GHSA-9m9w-gxf7-rh8m

### Summary

`Tesla.Middleware.FollowRedirects` is meant to strip the `Authorization` header when following a cross-origin redirect, but performs the check with a case-sensitive comparison against the lowercase string `"authorization"`. Because Tesla preserves header keys exactly as supplied by the caller, any application that sets the header with its RFC 7235 canonical casing (`"Authorization"`) bypasses the filter entirely, leaking bearer tokens or other credentials to whatever origin the redirect points at.

### Details

The filter list in `lib/tesla/middleware/follow_redirects.ex` is defined as `@filter_headers ["authorization", "host"]` and the membership check `k not in @filter_headers` compares the raw key string without case normalization. HTTP header names are case-insensitive per RFC 7230, but Tesla stores them verbatim. A header tuple `{"Authorization", "Bearer …"}` does not match `"authorization"`, so it passes through the filter and is forwarded to the redirect destination unchecked. The same defect applies to the `"Host"` entry.

An attacker who can control a `Location:` response seen by the victim client (their own endpoint, a redirect-open service, or a compromised upstream) receives the credential on the cross-origin follow. No special configuration is required beyond the victim using the standard header casing.

### PoC

1. Configure a Tesla client with `Tesla.Middleware.FollowRedirects` and set the `Authorization` header using canonical casing (`{"Authorization", "Bearer <token>"}`). 2. Make a request to an endpoint that returns a `302` redirect to a different origin. 3. Observe that the `Authorization` header with its value is present in the request delivered to the redirect destination.

### Impact

High severity (CVSS v4.0: 8.2). Any application using `tesla` 1.4.0 through 1.18.2 with `Tesla.Middleware.FollowRedirects` and a non-lowercase `Authorization` header is affected. The workaround is to use all-lowercase `"authorization"` as the header key until upgrading to 1.18.3.

### Workarounds

Normalize all header keys to lowercase before passing them to Tesla. Use `"authorization"` instead of `"Authorization"` when setting headers via `Tesla.put_header/3` or `Tesla.Middleware.Headers`.

### Resources

* Introduction commit: https://github.com/elixir-tesla/tesla/commit/2d937d5813d7cda5cd726f41824985fb655c920f * Patch commit: https://github.com/elixir-tesla/tesla/commit/db963dba67651b9abd1fc420a1d9679cf6efe182