Bandit trusts client-supplied URI scheme on plaintext connections
Summary
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.
Impact
Transport-state spoofing. Any unauthenticated client speaking plaintext HTTP/1.1 or h2c to a Bandit endpoint can cause the application to treat the connection as if it had been TLS-protected. Concrete consequences in real Phoenix/Plug stacks include:
Plug.SSL skipping its HTTP→HTTPS redirect because the request "already looks secure", letting plaintext requests bypass the redirect entirely.- Cookies emitted with
secure: true on a plaintext response, where a network attacker could capture them. - Audit logs recording requests as having arrived over HTTPS when they did not, breaking forensic and compliance assumptions.
- Application code that uses
conn.scheme to gate CSRF/SameSite policy, OAuth redirect URIs, or HSTS-related decisions making the wrong call.
The vulnerability is unauthenticated and trivially automatable; severity is medium because exploitation requires the deployment to expose a plaintext Bandit listener (or h2c) and to have downstream code that branches on conn.scheme.
Script and Logs
```elixir
# 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, 2
