fast-copy: Stack exhaustion in fast-copy when copying deeply-nested values

Summary

fast-copy traverses values recursively with no bound on depth. Copying a sufficiently deeply-nested value exhausts the JavaScript call stack and throws a native RangeError: Maximum call stack size exceeded from inside the library.

Details

Both copy and copyStrict recurse once per level of nesting. The value does not need to be circular (circular references were already handled correctly via an internal cache) and it does not need to be large. A plain object nested a few thousand levels deep is only a few kilobytes of equivalent JSON.

Measured depths at which copying begins to fail (Node.js, V8; the exact ceiling varies with JIT state and the stack available to the environment):

JSON.parse is iterative and parses deeply-nested input without difficulty, so a payload that deserializes cleanly can fail in a subsequent copy call.

Impact

The failure is a synchronous, catchable RangeError confined to the copy call that received the value. There are no memory safety concerns, no data exposure, and no effect on state outside that call. In a typical server the result is a failed request rather than a failed process.

Applications that call copy on externally-supplied data, such as request bodies, cached payloads, merged configuration, etc., do not expect a clone helper to throw may surface this as an unhandled error.

Patches

Fixed in 4.1.0, and backported to 3.1.0 and 2.2.0 so that every major line has a patch available without requiring a breaking upgrade.

Traversal is now bounded by a maxDepth option, defaulting to 1000, which sits below the native limit in standard environments. Exceeding it throws MaxDepthExceededError, which carries the limit that was exceeded and extends RangeError so existing handling continues to match.

The limit is configurable for consumers with legitimately deep data. On 3.x and 4.x it is set when creating a copier:

import { createCopier } from 'fast-copy';

export const copy = createCopier({ maxDepth: 5000 });

On 2.x it is passed per call, and the error is a property of the exported function rather than a named export:

import copy from 'fast-copy';

copy(value, { maxDepth: 5000 });
// detection: error instanceof copy.MaxDepthExceededError

In legacy environments without Object.setPrototypeOf, instanceof MaxDepthExceededError cannot be supported; check error.name === 'MaxDepthExceededError' or error instanceof RangeError instead.