GHSA-xcx6-vp38-8hr5: Scriban Scriban Has Uncontrolled Recursion Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-xcx6-vp38-8hr5",
  "modified" : "2026-07-06T13:09:17Z",
  "published" : "2026-03-24T22:15:13Z",
  "aliases" : [ ],
  "summary" : "Scriban has Uncontrolled Recursion in `object.to_json` Causing Unrecoverable Process Crash via StackOverflowException",
  "details" : "## Summary\n\nThe `object.to_json` builtin function in Scriban performs recursive JSON serialization via an internal `WriteValue()` static local function that has no depth limit, no circular reference detection, and no stack overflow guard. A Scriban template containing a self-referencing object passed to `object.to_json` triggers unbounded recursion, causing a `StackOverflowException` that terminates the hosting .NET process. This is a fatal, unrecoverable crash — `StackOverflowException` cannot be caught by user code in .NET.\n\n## Details\n\nThe vulnerable code is the `WriteValue()` static local function at `src/Scriban/Functions/ObjectFunctions.cs:494`:\n\n```csharp\nstatic void WriteValue(TemplateContext context, Utf8JsonWriter writer, object value)\n{\n    var type = value?.GetType() ?? typeof(object);\n    if (value is null || value is string || value is bool ||\n        type.IsPrimitiveOrDecimal() || value is IFormattable)\n    {\n        JsonSerializer.Serialize(writer, value, type);\n    }\n    else if (value is IList || type.IsArray) {\n        writer.WriteStartArray();\n        foreach (var x in context.ToList(context.CurrentSpan, value))\n        {\n            WriteValue(context, writer, x);  // recursive, no depth check\n        }\n        writer.WriteEndArray();\n    }\n    else {\n        writer.WriteStartObject();\n        var accessor = context.GetMemberAccessor(value);\n        foreach (var member in accessor.GetMembers(context, context.CurrentSpan, value))\n        {\n            if (accessor.TryGetValue(context, context.CurrentSpan, value, member, out var memberValue))\n            {\n                writer.WritePropertyName(member);\n                WriteValue(context, writer, memberValue);  // recursive, no depth check\n            }\n        }\n        writer.WriteEndObject();\n    }\n}\n```\n\nThis function has **none** of the safety mechanisms present in other recursive paths:\n\n- `ObjectToString()` at `TemplateContext.Helpers.cs:98` checks `ObjectRecursionLimit` (default 20)\n- `EnterRecursive()` at `TemplateContext.cs:957` calls `RuntimeHelpers.EnsureSufficientExecutionStack()`\n- `CheckAbort()` at `TemplateContext.cs:464` also calls `EnsureSufficientExecutionStack()`\n\nThe `WriteValue()` function bypasses all of these because it is a static local function that only takes the `TemplateContext` for member access — it never calls `EnterRecursive()`, never checks `ObjectRecursionLimit`, and never calls `EnsureSufficientExecutionStack()`.\n\n**Execution flow:**\n\n1. Template creates a ScriptObject: `{{ x = {} }}`\n2. Sets a self-reference: `x.self = x` — stores a reference in `ScriptObject.Store` dictionary\n3. Pipes to `object.to_json`: `x | object.to_json` → calls `ToJson()` at line 477\n4. `ToJson()` calls `WriteValue(context, writer, value)` at line 488\n5. `WriteValue` enters the `else` branch (line 515), gets members via accessor, finds \"self\"\n6. `TryGetValue` returns `x` itself, `WriteValue` recurses with the same object — infinite loop\n7. `StackOverflowException` is thrown — **fatal, cannot be caught, process terminates**\n\n## PoC\n\n```scriban\n{{ x = {}; x.self = x; x | object.to_json }}\n```\n\nIn a hosting application:\n\n```csharp\nusing Scriban;\n\n// This will crash the entire process with StackOverflowException\nvar template = Template.Parse(\"{{ x = {}; x.self = x; x | object.to_json }}\");\nvar result = template.Render(); // FATAL: process terminates here\n```\n\nEven without circular references, deeply nested objects can exhaust the stack since no depth limit is enforced:\n\n```scriban\n{{ a = {}\n   b = {inner: a}\n   c = {inner: b}\n   d = {inner: c}\n   # ... continue nesting ...\n   result = deepest | object.to_json }}\n```\n\n## Impact\n\n- **Process crash DoS**: Any application embedding Scriban for user-provided templates (CMS platforms, email template engines, report generators, static site generators) can be crashed by a single malicious template. The crash is unrecoverable — `StackOverflowException` terminates the .NET process.\n- **No try/catch protection possible**: Unlike most exceptions, `StackOverflowException` cannot be caught by application code. The hosting application cannot wrap `template.Render()` in a try/catch to survive this.\n- **No authentication required**: `object.to_json` is a default builtin function (registered in `BuiltinFunctions.cs`), available in all Scriban templates unless explicitly removed.\n- **Trivial to exploit**: The PoC is a single line of template code.\n\n## Recommended Fix\n\nAdd a depth counter parameter to `WriteValue()` and check it against `ObjectRecursionLimit`, consistent with how `ObjectToString` is protected. Also add `EnsureSufficientExecutionStack()` as a safety net:\n\n```csharp\nstatic void WriteValue(TemplateContext context, Utf8JsonWriter writer, object value, int depth = 0)\n{\n    if (context.ObjectRecursionLimit != 0 && depth > context.ObjectRecursionLimit)\n    {\n        throw new ScriptRuntimeException(context.CurrentSpan,\n            $\"Exceeding object recursion limit `{context.ObjectRecursionLimit}` in object.to_json\");\n    }\n\n    try\n    {\n        RuntimeHelpers.EnsureSufficientExecutionStack();\n    }\n    catch (InsufficientExecutionStackException)\n    {\n        throw new ScriptRuntimeException(context.CurrentSpan,\n            \"Exceeding recursive depth limit in object.to_json, near to stack overflow\");\n    }\n\n    var type = value?.GetType() ?? typeof(object);\n    if (value is null || value is string || value is bool ||\n        type.IsPrimitiveOrDecimal() || value is IFormattable)\n    {\n        JsonSerializer.Serialize(writer, value, type);\n    }\n    else if (value is IList || type.IsArray) {\n        writer.WriteStartArray();\n        foreach (var x in context.ToList(context.CurrentSpan, value))\n        {\n            WriteValue(context, writer, x, depth + 1);\n        }\n        writer.WriteEndArray();\n    }\n    else {\n        writer.WriteStartObject();\n        var accessor = context.GetMemberAccessor(value);\n        foreach (var member in accessor.GetMembers(context, context.CurrentSpan, value))\n        {\n            if (accessor.TryGetValue(context, context.CurrentSpan, value, member, out var memberValue))\n            {\n                writer.WritePropertyName(member);\n                WriteValue(context, writer, memberValue, depth + 1);\n            }\n        }\n        writer.WriteEndObject();\n    }\n}\n```",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/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-xcx6-vp38-8hr5"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/scriban/scriban"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-674" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-24T22:15:13Z",
    "nvd_published_at" : null
  }
}