Scriban: array * int (ScriptArray<T>.TryEvaluate) bypasses LoopLimit — incomplete fix for GHSA-c875-h985-hvrc, missed sibling of GHSA-24c8-4792-22hx

Summary

The array multiplication operator (array * integer) in Scriban allocates a result whose size is the product of the attacker-controlled integer and the array length, with no LoopLimit / LimitToString check and no overflow-safe arithmetic. A ~40-byte template forces a multi-gigabyte allocation, producing a denial-of-service.

This is the unguarded sibling of operations that were hardened against the same class of abuse: string * integer (gated by a LimitToString pre-check), array.insert_at (gated by StepLoop/LoopLimit — the GHSA-24c8-4792-22hx fix shipped in 7.2.0, scored 8.7 High), and the range/iteration paths covered by GHSA-c875-h985-hvrc ("Built-in operations bypass LoopLimit", fixed 7.0.0). The same LoopLimit-based hardening pattern was applied to those operations but never to array * integer.

This can be observed directly in 7.0.0, the release where GHSA-c875 was patched: (1..5) * 50000000 (and 1..N | array.size) correctly throws Exceeding number of iteration limit '1000', while [1,2,3,4,5] * 50000000 allocates ~2 GB with no limit. The LoopLimit control is enforced on the iteration path but not on the array * int allocation path, side by side, in the same version. The bug has been present since the operator was introduced in 3.0.0, survives all of the 6.6.0 / 7.0.0 / 7.2.0 DoS-hardening passes, and is still present in 7.2.0 (current) — i.e. it is both a missed sibling of GHSA-24c8 and an incomplete coverage of GHSA-c875's LoopLimit hardening.

Details

The array * int operator is handled in ScriptArray<T>.TryEvaluate:

// src/Scriban/Runtime/ScriptArray.cs:504-508  (Multiply case)
var newArray = new ScriptArray<T>(intModifier * array.Count);
for (int i = 0; i < intModifier; i++)
{
    newArray.AddRange(array);
}

intModifier is the attacker-supplied integer (context.ToInt(...), ScriptArray.cs:399). Two problems:

1. No resource limit. Neither new ScriptArray<T>(intModifier * array.Count) nor the AddRange loop consults LoopLimit, LimitToString, or calls context.StepLoop(...). A grep of the entire TryEvaluate method (ScriptArray.cs:360-560) finds no StepLoop / LoopLimit / Limit reference. LoopLimit (default 1000) is therefore not enforced: a template that requests 250,000,000 elements creates them all without any "iteration limit" error.

2. Integer overflow in the capacity. intModifier * array.Count is unchecked int arithmetic. The overflow-safe long cast used by the string sibling is absent here.

The DoS-hardening passes guarded the two sibling operations but not this one:

// src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs:341  (string * int — GUARDED)
if (context.LimitToString > 0 && value > 0 && leftText.Length > 0
        && (long)leftText.Length * value > context.LimitToString)   // long arithmetic, pre-check
{
    throw new ScriptRuntimeException(spanMultiplier, $"String multiplication exceeds LimitToString `{context.LimitToString}`.");
}

// src/Scriban/Functions/ArrayFunctions.cs:414  (array.insert_at — GUARDED, GHSA-24c8 fix in 7.2.0)
for (int i = array.Count; i < index; i++)
{
    context.StepLoop(span, ref loopStep);   // LoopLimit enforced
    array.Add(null);
}

array * int (ScriptArray.cs:504) received neither guard.

When the oversized allocation fails as a managed exception, it is wrapped by the binary-expression evaluator:

// src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs:241-243
catch (Exception ex) when (!(ex is ScriptRuntimeException))
{
    throw new ScriptRuntimeException(span, ex.Message);
}

So a host that wraps Render() in try/catch sees a ScriptRuntimeException carrying the original OutOfMemoryException message (or ArgumentOutOfRangeException on the integer-overflow path).

