GHSA-xw6w-9jjh-p9cr: Scriban Remote Code Execution Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-xw6w-9jjh-p9cr",
  "modified" : "2026-07-06T13:10:09Z",
  "published" : "2026-03-24T22:16:01Z",
  "aliases" : [ ],
  "summary" : "Scriban has Multiple Denial-of-Service Vectors via Unbounded Resource Consumption During Expression Evaluation",
  "details" : "## Summary\n\nScriban'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.\n\n## Details\n\n### Vector 1: Unbounded string multiplication\n\nIn `ScriptBinaryExpression.cs`, the `CalculateToString` method handles the `string * int` operator by looping without any upper bound:\n\n```csharp\n// src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs:319-334\nvar leftText = context.ObjectToString(left);\nvar builder = new StringBuilder();\nfor (int i = 0; i < value; i++)\n{\n    builder.Append(leftText);\n}\nreturn builder.ToString();\n```\n\nThe `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.\n\n### Vector 2: Unbounded BigInteger shift left\n\nThe `CalculateLongWithInt` and `CalculateBigIntegerNoFit` methods handle `ShiftLeft` without any bound on the shift amount:\n\n```csharp\n// src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs:710-711\ncase ScriptBinaryOperator.ShiftLeft:\n    return (BigInteger)left << (int)right;\n```\n\n```csharp\n// src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs:783-784\ncase ScriptBinaryOperator.ShiftLeft:\n    return left << (int)right;\n```\n\nIn 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.\n\n### Vector 3: LoopLimit bypass via range enumeration in builtin functions\n\nThe range operators `..` and `..<` produce lazy `IEnumerable<object>` iterators:\n\n```csharp\n// src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs:401-417\nprivate static IEnumerable<object> RangeInclude(BigInteger left, BigInteger right)\n{\n    if (left < right)\n    {\n        for (var i = left; i <= right; i++)\n        {\n            yield return FitToBestInteger(i);\n        }\n    }\n    // ...\n}\n```\n\nWhen 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:\n\n- `ArrayFunctions.Size` (line 609) calls `.Cast<object>().Count()`, fully enumerating the range\n- `ArrayFunctions.Join` (line 388) iterates with `foreach` and appends to a `StringBuilder` with no size limit\n\n## PoC\n\n### Vector 1 — String multiplication OOM:\n```csharp\nvar template = Template.Parse(\"{{ 'AAAA' * 500000000 }}\");\nvar context = new TemplateContext();\n// context.LimitToString is 1048576 by default — does NOT protect this path\ntemplate.Render(context); // OutOfMemoryException: attempts ~2GB allocation\n```\n\n### Vector 2 — BigInteger shift OOM:\n```csharp\nvar template = Template.Parse(\"{{ 1 << 100000000 }}\");\nvar context = new TemplateContext();\ntemplate.Render(context); // Allocates BigInteger with 100M bits (~12.5MB)\n// {{ 1 << 2000000000 }} attempts ~250MB\n```\n\n### Vector 3 — LoopLimit bypass via range + builtin:\n```csharp\nvar template = Template.Parse(\"{{ (0..1000000000) | array.size }}\");\nvar context = new TemplateContext();\n// context.LoopLimit is 1000 — does NOT protect builtin function iteration\ntemplate.Render(context); // CPU exhaustion: enumerates 1 billion items\n```\n\n```csharp\nvar template = Template.Parse(\"{{ (0..10000000) | array.join ',' }}\");\nvar context = new TemplateContext();\ntemplate.Render(context); // Memory exhaustion: builds ~80MB+ joined string\n```\n\n## Impact\n\nAn 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:\n\n1. 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.\n2. A single malicious template expression is sufficient — no complex template logic is required.\n3. The `OutOfMemoryException` in vectors 1 and 2 typically terminates the entire process, not just the template evaluation.\n\n## Recommended Fix\n\n### Vector 1 — String multiplication: Check `LimitToString` before the loop\n\n```csharp\n// src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs, before line 330\nvar leftText = context.ObjectToString(left);\nif (context.LimitToString > 0 && (long)value * leftText.Length > context.LimitToString)\n{\n    throw new ScriptRuntimeException(span,\n        $\"String multiplication would exceed LimitToString ({context.LimitToString} characters)\");\n}\nvar builder = new StringBuilder();\nfor (int i = 0; i < value; i++)\n```\n\n### Vector 2 — BigInteger shift: Cap the shift amount\n\n```csharp\n// src/Scriban/Syntax/Expressions/ScriptBinaryExpression.cs, lines 710-711 and 783-784\ncase ScriptBinaryOperator.ShiftLeft:\n    if (right > 1048576) // Same as MaxBigInteger bit count\n        throw new ScriptRuntimeException(span,\n            $\"Shift amount {right} exceeds maximum allowed (1048576)\");\n    return (BigInteger)left << (int)right;\n```\n\n### Vector 3 — Range + builtins: Add iteration counting to range iterators\n\nPass `TemplateContext` to `RangeInclude`/`RangeExclude` and enforce a limit:\n\n```csharp\nprivate static IEnumerable<object> RangeInclude(TemplateContext context, BigInteger left, BigInteger right)\n{\n    var maxRange = context.LoopLimit > 0 ? context.LoopLimit : int.MaxValue;\n    int count = 0;\n    if (left < right)\n    {\n        for (var i = left; i <= right; i++)\n        {\n            if (++count > maxRange)\n                throw new ScriptRuntimeException(context.CurrentNode.Span,\n                    $\"Range enumeration exceeds LoopLimit ({maxRange})\");\n            yield return FitToBestInteger(i);\n        }\n    }\n    // ... same for descending branch\n}\n```\n\nAlternatively, validate range size eagerly at creation time: `if (BigInteger.Abs(right - left) > maxRange) throw ...`",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "NuGet",
      "name" : "Scriban"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "7.0.0"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "NuGet",
      "name" : "Scriban.Signed"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "7.0.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/scriban/scriban/security/advisories/GHSA-xw6w-9jjh-p9cr"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/scriban/scriban"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-24T22:16:01Z",
    "nvd_published_at" : null
  }
}