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Permissions

Deno is secure by default. Therefore, unless you specifically enable it, a program run with Deno has no file, network, or environment access. Access to security sensitive functionality requires that permissions have been granted to an executing script through command line flags, or a runtime permission prompt. This is a major difference from Node, where dependencies are automatically granting full access to everything, introducing hidden vulnerabilities in your project.

Run untrusted code with confidence​

Since Deno provides no I/O access by default, it's useful for running untrusted code and auditing third-party code. If you're building or extending a platform that runs user generated code, you can use Deno for running third-party code securely and host this code through Deno Subhosting or any other cloud platform of your choice.

For the following example mod.ts has been granted read-only access to the file system. It cannot write to the file system, or perform any other security sensitive functions.

deno run --allow-read mod.ts

Permissions list​

The following permissions are available:

  • --allow-env=<VARIABLE_NAME> Allow environment access for things like getting and setting of environment variables. Since Deno 1.9, you can specify an optional, comma-separated list of environment variables to provide an allow-list of allowed environment variables.
  • --allow-sys=<API_NAME> Allow access to APIs that provide information about user's operating system, eg. Deno.osRelease() and Deno.systemMemoryInfo(). You can specify a comma-separated list of allowed interfaces from the following list: hostname, osRelease, osUptime, loadavg, networkInterfaces, systemMemoryInfo, uid, and gid. These strings map to functions in the Deno namespace that provide OS info, like Deno.systemMemoryInfo.
  • --allow-hrtime Allow high-resolution time measurement. High-resolution time can be used in timing attacks and fingerprinting.
  • --allow-net=<IP/HOSTNAME> Allow network access. You can specify an optional, comma-separated list of IP addresses or hostnames (optionally with ports) to provide an allow-list of allowed network addresses.
  • --allow-ffi=<PATH> Allow loading of dynamic libraries. You can specify an optional, comma-separated list of directories or files to provide an allow-list of allowed dynamic libraries to load. Be aware that dynamic libraries are not run in a sandbox and therefore do not have the same security restrictions as the Deno process. Therefore, use with caution. Please note that --allow-ffi is an unstable feature.
  • --allow-read=<PATH> Allow file system read access. You can specify an optional, comma-separated list of directories or files to provide an allow-list of allowed file system access.
  • --allow-run=<PROGRAM_NAME> Allow running subprocesses. Since Deno 1.9, You can specify an optional, comma-separated list of subprocesses to provide an allow-list of allowed subprocesses. Be aware that subprocesses are not run in a sandbox and therefore do not have the same security restrictions as the Deno process. Therefore, use with caution.
  • --allow-write=<PATH> Allow file system write access. You can specify an optional, comma-separated list of directories or files to provide an allow-list of allowed file system access.
  • -A, --allow-all Allow all permissions. This enables all security sensitive functions. Use with caution.

Starting with Deno 1.36 following flags are available:

  • --deny-env=<VARIABLE_NAME> Deny environment access for things like getting and setting of environment variables. You can specify an optional, comma-separated list of environment variables to provide an allow-list of allowed environment variables. Any environment variables specified here will be denied access, even if they are specified in --allow-env.
  • --deny-sys=<API_NAME> Deny access to APIs that provide information about user's operating system.
  • --deny-hrtime Disable high-resolution time measurement. High-resolution time can be used in timing attacks and fingerprinting.
  • --deny-net=<IP/HOSTNAME> Disable network access. You can specify an optional, comma-separated list of IP addresses or hostnames (optionally with ports) to provide a deny-list of network addresses. Any addresses specified here will be denied access, even if they are specified in --allow-net.
  • --deny-ffi=<PATH> Deny loading of dynamic libraries. You can specify an optional, comma-separated list of directories or files to provide a deny-list of allowed dynamic libraries to load. Any libraries specified here will be denied access, even if they are specified in --allow-ffi. Please note that --deny-ffi is an unstable feature.
  • --deny-read=<PATH> Deny file system read access. You can specify an optional, comma-separated list of directories or files to provide a deny-list of allowed file system access. Any paths specified here will be denied access, even if they are specified in --allow-read.
  • --deny-run=<PROGRAM_NAME> Deny running subprocesses. You can specify an optional, comma-separated list of subprocesses to provide a deny-list of allowed subprocesses. Be aware that subprocesses are not run in a sandbox and therefore do not have the same security restrictions as the Deno process. Therefore, use with caution. Any programs specified here will be denied access, even if they are specified in --allow-run.
  • --deny-write=<PATH> Deny file system write access. You can specify an optional, comma-separated list of directories or files to provide a deny-list of allowed file system access. Any paths specified here will be denied access, even if they are specified in --allow-write.

