GHSA-xw6w-9jjh-p9cr

## Summary Scriban's expression evaluation contains three distinct code paths that allow an attacker who can supply a template to cause denial of service through unbounded memory allocation or CPU exhaustion. The existing safety controls (`LimitToString`, `LoopLimit`) do not protect these paths, giving applications a false sense of safety when evaluating untrusted templates. ## Details ### Vector 1: Unbounded string multiplication In `ScriptBinaryExpression.cs`, the `CalculateToString` method handles the `string * int` operator by looping without any upper bound: ```csharp // src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs:319-334 var leftText = context.ObjectToString(left); var builder = new StringBuilder(); for (int i = 0; i < value; i++) { builder.Append(leftText); } return builder.ToString(); ``` The `LimitToString` safety control (default 1MB) does **not** protect this code path. It only applies to `ObjectToString` output conversions in `TemplateContext.Helpers.cs` (lines 101-121), not to intermediate string values constructed inside `CalculateToString`. The `LoopLimit` also does not apply because this is a C# `for` loop, not a template-level loop — `StepLoop()` is never called here. ### Vector 2: Unbounded BigInteger shift left The `CalculateLongWithInt` and `CalculateBigIntegerNoFit` methods handle `ShiftLeft` without any bound on the shift amount: ```csharp // src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs:710-711 case ScriptBinaryOperator.ShiftLeft: return (BigInteger)left << (int)right; ``` ```csharp // src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs:783-784 case ScriptBinaryOperator.ShiftLeft: return left << (int)right; ``` In contrast, the `Power` operator at lines 722 and 795 uses `BigInteger.ModPow(left, right, MaxBigInteger)` to cap results. The `MaxBigInteger` constant (`BigInteger.One << 1024 * 1024`, defined at line 690) already exists but is never applied to shift operations. ### Vector 3: LoopLi