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.
\nBandit.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.
"},{"base64":false,"type":"text/markdown","value":"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.\n\n`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.\n\nThis issue affects bandit: from 1.11.0 before 1.14.0."}],"value":"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.\n\nBandit.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.\n\nThis issue affects bandit: from 1.11.0 before 1.14.0."}],"impacts":[{"capecId":"CAPEC-130","descriptions":[{"lang":"en","supportingMedia":[{"base64":false,"type":"text/html","value":"An unauthenticated client can make one WebSocket connection hold about 150 times max_fragmented_message_size in server memory (about 82 MB for 3.5 MB of traffic in the reporter's test). This affects any Bandit server that accepts WebSocket upgrades with default settings, including Phoenix LiveView and Channels apps.
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.
1. Byte-only limit. In Bandit.WebSocket.Connection (lib/bandit/websocket/connection.ex), the fragment limit is checked against fragment_size, which only adds up payload bytes (IO.iodata_length(frame.data)).
2. Per-fragment accumulation. Each non-final continuation frame is stored as a new cons cell around its own binary, [connection.fragment_frame.data | frame.data]. Only zero-length fragments are kept out, so 1-byte fragments are accepted. Each costs 7 bytes on the wire but keeps about 150 to 170 bytes in memory. Against 1.12.5, 1M fragments kept about 169 MB and 3M kept about 450 MB, growing linearly, and about 56 MB of traffic reaches the 8 MB payload cap.
3. Kept alive. Ping frames sent between fragments are handled by handle_control_frame without touching the fragment state, so they keep the connection alive past the idle timeout.