From a87b09454c5d11ff5c12952bc56ee794689ae3f0 Mon Sep 17 00:00:00 2001
From: Matt Ludwigs <matt.ludwigs@river.com>
Date: Tue, 24 Jun 2025 20:33:26 -0700
Subject: [PATCH 1/2] Reject tokens with future signed_at timestamps

---
 lib/plug/crypto.ex        | 26 +++++++++++++++++++-------
 test/plug/crypto_test.exs |  5 +++++
 2 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/lib/plug/crypto.ex b/lib/plug/crypto.ex
index 682ec05..43a95cc 100644
--- a/lib/plug/crypto.ex
+++ b/lib/plug/crypto.ex
@@ -333,10 +333,9 @@ defmodule Plug.Crypto do
         %{data: data, signed: signed} -> {data, signed, 86400}
       end
 
-    if expired?(signed, Keyword.get(opts, :max_age, max_age)) do
-      {:error, :expired}
-    else
-      {:ok, data}
+    case validate_age(signed, Keyword.get(opts, :max_age, max_age)) do
+      :ok -> {:ok, data}
+      error -> error
     end
   end
 
@@ -351,9 +350,22 @@ defmodule Plug.Crypto do
     KeyGenerator.generate(secret_key_base, salt, iterations, length, digest, cache)
   end
 
-  defp expired?(_signed, :infinity), do: false
-  defp expired?(_signed, max_age_secs) when max_age_secs <= 0, do: true
-  defp expired?(signed, max_age_secs), do: signed + trunc(max_age_secs * 1000) < now_ms()
+  defp validate_age(_signed, :infinity), do: :ok
+  defp validate_age(_signed, max_age_secs) when max_age_secs <= 0, do: {:error, :expired}
+
+  defp validate_age(signed, max_age_secs) do
+    now = now_ms()
+
+    if signed > now do
+      {:error, :invalid}
+    else
+      if signed + trunc(max_age_secs * 1000) < now do
+        {:error, :expired}
+      else
+        :ok
+      end
+    end
+  end
 
   defp now_ms, do: System.os_time(:millisecond)
 end
diff --git a/test/plug/crypto_test.exs b/test/plug/crypto_test.exs
index 95d2bf5..1811ca1 100644
--- a/test/plug/crypto_test.exs
+++ b/test/plug/crypto_test.exs
@@ -200,6 +200,11 @@ defmodule Plug.CryptoTest do
                {:error, :expired}
     end
 
+    test "ensures signed_at is not in future while decrypting" do
+      token = encrypt(@key, "secret", 1, signed_at: System.os_time(:second) + 31_536_000)
+      assert {:error, :invalid} = decrypt(@key, "secret", token)
+    end
+
     test "passes key_iterations options to key generator" do
       signed1 = encrypt(@key, "secret", 1, signed_at: 0, key_iterations: 1)
       signed2 = encrypt(@key, "secret", 1, signed_at: 0, key_iterations: 2)

From fb96a38c98c8156e3542fc35b6fe90498f653666 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jos=C3=A9=20Valim?= <jose.valim@gmail.com>
Date: Wed, 25 Jun 2025 10:23:14 +0200
Subject: [PATCH 2/2] Update lib/plug/crypto.ex

---
 lib/plug/crypto.ex | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/lib/plug/crypto.ex b/lib/plug/crypto.ex
index 43a95cc..6208fe5 100644
--- a/lib/plug/crypto.ex
+++ b/lib/plug/crypto.ex
@@ -356,14 +356,10 @@ defmodule Plug.Crypto do
   defp validate_age(signed, max_age_secs) do
     now = now_ms()
 
-    if signed > now do
-      {:error, :invalid}
-    else
-      if signed + trunc(max_age_secs * 1000) < now do
-        {:error, :expired}
-      else
-        :ok
-      end
+    cond do
+      signed > now -> {:error, :invalid}
+      signed + trunc(max_age_secs * 1000) < now -> {:error, :expired}
+      true -> :ok
     end
   end