<!-- obsidian --><h2 data-heading="Description">Description</h2> <p>When a host pushes a CLR object into a Scriban <code>TemplateContext</code> via the standard, documented pattern —</p> <pre><code class="language-csharp">var so = new ScriptObject(); so["user"] = currentUser; // direct CLR reference context.PushGlobal(so); </code></pre> <p>— <code>TypedObjectAccessor</code> exposes every public-getter property for <strong>both reading and writing</strong>, and writes land on the live host object and <strong>persist after <code>Render()</code> returns</strong>. The write path performs no <code>CanWrite</code> and no setter-visibility check, producing two related but distinct weaknesses:</p> <p><strong>(A) Mass assignment of public setters — CWE-915 (originally F-002).</strong> Any <code>{ get; set; }</code> property is writable from template code (<code>{{ user.is_admin = true }}</code>, <code>{{ order.total_price = 0 }}</code>). This is "surprising but technically consistent with the setter being public" — and crucially, Scriban offers <strong>no way to expose such a property read-only</strong>, because <code>MemberFilter</code> is read/write-symmetric.</p> <p><strong>(B) Access-modifier bypass — CWE-284 (originally F-007).</strong> Properties the developer <strong>deliberately</strong> restricted are also writable, because reflection ignores C# accessibility:</p> Declaration | Developer intent | Actual behavior -- | -- | -- { get; set; } | writable | writable (mass assignment — A) { get; private set; } | only the owning class writes | template writes freely { get; internal set; } | only the declaring assembly writes | template writes freely { get; init; } | immutable after construction (C# 9 language guarantee) | template writes freely post-construction <p>The <code>init</code>-only post-construction write — the highest false-positive risk — was explicitly confirmed against the shipped 7.2.1 package.</p> <h2 data-heading="Affected Versions">Affected Versions</h2> <p>All releases that ship <code>TypedObjectAccessor</code> (<code>&#x3C;= 7.2.1</code>). <code>PrepareMembers</code> has used the getter-only filter since the accessor was introduced, and <code>TrySetValue</code> has never checked the setter. The <code>init</code> bypass applies on .NET 5+; <code>private set</code> / <code>internal set</code> apply on every supported runtime. No patched version exists.</p> <h2 data-heading="Steps to Reproduce">Steps to Reproduce</h2> <blockquote> <p>Copy-paste. Run from the engagement root (the folder containing both <code>scriban/</code> and <code>reports/</code>).</p> </blockquote> <p><strong>Prereqs:</strong></p> <pre><code class="language-bash">test -d scriban || { echo "scriban source missing"; exit 1; } ( command -v dotnet >/dev/null &#x26;&#x26; dotnet --list-sdks | grep -q '^10\.' ) \ || ( "$HOME/.dotnet/dotnet" --list-sdks | grep -q '^10\.' ) \ || { echo ".NET 10 SDK missing"; exit 1; } export PATH="$HOME/.dotnet:$PATH" </code></pre> <p><strong>Run both PoCs (native):</strong></p> <pre><code class="language-bash">( cd reports/f002/poc &#x26;&#x26; dotnet run -c Release ) # (A) public-setter mass assignment ( cd reports/f007/poc &#x26;&#x26; dotnet run -c Release ) # (B) private/internal/init bypass </code></pre> <p><strong>Docker fallback (no native SDK required):</strong></p> <pre><code class="language-bash">docker run --rm -v "$PWD":/work -w /work/reports/f007/poc \ mcr.microsoft.com/dotnet/sdk:10.0 bash -lc "dotnet run -c Release" </code></pre> <p><strong>Confirm the published package is affected (not just master):</strong> swap the <code>ProjectReference</code> in <code>reports/f007/poc/poc.csproj</code> for <code>&#x3C;PackageReference Include="Scriban" Version="7.2.1" /></code> and re-run — the four bypasses still succeed.</p> <p>Each PoC prints <code>[1]</code> original CLR values, <code>[2]</code> template output (reads originals → writes → reads back), and <code>[3]</code> the <strong>C#-side</strong> read after <code>Render()</code> proving the live host object was permanently altered.</p> <h2 data-heading="Remediation">Remediation</h2> <p>Fixes are listed flat. Note that (B) has a clean, clearly-correct code fix; (A) requires a <em>new control</em> because public-setter writes are otherwise by-design.</p> <ul> <li><strong>Fix 1 — block restricted setters in <code>TrySetValue</code> (<code>TypedObjectAccessor.cs</code> L108–L123). Fixes (B).</strong> Before the L120 <code>SetValue</code>, require a public, non-<code>init</code> setter: <pre><code class="language-csharp">var setM = propertyAccessor.GetSetMethod(nonPublic: false); if (setM is null) return false; // private / internal / protected setters if (setM.ReturnParameter.GetRequiredCustomModifiers() .Any(m => m.FullName == "System.Runtime.CompilerServices.IsExternalInit")) return false; // init-only: setter IS public, so the IsExternalInit check is REQUIRED </code></pre> A plain <code>GetSetMethod(

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