Scriban has Multiple Denial-of-Service Vectors via Unbounded Resource Consumption During Expression Evaluation

Summary

## 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: LoopLimit bypass via range enumeration in builtin functions The range operators `..` and `..<` produce lazy `IEnumerable<object>` iterators: ```csharp // src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs:401-417 private static IEnumerable<object> RangeInclude(BigInteger left, BigInteger right) { if (left < right) { for (var i = left; i <= right; i++) { yield return FitToBestInteger(i); } } // ... } ``` When these ranges are consumed by builtin functions, `LoopLimit` is completely bypassed because `StepLoop()` is only called in `ScriptForStatement` and `ScriptWhileStatement` — it is never called in any function under `src/Scriban/Functions/`. For example: - `ArrayFunctions.Size` (line 609) calls `.Cast<object>().Count()`, fully enumerating the range - `ArrayFunctions.Join` (line 388) iterates with `foreach` and appends to a `StringBuilder` with no size limit ## PoC ### Vector 1 — String multiplication OOM: ```csharp var template = Template.Parse("{{ 'AAAA' * 500000000 }}"); var context = new TemplateContext(); // context.LimitToString is 1048576 by default — does NOT protect this path template.Render(context); // OutOfMemoryException: attempts ~2GB allocation ``` ### Vector 2 — BigInteger shift OOM: ```csharp var template = Template.Parse("{{ 1 << 100000000 }}"); var context = new TemplateContext(); template.Render(context); // Allocates BigInteger with 100M bits (~12.5MB) // {{ 1 << 2000000000 }} attempts ~250MB ``` ### Vector 3 — LoopLimit bypass via range + builtin: ```csharp var template = Template.Parse("{{ (0..1000000000) | array.size }}"); var context = new TemplateContext(); // context.LoopLimit is 1000 — does NOT protect builtin function iteration template.Render(context); // CPU exhaustion: enumerates 1 billion items ``` ```csharp var template = Template.Parse("{{ (0..10000000) | array.join ',' }}"); var context = new TemplateContext(); template.Render(context); // Memory exhaustion: builds ~80MB+ joined string ``` ## Impact An attacker who can supply a Scriban template (common in CMS platforms, email templating systems, reporting tools, and other applications embedding Scriban) can cause denial of service by crashing the host process via `OutOfMemoryException` or exhausting CPU resources. This is particularly impactful because: 1. Applications relying on the default safety controls (`LoopLimit=1000`, `LimitToString=1MB`) believe they are protected against resource exhaustion from untrusted templates, but these controls have gaps. 2. A single malicious template expression is sufficient — no complex template logic is required. 3. The `OutOfMemoryException` in vectors 1 and 2 typically terminates the entire process, not just the template evaluation. ## Recommended Fix ### Vector 1 — String multiplication: Check `LimitToString` before the loop ```csharp // src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs, before line 330 var leftText = context.ObjectToString(left); if (context.LimitToString > 0 && (long)value * leftText.Length > context.LimitToString) { throw new ScriptRuntimeException(span, $"String multiplication would exceed LimitToString ({context.LimitToString} characters)"); } var builder = new StringBuilder(); for (int i = 0; i < value; i++) ``` ### Vector 2 — BigInteger shift: Cap the shift amount ```csharp // src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs, lines 710-711 and 783-784 case ScriptBinaryOperator.ShiftLeft: if (right > 1048576) // Same as MaxBigInteger bit count throw new ScriptRuntimeException(span, $"Shift amount {right} exceeds maximum allowed (1048576)"); return (BigInteger)left << (int)right; ``` ### Vector 3 — Range + builtins: Add iteration counting to range iterators Pass `TemplateContext` to `RangeInclude`/`RangeExclude` and enforce a limit: ```csharp private static IEnumerable<object> RangeInclude(TemplateContext context, BigInteger left, BigInteger right) { var maxRange = context.LoopLimit > 0 ? context.LoopLimit : int.MaxValue; int count = 0; if (left < right) { for (var i = left; i <= right; i++) { if (++count > maxRange) throw new ScriptRuntimeException(context.CurrentNode.Span, $"Range enumeration exceeds LoopLimit ({maxRange})"); yield return FitToBestInteger(i); } } // ... same for descending branch } ``` Alternatively, validate range size eagerly at creation time: `if (BigInteger.Abs(right - left) > maxRange) throw ...`