From a569dd992a70fa04af0bc12fb2364f1fa5541f22 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jos=C3=A9=20Valim?= <jose.valim@gmail.com>
Date: Tue, 9 Jun 2026 13:25:22 +0200
Subject: [PATCH 1/3] Cap width in strftime to 1024 characters

---
 lib/elixir/lib/calendar.ex               | 11 ++++++++--
 lib/elixir/test/elixir/calendar_test.exs | 26 ++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/lib/elixir/lib/calendar.ex b/lib/elixir/lib/calendar.ex
index 89160389b..35afb4663 100644
--- a/lib/elixir/lib/calendar.ex
+++ b/lib/elixir/lib/calendar.ex
@@ -3,6 +3,8 @@
 # SPDX-FileCopyrightText: 2012 Plataformatec
 
 defmodule Calendar do
+  @strftime_max_width 1024
+
   @moduledoc """
   This module defines the responsibilities for working with
   calendars, dates, times and datetimes in Elixir.
@@ -529,6 +531,7 @@ def get_time_zone_database() do
     * `%`: indicates the start of a formatted section
     * `<padding>`: set the padding (see below)
     * `<width>`: a number indicating the minimum size of the formatted section
+      (maximum #{@strftime_max_width})
     * `<format>`: the format itself (see below)
 
   ### Accepted padding options
@@ -667,9 +670,13 @@ defp parse_modifiers("_" <> rest, width, nil, parser_data) do
   end
 
   defp parse_modifiers(<<digit, rest::binary>>, width, pad, parser_data) when digit in ?0..?9 do
-    new_width = (width || 0) * 10 + (digit - ?0)
+    width = (width || 0) * 10 + (digit - ?0)
+
+    if width > @strftime_max_width do
+      raise ArgumentError, "invalid strftime format: width must be at most #{@strftime_max_width}"
+    end
 
-    parse_modifiers(rest, new_width, pad, parser_data)
+    parse_modifiers(rest, width, pad, parser_data)
   end
 
   # set default padding if none was specified
diff --git a/lib/elixir/test/elixir/calendar_test.exs b/lib/elixir/test/elixir/calendar_test.exs
index 54291ce3d..3fff584f8 100644
--- a/lib/elixir/test/elixir/calendar_test.exs
+++ b/lib/elixir/test/elixir/calendar_test.exs
@@ -339,6 +339,32 @@ test "handles `0` both as padding and as part of a width" do
       assert Calendar.strftime(~N[2019-08-15 17:07:57], "%010A") == "00Thursday"
     end
 
+    test "limits width to at most 1024 characters" do
+      assert Calendar.strftime(~D[2019-08-15], "%1024d") |> byte_size() == 1024
+
+      assert_raise ArgumentError, "invalid strftime format: width must be at most 1024", fn ->
+        Calendar.strftime(~D[2019-08-15], "%1025d")
+      end
+
+      assert_raise ArgumentError, "invalid strftime format: width must be at most 1024", fn ->
+        Calendar.strftime(~D[2019-08-15], "%10000d")
+      end
+    end
+
+    test "limits width in preferred formats" do
+      assert_raise ArgumentError, "invalid strftime format: width must be at most 1024", fn ->
+        Calendar.strftime(~N[2019-08-15 17:07:57], "%c", preferred_datetime: "%1025d")
+      end
+
+      assert_raise ArgumentError, "invalid strftime format: width must be at most 1024", fn ->
+        Calendar.strftime(~N[2019-08-15 17:07:57], "%x", preferred_date: "%1025d")
+      end
+
+      assert_raise ArgumentError, "invalid strftime format: width must be at most 1024", fn ->
+        Calendar.strftime(~N[2019-08-15 17:07:57], "%X", preferred_time: "%1025H")
+      end
+    end
+
     test "formats Epoch time with %s" do
       assert Calendar.strftime(~N[2019-08-15 17:07:57], "%s") == "1565888877"
 
-- 
2.52.0