GHSA-v66j-x4hw-fv9g Scriban: Uncontrolled Memory Allocation via string.pad_left/pad_right Allows Remote Denial of Service
SummaryThe 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: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 authenticationTemplate size limit of 1KB (line 71) — the payload fits in 39 bytesA 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)PoCSingle request to crash or degrade the AppService: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:The `string.pad_right` variant works identically: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 FixAdd 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: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.
CVSS v3.1 score: 7.5