postcss npm package: is it safe?

The hydrateInputs function constructs objects with literal '__proto__' properties: '{ ...input, __proto__: Input.prototype }' and '{ ...inputHydrated.map, __proto__: PreviousMap.prototype }'. While this appears intended to rehydrate class instances from JSON, setting '__proto__' explicitly is widely flagged as a dangerous pattern because it can be abused for prototype pollution. In this specific file the value is a hard-coded library prototype (not attacker-controlled), so it does not directly enable pollution, but it is a fragile and security-sensitive construct worth noting.

constructNode does 'let defaults = { ...json }' on data coming from JSON (potentially untrusted input to fromJSON). If any nested property named '__proto__' or 'constructor' is present in the JSON, spread/merge semantics could interact with prototype chains depending on runtime and downstream constructors. This mirrors known prototype-pollution risks in deserialization helpers.

The code does not make any network requests. It only manipulates paths and URLs locally.

The code uses path manipulation functions (dirname, relative, resolve) but does not perform any file system operations like reading or writing files. It only computes paths.

No environment variables are accessed or harvested in this code.

No child process spawning or shell command execution is present.

The code uses require() with static paths only. No dynamic imports based on computed input.

The code uses Buffer.from() and window.btoa() which are not dynamic code execution functions. The toBase64 function heavily relies on Buffer but this is a standard Node.js API for encoding strings to base64, not an eval or Function constructor.