDIR — it reads java.io.tmpdir from confstr(_CS_DARWIN_USER_TEMP_DIR) which always returns /var/folders/.... cplt automatically injects -Djava.io.tmpdir=<scratch> -Djansi.tmpdir=<scratch> -Djava.rmi.server.hostname=localhost via JAVA_TOOL_OPTIONS so that Maven Surefire forks, the Kotlin compiler daemon, and Jansi native lib extraction all use the scratch dir. The RMI hostname flag ensures the Kotlin daemon's Java RMI communication stays on localhost (without it, InetAddress.getLocalHost() may resolve to a non-loopback IP via mDNS, which the sandbox blocks). Override with --pass-env JAVA_TOOL_OPTIONS if you need custom JVM flags. For inline mocking (MockK, Mockito, ByteBuddy), also add --allow-jvm-attach — see JVM Attach API.

Gradle/JVM still failing? Some JVM native libraries (e.g. libjli.dylib, JNI libs) use dlopen from the system temp dir before JAVA_TOOL_OPTIONS takes effect. If you see "Operation not permitted" during JVM startup itself (not Gradle build), add --allow-tmp-exec:

# Recommended for Gradle projects (localhost + tmp exec + JVM attach):
cplt --allow-localhost-any --allow-tmp-exec --allow-jvm-attach -- -p "run tests"

# Or set permanently:
cplt config set sandbox.allow_localhost_any true
cplt config set sandbox.allow_tmp_exec true
cplt config set sandbox.allow_jvm_attach true

If you're still seeing this error, check that you haven't set scratch_dir = false in your config:

cplt config explain sandbox.scratch_dir

Cache exec (Playwright, pnpm dlx, etc.)

Some tools unpack and execute binaries directly from ~/Library/Caches, which is exec-blocked by default:

# Allow Playwright browser binaries
cplt --allow-cache-exec ms-playwright -- -p "run the e2e tests"

# Allow pnpm dlx-cached binaries
cplt --allow-cache-exec pnpm/dlx -- -p "run the scripts"

# Both at once
cplt --allow-cache-exec ms-playwright --allow-cache-exec pnpm/dlx -- -p "run tests"

Or set permanently in config:

[sandbox]
allow_cache_exec = ["ms-playwright", "pnpm/dlx"]

--allow-cache-exec-any opens exec for all of ~/Library/Caches — use only as a last resort.

Localhost blocking

Localhost outbound is blocked by default, which prevents sandboxed processes from connecting to local services:

Fix: Use --allow-localhost <PORT> for specific services, or --allow-localhost-any for build tools that use random ports (Next.js, Vite, esbuild).

Docker and Testcontainers

Docker is intentionally blocked — ~/.docker is denied and the Docker socket is not accessible. This is by design: Docker gives near-root access to the host system, which defeats the purpose of sandboxing.

• Docker commands, docker compose, and Testcontainers will fail
• Local databases via Docker Compose need --allow-localhost <PORT> for the exposed port (the database container runs outside the sandbox)
• Consider running database/Kafka containers before starting cplt, then use --allow-localhost for the ports

Opting in (⚠️ dangerous): If you understand the risks (container mounts bypass the sandbox entirely), you can allow Docker access:

cplt config set sandbox.allow_docker true
# or per-session:
cplt --allow-docker

SSH agent blocking

SSH agent access is blocked (unix socket denied), which means:

• git clone over SSH will fail — use HTTPS clones instead
• ssh commands spawned by the agent will fail
• gh CLI uses HTTPS by default and is unaffected

macOS protected folders (Desktop, Documents)

macOS TCC (Transparency, Consent, and Control) protects certain folders at the kernel level. Without Full Disk Access, Copilot CLI cannot access ~/Desktop or ~/Documents with or without cplt — this is a macOS restriction, not a sandbox limitation. The cplt sandbox remains fully active regardless of FDA status.

Fix: Grant Full Disk Access to your terminal (recommended):

1. Open System Settings → Privacy & Security → Full Disk Access
2. Enable your terminal app (Terminal.app, iTerm2, Ghostty, etc.)
3. Restart the terminal — TCC grants only take effect for new processes

This lifts TCC restrictions for all child processes while the cplt sandbox continues to enforce its own deny-by-default rules (write protection, network filtering, dotfile access, etc.).

Alternatives (if you prefer not to grant FDA):

1. Copy files into your project:

   cp ~/Desktop/screenshot.png .

2. Use a non-protected folder for screenshots:

   defaults write com.apple.screencapture location ~/Screenshots
   mkdir -p ~/Screenshots

   Then add to config:

   [sandbox]
   allow_read = ["~/Screenshots"]

Git workflow (commit & push)

Git commit and push work out of the box over HTTPS — no extra flags needed.

