Fetch: refuse a redirect from https to http (#1531)

crates/fetch/README.md

@@ -2,6 +2,6 @@
 
 Native model downloads in pure Rust (L0, Apache-2.0): HTTP/1.1 over std sockets and rustls with the RustCrypto provider, without ring, aws-lc, C or assembly build dependencies. The wasm32 crate is empty.
 
-Call `download` with bounded file specifications and ordered mirrors, or `download_url` for an exact catalog URL (including its query). Each file has a local filename independent of the URL, a size cap and optional pinned size/SHA-256. Transfers resume pinned `.part` files, verify before renaming, report progress and check cancellation during reads. Run on a worker thread. No proxy support; DNS resolution uses the platform resolver. Tokens are off by default and never forwarded to a different redirect host.
+Call `download` with bounded file specifications and ordered mirrors, or `download_url` for an exact catalog URL (including its query). Each file has a local filename independent of the URL, a size cap and optional pinned size/SHA-256. Transfers resume pinned `.part` files, verify before renaming, report progress and check cancellation during reads. Run on a worker thread. No proxy support; DNS resolution uses the platform resolver. Tokens are off by default and never forwarded to a different redirect host. A redirect from https to http is refused.
 
 Tests use local synthetic HTTP servers for redirects, resume, truncation, cancellation, stalls, hashes, size limits and hostile specifications; they make no internet requests.

crates/fetch/src/http.rs

@@ -110,7 +110,11 @@ impl Url {
     pub fn join(&self, location: &str) -> Result<Url, HttpError> {
         let l = location.trim();
         if l.starts_with("http://") || l.starts_with("https://") {
-            return Url::parse(l);
+            let url = Url::parse(l)?;
+            if self.tls && !url.tls {
+                return Err(HttpError::BadUrl("redirect from https to http refused".into()));
+            }
+            return Ok(url);
         }
         // another scheme (`ftp:`, `file:`, `data:`…)
         if let Some((scheme, _)) = l.split_once(':')
@@ -541,6 +545,11 @@ mod tests {
         assert_eq!(u.join("other").unwrap().path, "/x/y/other");
         assert_eq!(u.join("//c.example/p").unwrap().host, "c.example");
         assert!(u.join("ftp://nope").is_err());
+        // no downgrade from https to http, however the redirect is written
+        assert!(u.join("http://a.example/z").is_err());
+        let plain = Url::parse("http://127.0.0.1:8080/a").unwrap();
+        assert!(!plain.join("http://127.0.0.1:9/b").unwrap().tls);
+        assert!(plain.join("https://b.example/z").unwrap().tls);
     }
 
     #[test]