fix(providers): validate Vercel::with_base_url's api root, not just connect_to's

src/providers/mod.rs

@@ -187,7 +187,13 @@ pub fn connect_to(
 /// `localhost`, `127.0.0.1`/`127.x.x.x`, or `::1` — a mock server's or an
 /// egress proxy's loopback listener never leaves the machine, so plain HTTP
 /// there costs nothing a caller could intercept.
-fn is_secure_base_url(base_url: &str) -> bool {
+///
+/// `pub(crate)` because [`vercel::Vercel::with_base_url`] applies the same
+/// check: it is a second, lower-level way to reach the same client
+/// [`connect_to`] builds, and skipping the check there would just move the
+/// cleartext-credential exposure this guards against to a different entry
+/// point rather than closing it.
+pub(crate) fn is_secure_base_url(base_url: &str) -> bool {
     if let Some(rest) = base_url.strip_prefix("https://") {
         return !rest.is_empty();
     }

src/providers/vercel/mod.rs

@@ -70,8 +70,18 @@ impl Vercel {
     ///
     /// # Errors
     ///
-    /// Returns [`Error::Transport`] when the HTTP client cannot be built.
+    /// Returns [`Error::InsecureBaseUrl`] when `base_url` is `http://` against a
+    /// non-loopback host — this constructor sends the bearer credential to
+    /// every request `base_url` produces, the same as
+    /// [`connect_to`](crate::providers::connect_to), and applies the same
+    /// check. Returns [`Error::Transport`] when the HTTP client cannot be
+    /// built.
     pub fn with_base_url(credentials: Credentials, base_url: impl Into<String>) -> Result<Self> {
+        let base_url = base_url.into();
+        if !crate::providers::is_secure_base_url(&base_url) {
+            return Err(Error::InsecureBaseUrl { base_url });
+        }
+
         Ok(Self {
             http: Http::new(credentials, base_url)?,
         })

src/providers/vercel/test.rs

@@ -1130,15 +1130,30 @@ async fn a_team_scope_is_applied_to_every_request() {
 
 #[test]
 fn the_client_reports_itself_without_its_token() {
-    let host =
-        Vercel::with_base_url(Credentials::new("super-secret").unwrap(), "http://x").unwrap();
+    let host = Vercel::with_base_url(
+        Credentials::new("super-secret").unwrap(),
+        "http://127.0.0.1:1",
+    )
+    .unwrap();
     let rendered = format!("{host:?}");
 
     assert!(!rendered.contains("super-secret"), "{rendered}");
     assert!(rendered.contains("<redacted>"), "{rendered}");
     assert_eq!(host.kind(), ProviderKind::Vercel);
 }
 
+#[test]
+fn with_base_url_rejects_plain_http_against_a_non_loopback_host() {
+    let error = Vercel::with_base_url(Credentials::new("token").unwrap(), "http://x").unwrap_err();
+
+    assert_eq!(
+        error,
+        Error::InsecureBaseUrl {
+            base_url: "http://x".to_owned()
+        }
+    );
+}
+
 #[test]
 fn the_digest_is_sha_1() {
     // The published SHA-1 of "abc". Vercel's `x-vercel-digest` defines the