From 80b8509bfc20015bb178ecbcd80bce348b743076 Mon Sep 17 00:00:00 2001
From: Tom Edwards <109803929+TomEdwardsEnscape@users.noreply.github.com>
Date: Mon, 16 Mar 2026 13:20:32 +0100
Subject: [PATCH] Validate layoutable Thickness properties to block NaN or
 infinite values (#20899)

* Validate layoutable Thickness properties to block NaN or infinite values

* Use double.IsFinite
---
 src/Avalonia.Base/Layout/Layoutable.cs        |  4 ++-
 src/Avalonia.Controls/Border.cs               |  2 +-
 .../Chrome/WindowDrawnDecorations.cs          |  4 +--
 src/Avalonia.Controls/Decorator.cs            |  2 +-
 src/Avalonia.Controls/Page/Page.cs            |  2 +-
 .../BorderTests.cs                            | 28 ++++++++++++++-----
 6 files changed, 29 insertions(+), 13 deletions(-)

diff --git a/src/Avalonia.Base/Layout/Layoutable.cs b/src/Avalonia.Base/Layout/Layoutable.cs
index fedea332b6..e0c316c60a 100644
--- a/src/Avalonia.Base/Layout/Layoutable.cs
+++ b/src/Avalonia.Base/Layout/Layoutable.cs
@@ -113,7 +113,7 @@ namespace Avalonia.Layout
         /// Defines the <see cref="Margin"/> property.
         /// </summary>
         public static readonly StyledProperty<Thickness> MarginProperty =
-            AvaloniaProperty.Register<Layoutable, Thickness>(nameof(Margin));
+            AvaloniaProperty.Register<Layoutable, Thickness>(nameof(Margin), validate: ValidateThickness);
 
         /// <summary>
         /// Defines the <see cref="HorizontalAlignment"/> property.
@@ -161,6 +161,8 @@ namespace Avalonia.Layout
         private static bool ValidateMinimumDimension(double value) => !double.IsPositiveInfinity(value) && ValidateMaximumDimension(value);
         private static bool ValidateMaximumDimension(double value) => value >= 0;
 
+        private static bool ValidateThickness(Thickness value) => double.IsFinite(value.Left) && double.IsFinite(value.Top) && double.IsFinite(value.Right) && double.IsFinite(value.Bottom);
+
         /// <summary>
         /// Occurs when the element's effective viewport changes.
         /// </summary>
diff --git a/src/Avalonia.Controls/Border.cs b/src/Avalonia.Controls/Border.cs
index b816858632..29a31d8070 100644
--- a/src/Avalonia.Controls/Border.cs
+++ b/src/Avalonia.Controls/Border.cs
@@ -38,7 +38,7 @@ namespace Avalonia.Controls
         /// Defines the <see cref="BorderThickness"/> property.
         /// </summary>
         public static readonly StyledProperty<Thickness> BorderThicknessProperty =
-            AvaloniaProperty.Register<Border, Thickness>(nameof(BorderThickness));
+            AvaloniaProperty.Register<Border, Thickness>(nameof(BorderThickness), validate: MarginProperty.ValidateValue);
 
         /// <summary>
         /// Defines the <see cref="CornerRadius"/> property.
diff --git a/src/Avalonia.Controls/Chrome/WindowDrawnDecorations.cs b/src/Avalonia.Controls/Chrome/WindowDrawnDecorations.cs
index ae279d6ab3..48847b5f59 100644
--- a/src/Avalonia.Controls/Chrome/WindowDrawnDecorations.cs
+++ b/src/Avalonia.Controls/Chrome/WindowDrawnDecorations.cs
@@ -58,13 +58,13 @@ public class WindowDrawnDecorations : StyledElement
     /// Defines the <see cref="DefaultFrameThickness"/> property.
     /// </summary>
     public static readonly StyledProperty<Thickness> DefaultFrameThicknessProperty =
-        AvaloniaProperty.Register<WindowDrawnDecorations, Thickness>(nameof(DefaultFrameThickness));
+        AvaloniaProperty.Register<WindowDrawnDecorations, Thickness>(nameof(DefaultFrameThickness), validate: Border.BorderThicknessProperty.ValidateValue);
 
