Scriban has Uncontrolled Recursion in `object.to_json` Causing Unrecoverable Process Crash via StackOverflowException

Summary

## Summary The `object.to_json` builtin function in Scriban performs recursive JSON serialization via an internal `WriteValue()` static local function that has no depth limit, no circular reference detection, and no stack overflow guard. A Scriban template containing a self-referencing object passed to `object.to_json` triggers unbounded recursion, causing a `StackOverflowException` that terminates the hosting .NET process. This is a fatal, unrecoverable crash — `StackOverflowException` cannot be caught by user code in .NET. ## Details The vulnerable code is the `WriteValue()` static local function at `src/Scriban/Functions/ObjectFunctions.cs:494`: ```csharp static void WriteValue(TemplateContext context, Utf8JsonWriter writer, object value) { var type = value?.GetType() ?? typeof(object); if (value is null || value is string || value is bool || type.IsPrimitiveOrDecimal() || value is IFormattable) { JsonSerializer.Serialize(writer, value, type); } else if (value is IList || type.IsArray) { writer.WriteStartArray(); foreach (var x in context.ToList(context.CurrentSpan, value)) { WriteValue(context, writer, x); // recursive, no depth check } writer.WriteEndArray(); } else { writer.WriteStartObject(); var accessor = context.GetMemberAccessor(value); foreach (var member in accessor.GetMembers(context, context.CurrentSpan, value)) { if (accessor.TryGetValue(context, context.CurrentSpan, value, member, out var memberValue)) { writer.WritePropertyName(member); WriteValue(context, writer, memberValue); // recursive, no depth check } } writer.WriteEndObject(); } } ``` This function has **none** of the safety mechanisms present in other recursive paths: - `ObjectToString()` at `TemplateContext.Helpers.cs:98` checks `ObjectRecursionLimit` (default 20) - `EnterRecursive()` at `TemplateContext.cs:957` calls `RuntimeHelpers.EnsureSufficientExecutionStack()` - `CheckAbort()` at `TemplateContext.cs:464` also calls `EnsureSufficientExecutionStack()` The `WriteValue()` function bypasses all of these because it is a static local function that only takes the `TemplateContext` for member access — it never calls `EnterRecursive()`, never checks `ObjectRecursionLimit`, and never calls `EnsureSufficientExecutionStack()`. **Execution flow:** 1. Template creates a ScriptObject: `{{ x = {} }}` 2. Sets a self-reference: `x.self = x` — stores a reference in `ScriptObject.Store` dictionary 3. Pipes to `object.to_json`: `x | object.to_json` → calls `ToJson()` at line 477 4. `ToJson()` calls `WriteValue(context, writer, value)` at line 488 5. `WriteValue` enters the `else` branch (line 515), gets members via accessor, finds "self" 6. `TryGetValue` returns `x` itself, `WriteValue` recurses with the same object — infinite loop 7. `StackOverflowException` is thrown — **fatal, cannot be caught, process terminates** ## PoC ```scriban {{ x = {}; x.self = x; x | object.to_json }} ``` In a hosting application: ```csharp using Scriban; // This will crash the entire process with StackOverflowException var template = Template.Parse("{{ x = {}; x.self = x; x | object.to_json }}"); var result = template.Render(); // FATAL: process terminates here ``` Even without circular references, deeply nested objects can exhaust the stack since no depth limit is enforced: ```scriban {{ a = {} b = {inner: a} c = {inner: b} d = {inner: c} # ... continue nesting ... result = deepest | object.to_json }} ``` ## Impact - **Process crash DoS**: Any application embedding Scriban for user-provided templates (CMS platforms, email template engines, report generators, static site generators) can be crashed by a single malicious template. The crash is unrecoverable — `StackOverflowException` terminates the .NET process. - **No try/catch protection possible**: Unlike most exceptions, `StackOverflowException` cannot be caught by application code. The hosting application cannot wrap `template.Render()` in a try/catch to survive this. - **No authentication required**: `object.to_json` is a default builtin function (registered in `BuiltinFunctions.cs`), available in all Scriban templates unless explicitly removed. - **Trivial to exploit**: The PoC is a single line of template code. ## Recommended Fix Add a depth counter parameter to `WriteValue()` and check it against `ObjectRecursionLimit`, consistent with how `ObjectToString` is protected. Also add `EnsureSufficientExecutionStack()` as a safety net: ```csharp static void WriteValue(TemplateContext context, Utf8JsonWriter writer, object value, int depth = 0) { if (context.ObjectRecursionLimit != 0 && depth > context.ObjectRecursionLimit) { throw new ScriptRuntimeException(context.CurrentSpan, $"Exceeding object recursion limit `{context.ObjectRecursionLimit}` in object.to_json"); } try { RuntimeHelpers.EnsureSufficientExecutionStack(); } catch (InsufficientExecutionStackException) { throw new ScriptRuntimeException(context.CurrentSpan, "Exceeding recursive depth limit in object.to_json, near to stack overflow"); } var type = value?.GetType() ?? typeof(object); if (value is null || value is string || value is bool || type.IsPrimitiveOrDecimal() || value is IFormattable) { JsonSerializer.Serialize(writer, value, type); } else if (value is IList || type.IsArray) { writer.WriteStartArray(); foreach (var x in context.ToList(context.CurrentSpan, value)) { WriteValue(context, writer, x, depth + 1); } writer.WriteEndArray(); } else { writer.WriteStartObject(); var accessor = context.GetMemberAccessor(value); foreach (var member in accessor.GetMembers(context, context.CurrentSpan, value)) { if (accessor.TryGetValue(context, context.CurrentSpan, value, member, out var memberValue)) { writer.WritePropertyName(member); WriteValue(context, writer, memberValue, depth + 1); } } writer.WriteEndObject(); } } ```