GHSA-grr9-747v-xvcp: 
C# vulnerability analysis and mitigation

When Scriban renders an object that contains a circular reference, it traverses the object's members infinitely. Because the ObjectRecursionLimit property defaults to unlimited, this behavior exhausts the thread's stack space, triggering an uncatchable StackOverflowException that immediately terminates the hosting process. When rendering objects (e.g., {{ obj }}), the Scriban rendering engine recursively inspects and formats the object's properties. To prevent infinite loops caused by deeply nested or circular data structures, TemplateContext contains an ObjectRecursionLimit property. However, this property currently defaults to 0 (unlimited). If the data context pushed into the template contains a circular reference, the renderer will recurse indefinitely. This is especially dangerous for web applications that map user-controlled payloads (like JSON) directly to rendering contexts, or for applications that pass ORM objects (like Entity Framework models, which frequently contain circular navigation properties) into the template.

Proof of Concept (PoC)

The following C# code demonstrates the vulnerability. Executing this will cause an immediate, fatal StackOverflowException, bypassing any standard error handling.

using Scriban;
using Scriban.Runtime;
var template = Template.Parse("{{ a }}");
var context = new TemplateContext();
var a = new ScriptObject();
// Introduce a cycle
a["self"] = a;
context.PushGlobal(new ScriptObject { { "a", a } });
try {
  // This crashes the entire process immediately
  template.Render(context);
} catch (Exception ex) {
  // This will never execute because StackOverflowException
  Console.WriteLine("Caught exception: " + ex.Message);
}

Impact

This vulnerability allows a Denial of Service (DoS) attack. If a malicious user can manipulate the data structure passed to the renderer to include a cyclic reference, or if the application passes a complex object graph to an untrusted template, the entire .NET hosting process will crash.

Suggested Remediation

Update TemplateContext.cs to set ObjectRecursionLimit to a safe default, such as 20.

public int ObjectRecursionLimit { get; set; } = 20;

By implementing this default, circular references will gracefully result in a catchable ScriptRuntimeException rather than a fatal process crash.