     /// <summary>
     /// Defines the <see cref="DefaultShadowThickness"/> property.
     /// </summary>
     public static readonly StyledProperty<Thickness> DefaultShadowThicknessProperty =
-        AvaloniaProperty.Register<WindowDrawnDecorations, Thickness>(nameof(DefaultShadowThickness));
+        AvaloniaProperty.Register<WindowDrawnDecorations, Thickness>(nameof(DefaultShadowThickness), validate: Border.BorderThicknessProperty.ValidateValue);
 
     /// <summary>
     /// Defines the <see cref="TitleBarHeight"/> property.
diff --git a/src/Avalonia.Controls/Decorator.cs b/src/Avalonia.Controls/Decorator.cs
index e62ca0000b..8cd1916718 100644
--- a/src/Avalonia.Controls/Decorator.cs
+++ b/src/Avalonia.Controls/Decorator.cs
@@ -20,7 +20,7 @@ namespace Avalonia.Controls
         /// Defines the <see cref="Padding"/> property.
         /// </summary>
         public static readonly StyledProperty<Thickness> PaddingProperty =
-            AvaloniaProperty.Register<Decorator, Thickness>(nameof(Padding));
+            AvaloniaProperty.Register<Decorator, Thickness>(nameof(Padding), validate: MarginProperty.ValidateValue);
 
         /// <summary>
         /// Initializes static members of the <see cref="Decorator"/> class.
diff --git a/src/Avalonia.Controls/Page/Page.cs b/src/Avalonia.Controls/Page/Page.cs
index 601af92580..48b7bd1b0c 100644
--- a/src/Avalonia.Controls/Page/Page.cs
+++ b/src/Avalonia.Controls/Page/Page.cs
@@ -17,7 +17,7 @@ namespace Avalonia.Controls
         /// Defines the <see cref="SafeAreaPadding"/> property.
         /// </summary>
         public static readonly StyledProperty<Thickness> SafeAreaPaddingProperty =
-            AvaloniaProperty.Register<Page, Thickness>(nameof(SafeAreaPadding));
+            AvaloniaProperty.Register<Page, Thickness>(nameof(SafeAreaPadding), validate: PaddingProperty.ValidateValue);
 
         /// <summary>
         /// Defines the <see cref="Header"/> property.
diff --git a/tests/Avalonia.Controls.UnitTests/BorderTests.cs b/tests/Avalonia.Controls.UnitTests/BorderTests.cs
index e31eb08964..df80998b05 100644
--- a/tests/Avalonia.Controls.UnitTests/BorderTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/BorderTests.cs
@@ -1,9 +1,6 @@
+using System;
 using Avalonia.Layout;
-using Avalonia.Media;
-using Avalonia.Rendering;
 using Avalonia.UnitTests;
-using Avalonia.VisualTree;
-using Moq;
 using Xunit;
 
 namespace Avalonia.Controls.UnitTests
@@ -45,14 +42,31 @@ namespace Avalonia.Controls.UnitTests
 
             Assert.Equal(new Rect(6, 6, 0, 0), content.Bounds);
         }
-        
+
+        [Fact]
+        public void Should_Reject_NaN_Or_Infinite_Thicknesses()
+        {
+            var target = new Border();
+
+            SetValues(target, Layoutable.MarginProperty);
+            SetValues(target, Decorator.PaddingProperty);
+            SetValues(target, Border.BorderThicknessProperty);
+
+            static void SetValues(Border target, AvaloniaProperty<Thickness> property)
+            {
+                Assert.Throws<ArgumentException>(() => target.SetValue(property, new Thickness(0, 0, 0, double.NaN)));
+                Assert.Throws<ArgumentException>(() => target.SetValue(property, new Thickness(0, 0, 0, double.PositiveInfinity)));
+                Assert.Throws<ArgumentException>(() => target.SetValue(property, new Thickness(0, 0, 0, double.NegativeInfinity)));
+            }
+        }
+
         public class UseLayoutRounding : ScopedTestBase
         {
             [Fact]
             public void Measure_Rounds_Padding()
             {
-                var target = new Border 
-                { 
+                var target = new Border
+                {
                     Padding = new Thickness(1),
                     Child = new Canvas
                     {