Hono: Unbounded dot-notation nesting in `parseBody()` can cause memory exhaustion

Summary

When parseBody() expands dot-separated form field names into nested objects, it does not limit the nesting depth or the total number of objects created. A request body well within a normal size limit can therefore allocate an object graph far larger than the request itself, and concurrent requests can exhaust the heap and terminate the process.

Details

Each dot-separated segment of a field name creates an intermediate object. Neither the segments within a single field name nor the total across a request was bounded, and empty segments were preserved, so a field name could encode one nesting level per byte.

Both shapes produce the effect: a single deeply dotted field name, and a large number of shallowly dotted ones within one body. A request body size limit does not prevent it, because the amplification happens after the body has been accepted.

Dot-notation parsing is not enabled by default.

Impact

An attacker who can reach an endpoint that parses request bodies with dot-notation enabled can send concurrent requests whose memory cost is disproportionate to their size.

This may lead to:

• exhaustion of the JavaScript heap and termination of the server process
• the service remaining unavailable until it is restarted

This issue affects applications that explicitly enable dot-notation parsing. Applications using the default behaviour are not affected.