PoC

A single console project reproduces it on the released NuGet package.

poc.csproj:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <!-- If only the .NET 9 SDK is installed, change to net9.0. Behavior is identical. -->
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Scriban" Version="7.2.0" />
  </ItemGroup>
</Project>

Program.cs:

using Scriban;

// ~41-byte template requests 5 * 200,000,000 = 1,000,000,000 elements
string tpl = "{{ x = [1,2,3,4,5] * 200000000; x.size }}";

System.Console.WriteLine("Rendering...");
var sw = System.Diagnostics.Stopwatch.StartNew();
var result = Template.Parse(tpl).Render();           // allocates ~7.7 GB
System.Console.WriteLine($"size={result.Trim()} peakWS="
    + System.Diagnostics.Process.GetCurrentProcess().PeakWorkingSet64 / (1024 * 1024)
    + "MB elapsed=" + sw.ElapsedMilliseconds + "ms");

Run:

dotnet run -c Release

Measured peak working set on Scriban 7.2.0 (net8.0, .NET 9 runtime, Linux), varying only the multiplier:

LoopLimit (default 1000) is demonstrably not enforced: 250,000,000 elements are created with no "iteration limit" error. Reproduced identically on released NuGet 6.6.0, 7.0.0, 7.1.0, and 7.2.0, and on 3.0.0, 4.0.0, 5.0.0, 5.10.0, 6.0.0, 6.2.1, 6.5.8 (~2 GB at multiplier 50,000,000). Version 2.1.4 and earlier are NOT affected — the operator did not exist (Unable to convert type ScriptArray to int).

Impact

• Type: Denial of service via uncontrolled memory allocation (CWE-789 / CWE-1284). The result size is intModifier * array.Count, attacker-controlled, with no limit and no overflow-safe arithmetic.
• Severity: CVSS 4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N = 8.7 (High) — the same vector and score GitHub/Scriban assigned to the sibling advisory GHSA-24c8-4792-22hx. CVSS 3.1 equivalent AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H = 7.5 (High).
• Who is impacted: any application that renders a template whose text is wholly or partially attacker-controlled (the documented server-side template scenario), or that passes attacker-controlled strings to object.eval / object.eval_template. No MemberFilter interaction is required — this is a pure language operation.
• Outcome (deployment-dependent, stated honestly): On systems with sufficient memory, the runtime catches the allocation failure and the host sees a ScriptRuntimeException wrapping OutOfMemoryException (or ArgumentOutOfRangeException on the integer-overflow path) — recoverable per request. On systems where the multi-GB allocation exceeds available memory, the OS OOM-killer can terminate the process before the managed exception fires (this outcome is deployment-dependent and was not reproduced in our 20 GB + swap test environment). In all cases, a ~40-byte template forces a multi-GB allocation and seconds of pegged CPU/GC — a real per-request availability degradation and resource amplification.
• Why the existing mitigation does not help: LoopLimit (default 1000) is the documented control for unbounded iteration/allocation, but the array * int path never consults it, so a defender running default configuration is not protected.
• Affected versions: 3.0.0 – 7.2.0 (every release containing the array * int operator). 2.1.4 and earlier are not affected.

Suggested remediation

Apply the same hardening already used on the sibling operations, in ScriptArray.cs (Multiply case, :504-508):

• Mirror array.insert_at: call context.StepLoop(span, ref loopStep) inside the fill loop so LoopLimit is enforced; or
• Mirror string * int: pre-check the result size with overflow-safe arithmetic before allocating, e.g. if (context.LimitToString > 0 && (long)intModifier * array.Count > context.LimitToString) throw new ScriptRuntimeException(...), and compute the capacity as long (or reject negative/overflowing products) to remove the integer-overflow path.

Add a regression test that asserts a graceful ScriptRuntimeException for a large multiplier (e.g. [1,2,3,4,5] * 50000000) rather than allowing the allocation to proceed.