Prerequisites:

1. Use HTTPS remotes (not SSH). Check with git remote -v:

   # If you see git@github.com:org/repo.git, switch to HTTPS:
   git remote set-url origin https://github.com/org/repo.git

   Or rewrite globally for all repos (no remote changes needed):

   git config --global url."https://github.com/".insteadOf "git@github.com:"

   This makes git transparently use HTTPS even when remotes are configured as SSH. The rewrite is read from ~/.gitconfig which is readable inside the sandbox.
2. Authenticate with gh — cplt allows the agent to read gh auth token:

   gh auth login   # one-time setup outside the sandbox

3. Configure git credential helper (if not already set by gh auth setup-git):

   gh auth setup-git   # sets credential.helper to use gh

That's it. The agent can now git add, git commit, git push, create branches, and fetch — all inside the sandbox.

Optional: signed commits — add --allow-gpg-signing (see GPG signing).

Why is SSH blocked? The SSH agent socket gives access to all loaded keys, which could authenticate to any host. HTTPS with gh auth token is scoped to GitHub only. See SSH agent blocking.

Tip: Protect your main branch with branch protection rules to prevent the agent from pushing directly to main or force-pushing. This is good practice regardless of cplt.

Git restrictions

Certain git operations are blocked to prevent persistence attacks that survive the sandbox session:

Global git hooks: If core.hooksPath is set in ~/.gitconfig, cplt auto-detects the hooks directory and allows reading it so git operations succeed. Write access is explicitly denied to prevent persistence attacks. The hooks path must be under $HOME with at least 3 path components (e.g. ~/.config/git/hooks) to prevent overly broad read access.

Commit signing: ~/.ssh and ~/.gnupg are blocked, so GPG/SSH signing would fail. Instead of opening private key directories, cplt injects GIT_CONFIG_COUNT/GIT_CONFIG_KEY_N/GIT_CONFIG_VALUE_N env vars to disable commit.gpgsign and tag.gpgsign inside the sandbox. Commits made by Copilot are unsigned — this is expected since users typically re-sign on merge/squash. Use --allow-gpg-signing to override this (see GPG signing).

GPG commit signing

GPG commit/tag signing is disabled by default because ~/.gnupg is blocked. Copilot commits are unsigned — you re-sign on merge/squash.

If you want Copilot commits to be signed (e.g. branch protection requires signatures), use --allow-gpg-signing:

cplt --allow-gpg-signing -- -p "commit your changes"

Or set it permanently in config:

[sandbox]
allow_gpg_signing = true

Setup checklist:

Before using this flag, verify GPG signing works outside the sandbox:

# 1. Check your signing key is configured
git config --get user.signingkey          # should show your key ID

# 2. Check gpg-agent is running
gpg-connect-agent 'GETINFO version' /bye  # should print version + OK

# 3. Cache your passphrase (so signing doesn't hang)
echo "test" | gpg --clearsign > /dev/null  # triggers passphrase prompt

# 4. Verify git signing works
git commit --allow-empty -S -m "test signed commit"
git log --show-signature -1               # should show "Good signature"
git reset HEAD~1                          # undo the test commit

If all of that works, cplt --allow-gpg-signing will work too. The gpg-agent runs outside the sandbox, so pinentry prompts appear normally — the sandbox only needs to reach the agent socket.

Note: Signature verification (git log --show-signature) won't work inside the sandbox because GPG opens trustdb.gpg for writing during verification. This is harmless — signing works correctly, and signatures can be verified outside the sandbox or in CI.

Troubleshooting:

What this does:

Security notes:

• Private keys are NOT exposed. GPG agent holds keys in memory — the Assuan IPC protocol has no command to export private key material. The private-keys-v1.d/ directory remains denied even with this flag.
• Risk: signature impersonation and decryption. A compromised process with agent socket access can request signatures on arbitrary data (adding a "Verified" badge) and, if an encryption subkey exists, decrypt arbitrary ciphertext. This is the same level of impersonation Copilot already has for unsigned commits — signing just adds the badge.
• GPG-only. This flag does not enable SSH signing (gpg.format=ssh). SSH keys and SSH_AUTH_SOCK remain blocked.
• --deny-path wins. If you specify --deny-path ~/.gnupg alongside --allow-gpg-signing, the deny takes precedence — all GPG allows are suppressed.
• GNUPGHOME is not supported yet — only the default ~/.gnupg location is allowed.

JVM Attach API

JVM testing frameworks like MockK (inline mocking), Mockito (inline agents), and ByteBuddy use the JVM Attach API for runtime class instrumentation. This API creates a Unix domain socket at /tmp/.java_pid<PID> — which the sandbox blocks by default.

