Inconsistent Interpretation of HTTP Requests vulnerability in mtrudel bandit allows HTTP request smuggling via duplicate Content-Length headers.

'Elixir.Bandit.Headers':get_content_length/1 in lib/bandit/headers.ex uses List.keyfind/3, which returns only the first matching header. When a request contains two Content-Length headers with different values, Bandit silently accepts it, uses the first value to read the body, and dispatches the remaining bytes as a second pipelined request on the same keep-alive connection. RFC 9112 §6.3 requires recipients to treat this as an unrecoverable framing error.

When Bandit sits behind a proxy that picks the last Content-Length value and forwards the request rather than rejecting it, an unauthenticated attacker can smuggle requests past edge WAF rules, path-based ACLs, rate limiting, and audit logging.

This issue affects bandit: before 1.11.0.

Affected: Hex: bandit

### Summary

Bandit is vulnerable to CL.CL HTTP request smuggling: it silently accepts requests with two `Content-Length` headers whose values differ, takes the first value, and dispatches the body bytes as a second pipelined request on the same keep-alive connection. RFC 9110 §5.3 prohibits multiple lines for singleton fields like `Content-Length`, and RFC 9112 §6.3 item 5 requires the recipient to treat invalid `Content-Length` as an unrecoverable framing error. When Bandit sits behind a proxy that picks the *last* `Content-Length` and forwards rather than rejects, an unauthenticated attacker can smuggle requests past edge WAF rules, path-based ACLs, rate limiting, and audit logging.

The vulnerability was introduced prior to `v0.1.0 (released Nov 5, 2020)` on Nov 16, 2019: https://github.com/mtrudel/bandit/commit/e5270b1b19e9f3574aa0f87ec76851d66c38c0af

### Details

`Bandit.Headers.get_content_length/1` (`lib/bandit/headers.ex`) calls `List.keyfind/3`, which returns only the first matching header. Bandit already correctly rejects the comma-separated form (`Content-Length: 0, 43`) when values differ; the bug is that the multi-line form never reaches that check.

**Fix:** collect every `Content-Length` value from the header list and reject unless all values parse and are byte-identical — extending the existing rejection to the multi-line case.

### PoC

The script below boots a local Bandit server with a Plug that echoes the dispatched method and path, then sends a POST wi…