Fix symlink escape in Gopher public file resolver

internal/gopher/gopher_test.go

@@ -159,6 +159,38 @@ func TestHomepageFileAndDir(t *testing.T) {
 	}
 }
 
+func TestSymlinkEscapeRefused(t *testing.T) {
+	srv, dataDir := newTestServer(t)
+	// Create a symlink inside alice's public_html that points to secret.txt
+	// outside the member area. The selector should be refused.
+	link := filepath.Join(dataDir, "users", "alice", "public_html", "leak.txt")
+	if err := os.Symlink(filepath.Join(dataDir, "secret.txt"), link); err != nil {
+		t.Fatal(err)
+	}
+	r := srv.Resolve("/~alice/leak.txt", false, "")
+	if r.Kind == KindText && strings.Contains(r.Text, "TOP SECRET") {
+		t.Fatal("symlink escape: resolved path pointed outside member area but content was served")
+	}
+	// Symlink to a subdirectory should still work.
+	subdir := filepath.Join(dataDir, "users", "alice", "public_html", "sub")
+	if err := os.Mkdir(subdir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	inner := filepath.Join(subdir, "hello.txt")
+	if err := os.WriteFile(inner, []byte("inside"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	// Symlink inside the area pointing to another path within the area is fine.
+	sym := filepath.Join(dataDir, "users", "alice", "public_html", "ok-link")
+	if err := os.Symlink(inner, sym); err != nil {
+		t.Fatal(err)
+	}
+	r2 := srv.Resolve("/~alice/ok-link", false, "")
+	if r2.Kind != KindText || !strings.Contains(r2.Text, "inside") {
+		t.Fatalf("symlink within area should be allowed: got %+v", r2)
+	}
+}
+
 func TestPathTraversalRefused(t *testing.T) {
 	srv, _ := newTestServer(t)
 	for _, sel := range []string{

internal/gopher/server.go

@@ -310,6 +310,14 @@ func (s *Server) userTree(prefix, sel, area string) Response {
 	if target != base && !strings.HasPrefix(target, base+string(os.PathSeparator)) {
 		return errResp("forbidden")
 	}
+	// Symlink escape guard: resolve symlinks and re-check the real path stays
+	// within the member's area. A symlink created inside public_html that
+	// points outside would pass the lexical check but leak external content.
+	if resolved, err := filepath.EvalSymlinks(target); err == nil {
+		if resolved != base && !strings.HasPrefix(resolved, base+string(os.PathSeparator)) {
+			return errResp("forbidden")
+		}
+	}
 
 	info, err := os.Stat(target)
 	if err != nil {