Enable it with --allow-jvm-attach:

cplt --allow-jvm-attach -- -p "run the tests"

Or permanently in config:

cplt config set sandbox.allow_jvm_attach true

When to enable:

• Kotlin/Java projects using MockK with mockk() or mockkStatic() inline mocking
• Projects using Mockito with Mockito.mock() on final classes (requires ByteBuddy agent)
• Any test suite that gets "Could not self-attach to current VM using external process" errors
• JMX monitoring tools that attach to running JVMs

How it works: The JVM creates a socket at /tmp/.java_pid<PID> (hardcoded path, not affected by java.io.tmpdir). A helper JVM process connects to this socket to load an instrumentation agent. The sandbox rule uses a regex pattern that only allows sockets matching .java_pid<PID> — all other Unix sockets in /tmp (including SSH agent, tmux, PostgreSQL) remain blocked.

Security note: This opens a narrow IPC channel for .java_pid*-named sockets only. SSH agent access (SSH_AUTH_SOCK) is NOT exposed — on macOS it lives at /private/tmp/com.apple.launchd.*/Listeners which does not match the pattern.

Port restriction

Only port 443 is allowed by default. Services on other ports need --allow-port:

• npm install from private registries on non-standard ports
• API calls to services not on 443
• FTP, SMTP, or other protocol connections

Private registries

Registry credential files are blocked by default because they typically contain passwords or tokens that a rogue agent could exfiltrate:

For Maven, Gradle, and Cargo files, you can override this with --allow-read:

# Per session — Maven
cplt --allow-read ~/.m2/settings.xml -- -p "build with Maven"

# Per session — Gradle
cplt --allow-read ~/.gradle/gradle.properties -- -p "build with Gradle"

# Multiple files
cplt --allow-read ~/.m2/settings.xml --allow-read ~/.gradle/gradle.properties -- -p "build"

To allow permanently, add to ~/.config/cplt/config.toml:

[allow]
read = ["~/.m2/settings.xml", "~/.gradle/gradle.properties"]

Note: .npmrc cannot be overridden — it is in the hard-deny list alongside .netrc and .pypirc. If you need npm private registry access, consider using project-level .npmrc (which is readable as part of the project directory) with a token injected via environment variable.

Linux limitation: These file-level denials are only enforced on macOS (via SBPL literal deny rules). On Linux, Landlock cannot deny individual files within an allowed directory — the parent dirs (.m2, .gradle, .cargo) remain fully readable for dependency resolution.

Limitations

macOS

• sandbox-exec is deprecated — Apple has not removed it but may in future macOS versions
• SBPL has no domain-based filtering — the optional CONNECT proxy provides domain blocking
• Keychain access required — Copilot stores auth tokens in macOS Keychain

Linux

• Kernel 5.13+ required — Landlock LSM must be enabled (cat /sys/kernel/security/lsm)
• TCP port filtering requires kernel 6.7+ — older kernels get filesystem-only enforcement; network security via proxy only
• Landlock network rules are port-based only — cannot distinguish localhost from remote. When --allow-localhost-any is set, kernel TCP connect filtering is disabled entirely (the proxy still enforces domain filtering and port restrictions for remote connections)
• Gradle/JVM on Linux — Gradle daemon uses ephemeral localhost ports. Use --allow-localhost-any or cplt config set sandbox.allow_localhost_any true to allow Gradle client↔daemon communication. If JVM startup itself fails, also add --allow-tmp-exec (native lib loading from temp)
• Landlock cannot deny subpaths within allowed paths — unlike macOS Seatbelt, Landlock cannot deny .env reads or .git/hooks writes inside the project directory at the kernel level. Defense-in-depth comes from the proxy (blocks exfiltration) and env hardening (GIT_CONFIG_NOSYSTEM, etc.)
• --deny-path has no effect — Landlock is allowlist-only; a runtime warning is emitted
• Some macOS flags are not applicable — --allow-docker, --allow-jvm-attach, --allow-cache-exec emit warnings and are ignored on Linux
• No audit logs — --show-denials is macOS-only; use strace -f -e trace=file,network for debugging
• Auth scoped to env + gh CLI — no D-Bus/Secret Service integration for v1

Both platforms

• No TLS inspection — the proxy sees domain names (via CONNECT) but not request bodies

For known attack vectors, out-of-scope threats, and prior art, see SECURITY.md.

Contributing

Contributions are welcome! To get started:

git clone https://github.com/navikt/cplt.git && cd cplt
git config core.hooksPath hack    # enables pre-commit fmt + clippy checks
mise run check                    # runs fmt, clippy, and tests

Please open an issue before starting large changes. All PRs must pass CI (fmt, clippy, tests).

References

• SECURITY.md — Full security model, threat analysis, test strategy, and prior art
• Apple sandbox-exec(1)
• Chromium Seatbelt V2 Design
• Landlock LSM documentation
• seccomp-BPF documentation
• OWASP SSRF Prevention Cheat Sheet
• michaelneale/agent-seatbelt-sandbox

License

MIT