Reliance on Untrusted Inputs in a Security Decision vulnerability in mtrudel bandit allows unauthenticated transport-state spoofing on plaintext HTTP connections.

'Elixir.Bandit.Pipeline':determine_scheme/2 in lib/bandit/pipeline.ex returns the client-supplied URI scheme verbatim, ignoring the transport's secure? flag. HTTP/1.1 absolute-form request targets (e.g. GET https://victim/path HTTP/1.1) and the HTTP/2 :scheme pseudo-header are both attacker-controlled strings that flow through this function. Over a plaintext TCP connection, a client can declare https and Bandit will set conn.scheme = :https even though no TLS was negotiated.

Downstream Plug consumers that branch on conn.scheme are silently misled: Plug.SSL's already-secure branch skips its HTTP→HTTPS redirect, cookies emitted with secure: true are sent over plaintext, audit logs record requests as having arrived over HTTPS, and CSRF/SameSite gating may make incorrect decisions.

This issue affects bandit: from 1.0.0 before 1.11.0.

Affected: Hex: bandit

### 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 tra…