Fix GHSA-rvv3-g6hj-g44x: apply default MaxDepth of 64 for self-refere…

docs/source/5.0-Upgrade-Guide.md

@@ -84,7 +84,8 @@ cfg.CreateMap<Category, CategoryDto>().MaxDepth(3);
 cfg.CreateMap<User, UserDto>().PreserveReferences();
 ```
 
-Starting from 6.1.0 PreserveReferences is set automatically at config time whenever the recursion can be detected statically. If you're still getting `StackOverflowException`, open an issue with a full repro and we'll look into it.
+Starting from 6.1.0 PreserveReferences is set automatically at config time whenever the recursion can be detected statically.
+Starting from 16.x / MagicMapper patch, a default MaxDepth of 64 is also applied automatically, preventing a StackOverflowException from deeply nested (but non-circular) object graphs (see GHSA-rvv3-g6hj-g44x). If you need deeper mapping, call `.MaxDepth(n)` explicitly. To rely solely on object-identity caching without a depth limit, call `.PreserveReferences()` explicitly.
 
 ## UseDestinationValue
 

docs/source/Configuration.md

@@ -224,4 +224,20 @@ Compilation times increase with the size of the execution plan and that depends
 You can set `MapAtRuntime` per member or `MaxExecutionPlanDepth` globally (the default is one, set it to zero).
 
 These will reduce the size of the execution plan by replacing the execution plan for a child object with a method call. The compilation will be faster, but the mapping itself might be slower. Search the repo for more details and use a profiler to better understand the effect.
-Avoiding `PreserveReferences` and `MaxDepth` also helps.
+Avoiding `PreserveReferences` and `MaxDepth` also helps.
+
+## Circular and Self-Referential Types
+
+When MagicMapper detects a self-referential type mapping (e.g., `CreateMap<Node, Node>()` where `Node` has a `Node` property), it automatically enables `PreserveReferences` to avoid re-mapping the same object instance. It also applies a default `MaxDepth` of **64** — matching System.Text.Json and Newtonsoft.Json — to prevent a Denial-of-Service condition from deeply nested object graphs (see GHSA-rvv3-g6hj-g44x).
+
+If your object graphs legitimately exceed 64 levels, increase the limit explicitly:
+
+```c#
+cfg.CreateMap<Node, Node>().MaxDepth(128);
+```
+
+To disable the depth limit entirely and rely solely on object-identity caching, call `.PreserveReferences()` explicitly:
+
+```c#
+cfg.CreateMap<Node, Node>().PreserveReferences();
+```

src/UnitTests/Bug/DeepNestingStackOverflow.cs

@@ -0,0 +1,47 @@
+namespace AutoMapper.UnitTests.Bug;
+
+public class DeepNestingStackOverflow
+{
+    class Circular { public Circular Self { get; set; } }
+
+    // Verifies that mapping a deeply nested self-referential object does not
+    // crash the process with a StackOverflowException (GHSA-rvv3-g6hj-g44x).
+    // AutoMapper auto-applies a default MaxDepth of 64 (matching System.Text.Json
+    // and Newtonsoft.Json) when it detects a self-referential type mapping.
+    [Fact]
+    public void Mapping_deeply_nested_self_referential_object_should_not_stackoverflow()
+    {
+        var config = new MapperConfiguration(cfg => cfg.CreateMap<Circular, Circular>());
+        var mapper = config.CreateMapper();
+
+        var root = new Circular();
+        var current = root;
+        for (int i = 0; i < 30_000; i++)
+        {
+            current.Self = new Circular();
+            current = current.Self;
+        }
+
+        // Should complete without crashing; mapping is truncated at default MaxDepth (64)
+        var result = mapper.Map<Circular>(root);
+        result.ShouldNotBeNull();
+
+        int depth = 0;
+        current = result;
+        while (current.Self != null)
+        {
+            depth++;
+            current = current.Self;
+        }
+        depth.ShouldBeLessThanOrEqualTo(64);
+    }
+
+    // Verifies that configuration validation does not detect the vulnerability —
+    // only the runtime mapping is affected, not the configuration itself.
+    [Fact]
+    public void AssertConfigurationIsValid_does_not_detect_deep_nesting_vulnerability()
+    {
+        var config = new MapperConfiguration(cfg => cfg.CreateMap<Circular, Circular>());
+        config.AssertConfigurationIsValid();
+    }
+}