Scriban has Uncontrolled Recursion in Parser Leads to Stack Overflow and Process Crash (Denial of Service)

Scriban is vulnerable to an uncontrolled process crash resulting in a Denial of Service. Because the recursive-descent parser does not enforce a default limit on expression depth, an attacker who controls template input can craft a heavily nested template that triggers a StackOverflowException. In .NET, a StackOverflowException cannot be caught by standard try-catch blocks, resulting in the immediate and ungraceful termination of the entire hosting process.

Scriban utilizes a recursive-descent parser to process template expressions. While the library exposes an ExpressionDepthLimit property in its ParserOptions, this property defaults to null (disabled).

If an application accepts user-supplied templates (or dynamically constructs templates from untrusted input), an attacker can supply thousands of nested parentheses or blocks. As the parser recursively evaluates each nested layer, it consumes thread stack space until it exceeds the limits of the host OS, triggering a fatal crash.

Impact

An attacker can supply crafted input that triggers a StackOverflowException, causing immediate termination of the hosting process and resulting in a Denial of Service. In applications that process untrusted or user-controlled templates (e.g., web applications or APIs), this can be exploited remotely without authentication. The failure is not recoverable, requiring a full process restart and leading to service disruption.

Proof of Concept (PoC)

The following C# code demonstrates the vulnerability. Executing this code will immediately terminate the application process.

using Scriban;

// Creates a deeply nested expression: (((( ... (1) ... ))))
string nested = new string('(', 10000) + "1" + new string(')', 10000);

try {
  // This will crash the entire process immediately
  Scriban.Template.Parse("{{ " + nested + " }}");
} catch (Exception ex) {
  // This catch block will never execute because StackOverflowException
  Console.WriteLine("Caught exception: " + ex.Message);
}

Suggested Remediation

Update the ParserOptions constructor (or the internal parser initialization) to set a default value for ExpressionDepthLimit. A limit of 1000 (or even lower, such as 250 or 500) is generally more than enough for legitimate templates while safely preventing stack exhaustion.

public int? ExpressionDepthLimit { get; set; } = 250;

Alternatively, document the risk heavily and warn developers to manually set ExpressionDepthLimit if evaluating untrusted templates, though a secure-by-default approach is strongly preferred.