CVE-2019-16676: Input Validation

Incorrect Access Control in filemethod? in lib/simpleform/formbuilder.rb; a user-supplied string is invoked as a method call

Impact For pages that build a form using user input, it is possible for an attacker to manipulate the input and send any method present in the form object. For example:

erb <%= simpleformfor @user do |f| %> <%= f.label @usersuppliedstring %> ... <% end %>

The string provided in the variable @usersuppliedstring would be invoked as a method call inside the @user object (unless the string contains any of the following: password, timezone, country, email, phone and url).

By manipulation that input, an attacker could do any of the following:

- Code execution (call actions like #destroy) - Denial of Service (by executing a computation intensive method) - Information Disclosure (check the presence of methods, leak user information)

Patches The problem was fixed in version 5.0. Although it's a major version, there should be no issues with upgrading for 4.x. The reason it was released in a major version is that the configuration SimpleForm.filemethods was deprecated in order to fix the problem.

Workarounds The issue only happens with pages that build forms based on user-provided input. If your application doesn't do that, you're not affected. A workaround is to explicitly pass which type you want for an input since the issue lies on Simple Form's automatically discovery of input types. This can be done using the as option, like the following: erb <%= form.input :avatar, as: :file %>

References [TDB]

For more information If you have any questions or comments about this advisory: Open an issue in https://github.com/plataformatec/simpleform Email us at opensource@plataformatec.com.br