fix(gopher): block symlink escapes (#85)

Co-authored-by: rissrice2105-agent <rissrice2105-agent@users.noreply.github.com>

internal/gopher/gopher_test.go

@@ -181,6 +181,23 @@ func TestPathTraversalRefused(t *testing.T) {
 	}
 }
 
+func TestSymlinkEscapeRefused(t *testing.T) {
+	srv, dataDir := newTestServer(t)
+	home := filepath.Join(dataDir, "users", "alice", "public_html")
+	link := filepath.Join(home, "leak.txt")
+	if err := os.Symlink(filepath.Join(dataDir, "secret.txt"), link); err != nil {
+		t.Skipf("symlink unavailable: %v", err)
+	}
+
+	r := srv.Resolve("/~alice/leak.txt", false, "")
+	if r.Kind != KindError {
+		t.Fatalf("symlink escape should be refused, got %v", r.Kind)
+	}
+	if strings.Contains(r.Text, "TOP SECRET") || strings.Contains(string(r.Data), "TOP SECRET") {
+		t.Fatalf("symlink escape leaked the secret file: %+v", r)
+	}
+}
+
 func TestNewsPublicVsAuthed(t *testing.T) {
 	srv, _ := newTestServer(t)
 

internal/gopher/server.go

@@ -303,8 +303,8 @@ func (s *Server) userTree(prefix, sel, area string) Response {
 	base := filepath.Join(s.dataDir, "users", name, area)
 
 	// Confine sub to base. Cleaning as an absolute path drops any leading ".."
-	// components; the HasPrefix re-check is belt-and-suspenders against symlink
+	// components; the HasPrefix re-check is belt-and-suspenders against edge
-	// or edge cases.
+	// cases before resolving symlinks below.
 	clean := filepath.Clean("/" + sub)
 	target := filepath.Join(base, clean)
 	if target != base && !strings.HasPrefix(target, base+string(os.PathSeparator)) {
@@ -315,6 +315,17 @@ func (s *Server) userTree(prefix, sel, area string) Response {
 	if err != nil {
 		return errResp("not found")
 	}
+	baseReal, err := filepath.EvalSymlinks(base)
+	if err != nil {
+		return errResp("not found")
+	}
+	targetReal, err := filepath.EvalSymlinks(target)
+	if err != nil {
+		return errResp("not found")
+	}
+	if targetReal != baseReal && !strings.HasPrefix(targetReal, baseReal+string(os.PathSeparator)) {
+		return errResp("forbidden")
+	}
 	linkBase := "/~" + name
 	if prefix == "files" {
 		linkBase = "/files/~" + name
@@ -322,9 +333,9 @@ func (s *Server) userTree(prefix, sel, area string) Response {
 	rel := strings.TrimPrefix(strings.TrimPrefix(target, base), string(os.PathSeparator))
 
 	if info.IsDir() {
-		return s.dirMenu(target, linkBase, rel, name)
+		return s.dirMenu(targetReal, linkBase, rel, name)
 	}
-	data, err := os.ReadFile(target)
+	data, err := os.ReadFile(targetReal)
 	if err != nil {
 		return errResp("unreadable")
 	}