fix(ci): reject path traversal in S3 prefix downloads (#2070)

tools/ci/src/common.rs

@@ -1069,7 +1069,7 @@ pub(crate) async fn download_s3_prefix(
         if relative.is_empty() {
             continue;
         }
-        let output = target.join(relative);
+        let output = safe_download_target(target, relative)?;
         if let Some(parent) = output.parent() {
             tokio::fs::create_dir_all(parent)
                 .await
@@ -1248,6 +1248,17 @@ pub(crate) fn path_to_s3_key(path: &Path) -> String {
         .join("/")
 }
 
+fn safe_download_target(target: &Path, relative: &str) -> Result<PathBuf> {
+    let candidate = Path::new(relative);
+    for component in candidate.components() {
+        ensure!(
+            matches!(component, std::path::Component::Normal(_)),
+            "Refusing to write S3 object outside download target: {relative}"
+        );
+    }
+    Ok(target.join(candidate))
+}
+
 pub(crate) async fn download_file(url: &str, path: &Path) -> Result<()> {
     let bytes = Client::new()
         .get(url)