Configurable permissions​

Some permissions allow you to grant access to a specific list of entities (files, servers, etc) rather than to everything.

File system access​

This example restricts file system access by allowing read-only access to the /usr directory. In consequence the execution fails as the process was attempting to read a file in the /etc directory:

$ deno run --allow-read=/usr https://deno.land/std@0.198.0/examples/cat.ts /etc/passwd
error: Uncaught PermissionDenied: read access to "/etc/passwd", run again with the --allow-read flag
► $deno$/dispatch_json.ts:40:11
    at DenoError ($deno$/errors.ts:20:5)
    ...

Try it out again with the correct permissions by allowing access to /etc instead:

deno run --allow-read=/etc https://deno.land/std@0.198.0/examples/cat.ts /etc/passwd

You can further restrict some sub-paths to not be accessible, using --deny-read flag:

deno run --allow-read=/etc --deny-read=/etc/hosts https://deno.land/std@0.198.0/examples/cat.ts /etc/passwd
deno run --allow-read=/etc --deny-read=/etc/hosts https://deno.land/std@0.198.0/examples/cat.ts /etc/hosts
error: Uncaught PermissionDenied: read access to "/etc/hosts"...

--allow-write works the same as --allow-read.

Note for Windows users: the /etc and /usr directories and the /etc/passwd file do not exist on Windows. If you want to run this example yourself, replace /etc/passwd with C:\Windows\System32\Drivers\etc\hosts, and /usr with C:\Users.

Network access​

// fetch.js
const result = await fetch("https://deno.land/");

This is an example of how to allow network access to specific hostnames or IP addresses, optionally locked to a specified port:

# Multiple hostnames, all ports allowed
deno run --allow-net=github.com,deno.land fetch.js

# A hostname at port 80:
deno run --allow-net=deno.land:80 fetch.js

# An IPv4 address on port 443
deno run --allow-net=1.1.1.1:443 fetch.js

# An IPv6 address, all ports allowed
deno run --allow-net=[2606:4700:4700::1111] fetch.js

You can restrict certain domains to never be accessible by using the --deny-net flag:

# Allow to make network connections to all addresses except myserver.com.
deno run --allow-net --deny-net=myserver.com fetch.js

If fetch.js tries to establish network connections to any hostname or IP not explicitly allowed, the relevant call will throw an exception.

Allow net calls to any hostname/IP:

deno run --allow-net fetch.js

Environment variables​

// env.js
Deno.env.get("HOME");

This is an example of how to allow access to environment variables:

# Allow all environment variables
deno run --allow-env env.js

# Allow access to only the HOME env var
deno run --allow-env=HOME env.js

Note for Windows users: environment variables are case insensitive on Windows, so Deno also matches them case insensitively (on Windows only).

You can restrict certain env vars to never be accessible by using the --deny-env flag:

# Allow all environment variables except AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY.
deno run --allow-env --deny-env=AWS_ACCESS_KEY_ID,AWS_SECRET_ACCESS_KEY env.js

Subprocess permissions​

Subprocesses are very powerful, and can be a little scary: they access system resources regardless of the permissions you granted to the Deno process that spawns them. The cat program on unix systems can be used to read files from disk. If you start this program through the Deno.run API it will be able to read files from disk even if the parent Deno process can not read the files directly. This is often referred to as privilege escalation.

Because of this, make sure you carefully consider if you want to grant a program --allow-run access: it essentially invalidates the Deno security sandbox. If you really need to spawn a specific executable, you can reduce the risk by limiting which programs a Deno process can start by passing specific executable names to the --allow-run flag.

// run.js
const proc = Deno.run({ cmd: ["whoami"] });
# Allow only spawning a `whoami` subprocess:
deno run --allow-run=whoami run.js

# Allow running any subprocess:
deno run --allow-run run.js

You can only limit the executables that are allowed; if permission is granted to execute it then any parameters can be passed. For example if you pass --allow-run=cat then the user can use cat to read any file.

You can restrict certain executables to never be accessible by using the --deny-run flag:

# Disallow spawning `git`.
deno run --allow-run --deny-run=git run.js