matrix-mcp: report the sanitized file name that download_file wrote

hive-matrix-mcp/src/handlers.rs

@@ -1128,6 +1128,17 @@ fn media_parts(
     Some((source.clone(), name.to_owned(), caption.map(str::to_owned)))
 }
 
+/// The last path segment of a sender-supplied attachment name, so it cannot
+/// point outside the temp dir; `matrix-attachment` when nothing usable is
+/// left.
+fn safe_file_name(name: &str) -> &str {
+    std::path::Path::new(name)
+        .file_name()
+        .and_then(|n| n.to_str())
+        .filter(|n| !n.is_empty())
+        .unwrap_or("matrix-attachment")
+}
+
 /// Download the media attachment carried by `event_id` in `room` and
 /// write it to a local file, returning the path. Resolves the event,
 /// extracts the `MediaSource` from its `m.file` / `m.image` / `m.audio`
@@ -1180,15 +1191,11 @@ pub async fn download_file(
         Ok(b) => b,
         Err(e) => return DaemonResponse::error(format!("download media for {eid}: {e}")),
     };
+    let filename = safe_file_name(&filename);
     let dest = if let Some(p) = dest_path {
         std::path::PathBuf::from(p)
     } else {
-        let name = std::path::Path::new(&filename)
+        std::env::temp_dir().join(filename)
-            .file_name()
-            .and_then(|n| n.to_str())
-            .filter(|n| !n.is_empty())
-            .unwrap_or("matrix-attachment");
-        std::env::temp_dir().join(name)
     };
     if let Err(e) = tokio::fs::write(&dest, &bytes).await {
         return DaemonResponse::error(format!("write {}: {e}", dest.display()));
@@ -1315,7 +1322,8 @@ mod utd_body_tests {
 mod unread_tests {
     use super::{
         ParsedEvent, TimelineEvent, attachment_marker, collapse_edits, extract_body_sync,
-        extract_in_reply_to, is_intentional, media_parts, replacement_of, unread_verdict,
+        extract_in_reply_to, is_intentional, media_parts, replacement_of, safe_file_name,
+        unread_verdict,
     };
     use matrix_sdk::ruma::events::AnySyncTimelineEvent;
     use matrix_sdk::ruma::events::room::message::MessageType;
@@ -1705,6 +1713,15 @@ mod unread_tests {
         assert!(media_parts(&msgtype(&json!({"msgtype": "m.text", "body": "x"}))).is_none());
     }
 
+    #[test]
+    fn an_attachment_name_is_reduced_to_its_last_segment() {
+        assert_eq!(safe_file_name("a.png"), "a.png");
+        assert_eq!(safe_file_name("../../etc/x.png"), "x.png");
+        assert_eq!(safe_file_name("dir/"), "dir");
+        assert_eq!(safe_file_name(""), "matrix-attachment");
+        assert_eq!(safe_file_name(".."), "matrix-attachment");
+    }
+
     #[test]
     fn attachments_render_as_typed_markers_and_text_as_itself() {
         let marker = |kind: &str, extra: serde_json::Value| {