diff --git a/ui/gtk/gtk_ui.cc b/ui/gtk/gtk_ui.cc
index 43dd02ec8f1be..cad973921fbb1 100644
--- a/ui/gtk/gtk_ui.cc
+++ b/ui/gtk/gtk_ui.cc
@@ -22,6 +22,7 @@
 #include "base/containers/flat_map.h"
 #include "base/debug/leak_annotations.h"
 #include "base/environment.h"
+#include "base/files/file_path.h"
 #include "base/functional/bind.h"
 #include "base/logging.h"
 #include "base/nix/mime_util_xdg.h"
@@ -495,6 +496,30 @@ void GtkUi::GetInactiveSelectionFgColor(SkColor* color) const {
 gfx::Image GtkUi::GetIconForContentType(const std::string& content_type,
                                         int dip_size,
                                         float scale) const {
+  // Under X11 a less-privileged process that retains an X11 socket (e.g. the
+  // sandboxed Linux GPU process) can become the _XSETTINGS_S0 manager and
+  // supply an arbitrary Net/IconThemeName, which GDK writes verbatim to
+  // gtk-icon-theme-name. GTK joins that value onto each icon search-path
+  // entry without validation, so a value containing "../" lets such a process
+  // point the icon theme at any on-disk directory and have gdk-pixbuf /
+  // librsvg decode files from it inside the unsandboxed browser process.
+  // Reject such theme names, mirroring IsValidCursorThemeName() in
+  // ui/base/x/x11_cursor_loader.cc.
+  {
+    gchar* raw_name = nullptr;
+    g_object_get(gtk_settings_get_default(), "gtk-icon-theme-name", &raw_name,
+                 nullptr);
+    std::string theme_name = raw_name ? raw_name : "";
+    g_free(raw_name);
+    base::FilePath theme_path(theme_name);
+    if (theme_name.empty() || theme_name == "." || theme_path.IsAbsolute() ||
+        theme_path.ReferencesParent() ||
+        theme_path.BaseName() != theme_path) {
+      LOG(ERROR) << "Rejecting unsafe gtk-icon-theme-name: " << theme_name;
+      return gfx::Image();
+    }
+  }
+
   // This call doesn't take a reference.
   GtkIconTheme* theme = GetDefaultIconTheme();