WebSocket 1-byte continuation frames bypass `max_fragmented_message_size` in Bandit

Overview

Allocation of Resources Without Limits or Throttling vulnerability in the WebSocket fragment reassembly of mtrudel bandit allows an unauthenticated remote attacker to exhaust server memory via a fragmented message made of many 1-byte continuation frames.

Bandit.WebSocket.Connection checks max_fragmented_message_size against the payload bytes received, but keeps every non-final continuation frame as a new list cell around its own binary, and nothing limits the number of fragments. A 1-byte fragment costs 7 bytes on the wire but about 150 to 170 bytes of server memory, so one connection can hold about 150 times the configured limit, roughly 1.2 to 1.4 GB at the default 8 MB. Ping frames between fragments keep the connection alive past the idle timeout.

This issue affects bandit: from 1.11.0 before 1.14.0.

Affected

• bandit >= 1.11.0 < 1.14.0
• mtrudel/bandit >= 21612c7c7b1ce43eccd36d3af3a2299d23513667 < 220b9dfd27f9a21305be2afc75cbe585ddc24e7a

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Workarounds

Lower websocket_options: [max_fragmented_message_size: ...] to the largest fragmented message the application needs, for example 65_536. Memory per connection stays at about 150 times this limit, so a lower limit lowers the ceiling.