Improper Handling of Case Sensitivity vulnerability in elixir-tesla tesla allows credential leakage to a third-party origin on cross-origin redirects.

Tesla.Middleware.FollowRedirects strips security-sensitive headers on cross-origin redirects using a case-sensitive string comparison against a lowercase filter list (@filter_headers ["authorization", "host"]). HTTP header names are case-insensitive per RFC 7230, but Tesla preserves header keys verbatim as supplied by the caller without normalizing case. A header set as {"Authorization", "Bearer …"} (the RFC 7235 canonical casing used by virtually all HTTP libraries and documentation) does not match the lowercase filter entry and is forwarded to the redirect destination. An attacker who can control or influence a Location: response seen by the client (via their own endpoint, a redirect-open upstream, or a compromised origin) receives the bearer token or other Authorization material on the cross-origin request.

This issue affects tesla: from 1.4.0 before 1.18.3.

Affected: Hex: tesla

### 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 (`{"Authoriz…