Implementation of robust data validation mechanisms in the source code
import 'package:shelf/shelf.dart' as shelf;
shelf.Response handleRequest(shelf.Request request) {
var regexPattern = request.url.queryParameters['pattern'];
var inputString = request.url.queryParameters['input'];
var regex = RegExp(regexPattern);
if (regex.hasMatch(inputString)) {
return shelf.Response.ok('Match found');
} else {
return shelf.Response.ok('No match found');
}
}
In the provided Dart code, we are handling a request in a backend application that uses the Shelf framework. The function
handleRequest retrieves two query parameters from the request URL:
pattern and
input.
The
pattern parameter is used to create a regular expression (
RegExp) without any sanitization or validation. This
RegExp is then used to check if it matches the
input parameter.
The vulnerability here lies in the lack of data validation for the
pattern parameter. An attacker could provide a complex or malicious regular expression as the
pattern parameter. This could lead to a Regular Expression Denial of Service (ReDoS) attack, where the server could be forced to spend a significant amount of time processing the regular expression, potentially leading to a server crash.
For example, an attacker could provide a regular expression that results in catastrophic backtracking. This occurs when a regular expression has multiple paths it could take, but none lead to a successful match, forcing it to backtrack and try every possible combination, which can take a very long time for complex patterns.
This vulnerability could be mitigated by implementing data validation for the
pattern parameter to ensure it is a safe and valid regular expression before it is used.
import 'package:shelf/shelf.dart' as shelf;
import 'package:timeout/timeout.dart';
shelf.Response handleRequest(shelf.Request request) {
var regexPattern = request.url.queryParameters['pattern'];
var inputString = request.url.queryParameters['input'];
if (regexPattern == null || regexPattern.isEmpty || inputString == null || inputString.isEmpty) {
return shelf.Response.badRequest('Invalid input');
}
if (regexPattern.length > 100) { // Limit the complexity of the regex
return shelf.Response.badRequest('Pattern too complex');
}
try {
var regex = RegExp(regexPattern);
// Implement a timeout for the regex evaluation
var hasMatch = timeout(Duration(seconds: 2), () {
return regex.hasMatch(inputString);
});
if (hasMatch) {
return shelf.Response.ok('Match found');
} else {
return shelf.Response.ok('No match found');
}
} catch (e) {
return shelf.Response.internalServerError('Error processing request');
}
}
The updated code includes several measures to mitigate the risk of server crashes due to dangerous regular expressions:
1. Input Validation: The 'pattern' and 'input' query parameters are checked to ensure they are not null or empty. If either is null or empty, a 400 Bad Request response is returned.
2. Limiting Regex Complexity: The complexity of the regular expressions allowed is limited by restricting the length of the 'pattern' query parameter to 100 characters. If the 'pattern' is longer than this, a 400 Bad Request response is returned.
3. Timeout for Regex Evaluation: A timeout is implemented for the regular expression evaluation using the
timeout function from the
timeout package. This prevents long-running operations that could cause a server crash. If the regex evaluation takes longer than 2 seconds, the
timeout function will throw a
TimeoutException, which is caught and handled by returning a 500 Internal Server Error response.
4. Error Handling: Any exceptions thrown by the
RegExp constructor or the
hasMatch method are caught and handled by returning a 500 Internal Server Error response. This prevents the server from crashing if an invalid regular expression is provided.