How does Bun's security compare to Deno's permission model

Deno's Security and Permission Model

Deno is designed with a strong emphasis on security, adopting a "secure by default" philosophy. This means that when running a Deno program, it has no access to sensitive APIs or system resources such as the file system, network, environment variables, or the ability to spawn subprocesses unless these accesses are explicitly permitted by the user. The main aspects of Deno's permission model include:

- No Implicit Permissions: By default, scripts have no access to critical resources. Attempting to access these resources without granted permissions results in runtime errors.

- Explicit Permission Flags: Access to resources is granted through command-line flags when invoking a script, such as `--allow-read` for file system reads, `--allow-net` for network access, and others for environment variables or subprocesses. This requires deliberate action from the developer or user to enable specific capabilities.

- Fine-Grained and Scoped Permissions: Permissions can be scoped narrowly. For example, a script may be allowed to read only specific files or access only particular network addresses rather than broad access. This granularity improves security by limiting resource exposure to only what is necessary.

- No Privilege Escalation: Code cannot escalate its privileges after execution starts. Any increase in permissions requires user consent through an interactive prompt or a new runtime invocation with changed flags.

- Static Module Import Permissions: Files that are included in the initial static module graph can be imported without explicit permissions, even if read access is not granted. However, dynamically imported modules still require appropriate permissions.

- Uniform Privilege Level: All code executing in the same thread shares the same privilege level; Deno does not support compartmentalizing privileges within a single runtime execution context. This restricts internal privilege escalation but emphasizes clear boundaries on the application level.

- No Access to I/O by Default: The system enforces strict no access to input/output operations unless explicitly allowed, effectively sandboxing scripts to minimize potential damage caused by malicious or buggy code.

- Permission Prompts: When a script requires a permission it does not have, the runtime prompts the user interactively for consent before granting this, helping maintain user awareness of the security posture.

This permission system is designed to provide developers and users with confidence when running third-party or untrusted code by minimizing the scope of potential security vulnerabilities. It is particularly suited for production environments or multi-tenant systems where controlling resource usage tightly is critical.

Bun's Security Model

In contrast to Deno, Bun adopts a more traditional and permissive security model, which is closer to Node.js. By default, Bun does not impose restrictions on access to system resources like the file system, network, or environment variables, giving full access to scripts unless developers implement their own safeguards. Key characteristics of Bun's security approach include:

- Permissive by Default: Bun allows unrestricted access to system resources by default, without requiring explicit permission flags. This lowers barriers for rapid development and prototyping but increases the risk of unauthorized resource access by code.

- Developer Responsibility: Security control is largely delegated to developers and users. There are no enforced runtime permission prompts as in Deno, meaning that scripts and dependencies can freely access sensitive APIs unless preventative measures are taken externally.

- Trusted Dependencies Setting: Bun offers a configuration called `trustedDependencies` in the `package.json` file, which is intended to limit some security risks by tracking trusted libraries. This is less robust than Deno's system and does not enforce comprehensive runtime permissions.

- Focus on Speed and Ease of Use: The lack of a strict security model aligns with Bun's goal of optimizing for performance and developer convenience. This flexibility and reduced friction come at the cost of potential security gaps if developers do not manually audit or sandbox their code.

- Traditional Node.js Paradigm: Bun continues the Node.js model of assuming scripts are trusted within the developer's environment, sacrificing some safety for speed and simplicity of use and deployment.

Security Philosophies

Deno's security model trades increased upfront configuration and potentially more complex deployment setups for a stronger security guarantee at runtime. It forces developers to explicitly consider which operations their code is capable of performing, mitigating abuse vectors and reducing the attack surface dramatically.

Bun's model prioritizes developer velocity and performance, assuming the runtime environment is trusted and thereby allowing all scripts to run without permission checks. While this lowers friction and accelerates development cycles, it potentially exposes users to greater risk if scripts or dependencies behave maliciously or erroneously.

Security Implications and Use Cases

Due to Deno's strict permissions and sandboxing, it is better suited for environments where security is paramount — such as multi-user environments, deploying services with sensitive data, or running untrusted code fetched from external sources. Its explicit permission system helps prevent accidental or malicious breaches.

Bun's permissiveness makes it ideal for rapid prototyping, local development, and performance-critical internal applications where the risk profile is lower or more controlled. It expects developers to manually secure their setups or rely on surrounding infrastructure for protection.

Known Security Challenges and Critiques

Despite Deno's rigorous security model, there are some known issues:

- The coarse-grained permission model at the application level does not yet support compartmentalization within different modules or components of the same application, meaning a single compromised module can still access all granted permissions.

- URL imports in Deno's permission system have been found to be exempt from certain permission checks, enabling potential unlawful network requests.

- Time-of-check to time-of-use (TOCTOU) vulnerabilities exist related to symbolic link handling in the file system, limiting the effectiveness of fine-grained file permissions in some scenarios.

- There is a tendency for developers to overuse or poorly document required permissions, undermining the system's benefits.

- Certain permissions like `--allow-sys` enable running binary code outside the permission system's control.

Bun's security model does not incorporate these protections to begin with, so while simpler, it places more trust in developers' diligence and the security of the execution environment.

Summary

Deno offers a strong, "secure by default" runtime security model centered on explicit, fine-grained permission grants that protect resources such as the file system, network, and environment variables at the runtime level. It enforces permission checks with interactive prompts to help users maintain control over script behavior.

Bun follows a more traditional, permissive approach without runtime-enforced permissions, akin to Node.js, favoring developer convenience and speed over built-in security safeguards. While Bun provides some limited options for trusted dependencies, it does not have a comprehensive permission system restricting resource access.