Bun Security Best Practices (2026)

Bun is an all-in-one JavaScript runtime, package manager, bundler, and test runner built for speed, and it is broadly Node.js-compatible. That compatibility is the first thing to understand about its security posture: unlike Deno, Bun has no built-in permission sandbox. Code you run under Bun has the same access to the file system, network, and environment as any Node process. Bun does not restrict what your program can do, so the security work moves to where it belongs in the Node world: controlling what enters your dependency graph and what code is allowed to execute during installation.

The short answer to running Bun securely: rely on Bun's default blocking of lifecycle scripts and keep the trustedDependencies allowlist small, install with a frozen lockfile, audit your dependency graph against known advisories, keep secrets out of your code, and use OS-level isolation (containers, seccomp) when you need to run genuinely untrusted code, because Bun will not sandbox it for you.

Lifecycle scripts are blocked by default; keep it that way

Bun's best security default is that it does not run postinstall and other lifecycle scripts for arbitrary dependencies. Malicious install scripts are the classic npm attack, so blocking them by default removes a major infection vector. Bun only runs lifecycle scripts for packages you explicitly list under trustedDependencies in package.json.

{
  "name": "acme-app",
  "trustedDependencies": ["esbuild", "sharp"]
}

Treat this list like a permission grant, because that is what it is. Add a package only when it genuinely needs a native build step, review what its install script does before adding it, and keep the list as short as possible. A bloated trustedDependencies quietly re-opens the door Bun's default closed.

Install with a frozen lockfile

Bun writes a lockfile (bun.lock) that records the exact resolved version and integrity hash of every package. In CI and production builds, install with the lockfile frozen so a build fails rather than silently resolving new or different versions.

# CI / production: fail if the lockfile does not match package.json
bun install --frozen-lockfile

# Production-only install, omitting devDependencies
bun install --frozen-lockfile --production

A frozen install is what stops a compromised or yanked upstream version from sneaking in between the moment a developer resolves dependencies and the moment CI builds the artifact. Commit the lockfile and require it to be current in code review.

Audit the dependency graph and update the runtime

Bun's default script blocking does nothing about vulnerable code inside a package that you import and call. Bun can surface known advisories for your installed packages against the npm advisory database, but built-in auditing tends to be presence-based: it tells you a vulnerable version exists somewhere in the tree, not whether your application actually reaches the vulnerable function. That gap between "present" and "exploitable" is where most triage time is wasted, so pair Bun's audit output with reachability-aware software composition analysis.

Keep the runtime itself patched, too. Bun ships frequently, and runtime fixes, including security fixes, land in point releases, so pinning to an old Bun version to avoid churn also pins you to any unpatched runtime bugs.

Bun hardening checklist

• Understand Bun has no permission sandbox; do not rely on it to isolate untrusted code.
• Keep trustedDependencies minimal and review each entry's install script.
• Install with --frozen-lockfile in CI and production.
• Commit bun.lock and require it current in review.
• Audit the full dependency graph for known CVEs, not just presence.
• Keep secrets in the environment or a secrets manager, never in source.
• Use containers or OS sandboxing when executing untrusted code.
• Keep the Bun runtime on a current patched release.

How Safeguard helps

Because Bun does not sandbox your code, the dependency graph is where your real supply chain risk concentrates. Safeguard's software composition analysis resolves your full Bun dependency tree and adds call-path reachability, so a vulnerable transitive package that your application never actually invokes is separated from one on a live, exploitable path. Griffin AI explains why a given finding matters in your context, and autonomous auto-fix opens a tested pull request when a safe upgrade is available. Developers get the same results locally and in CI through the Safeguard CLI, and teams evaluating alternatives can read our comparison with Snyk.

Bring prioritized dependency analysis to your Bun projects: get started free or read the documentation.

Frequently Asked Questions

Does Bun sandbox code like Deno does?

No. Bun has no permission model; code running under Bun has the same file, network, and environment access as any Node.js process. If you need to run untrusted code in isolation, use OS-level controls such as containers or seccomp, because Bun will not restrict it for you.

How does Bun handle npm install scripts?

Bun blocks postinstall and other lifecycle scripts by default and only runs them for packages listed in trustedDependencies in your package.json. Keep that list short and review each package's install script before adding it, since every entry is effectively a grant to execute code during install.

Is bun install --frozen-lockfile important for security?

Yes. A frozen install fails the build if the lockfile and manifest disagree, which prevents a compromised, swapped, or yanked upstream version from being resolved between development and the CI build. Use it everywhere except when you are intentionally updating dependencies.

Does Bun's built-in audit replace a dedicated scanner?

Not fully. Built-in auditing checks installed packages against the npm advisory database but is largely presence-based, flagging that a vulnerable version exists without telling you whether your code reaches it. Reachability-aware SCA prioritizes the findings that are genuinely exploitable in your application.