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Regular expression injection

ID: rb/regexp-injection
Kind: path-problem
Security severity: 7.5
Severity: error
Precision: high
Tags:
   - security
   - external/cwe/cwe-1333
   - external/cwe/cwe-730
   - external/cwe/cwe-400
Query suites:
   - ruby-code-scanning.qls
   - ruby-security-extended.qls
   - ruby-security-and-quality.qls

Click to see the query in the CodeQL repository

Constructing a regular expression with unsanitized user input is dangerous, since a malicious user may be able to modify the meaning of the expression. In particular, such a user may be able to provide a regular expression fragment that takes exponential time in the worst case, and use that to perform a Denial of Service attack.

Recommendation

Before embedding user input into a regular expression, use a sanitization function such as Regexp.escape to escape meta-characters that have special meaning.

Example

The following examples construct regular expressions from an HTTP request parameter without sanitizing it first:

class UsersController < ActionController::Base
  def first_example
    # BAD: Unsanitized user input is used to construct a regular expression
    regex = /#{ params[:key] }/
  end

  def second_example
    # BAD: Unsanitized user input is used to construct a regular expression
    regex = Regexp.new(params[:key])
  end
end

Instead, the request parameter should be sanitized first. This ensures that the user cannot insert characters that have special meanings in regular expressions.

class UsersController < ActionController::Base
  def example
    # GOOD: User input is sanitized before constructing the regular expression
    regex = Regexp.new(Regex.escape(params[:key]))
  end
end

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