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Asymmetric denial of service - ReDoS

Description​

Within the source code there is evidence of the use of dangerous regular expressions, because they make use of complex operations to find matches, which can lead to an attacker sending a specific string of data that could cause the server to crash when evaluating the string with the dangerous regular expression.

Impact​

Generate server crash.

Recommendation​

Use optimized regular expressions to perform functions without much computational overhead.

Threat​

Anonymous attacker from the Internet.

Expected Remediation Time​

⌚ 60 minutes.

Score​

Default score using CVSS 3.1. It may change depending on the context of the vulnerability.

Base​

  • Attack vector: N
  • Attack complexity: L
  • Privileges required: N
  • User interaction: N
  • Scope: U
  • Confidentiality: N
  • Integrity: N
  • Availability: L

Temporal​

  • Exploit code madurity: U
  • Remediation level: X
  • Report confidence: R

Result​

  • Vector string: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L/E:U/RL:X/RC:R
  • Score:
    • Base: 5.3
    • Temporal: 4.7
  • Severity:
    • Base: Medium
    • Temporal: Medium

Code Examples​

Compliant code​

The application simplifies the regular expressions used to compare input strings

using System.Text.RegularExpressions;
//use standard regex to verify and sanitize input
namespace Controllers {
  public class Controller : Controller {
    public IActionResult Validate(string regex, string input) {
      var expreg = ^\+[0-9]{11};
      Regex reg = new Regex(pat, RegexOptions.IgnoreCase);
      bool match = reg.IsMatch(input, reg.Escape(regex));
    }
  }
}

Non compliant code​

Complex regular expressions can cause a server overload

using System.Text.RegularExpressions;
namespace Controllers {
  public class Controller : Controller {
    public IActionResult Validate(string regex, string input) {
      var expreg = "^(http|https|ftp):[\/]{2}([a-zA-Z0-9\-\.]+\.[a-zA-Z]{2,4})(:[0-9]+)?\/?([a-zA-Z0-9\-\._\?\,\'\/\\+&%\$#\=~]*);
      Regex reg = new Regex(pat, RegexOptions.IgnoreCase);
      bool match2 = reg.IsMatch(input, expreg);
    }
  }
}

Example where the application uses unverified input to search in regex

using System;
using System.Text.RegularExpressions;
public partial class WebForm : System.Web.UI.Page{
  protected void Page_Load(object sender, EventArgs e){
    string findTerm = Request.Form["findTerm"];
    Match m = Regex.Match(SearchableText, "^term=" + findTerm);
  }
}

Requirements​

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