Ruby Security

The Ruby programming language is large and complex and there are many security pitfalls often encountered by newcomers and experienced Rubyists alike.

This document aims to discuss many of these pitfalls and provide more secure alternatives where applicable.

Please check the full list of publicly known CVEs and how to correctly report a security vulnerability, at: www.ruby-lang.org/en/security/ Japanese version is here: www.ruby-lang.org/ja/security/

Security vulnerabilities should be reported via an email to security@ruby-lang.org (the PGP public key), which is a private mailing list. Reported problems will be published after fixes.

Marshal.load

Ruby’s Marshal module provides methods for serializing and deserializing Ruby object trees to and from a binary data format.

Never use Marshal.load to deserialize untrusted or user supplied data. Because Marshal can deserialize to almost any Ruby object and has full control over instance variables, it is possible to craft a malicious payload that executes code shortly after deserialization.

If you need to deserialize untrusted data, you should use JSON as it is only capable of returning ‘primitive’ types such as strings, arrays, hashes, numbers and nil. If you need to deserialize other classes, you should handle this manually. Never deserialize to a user specified class.

YAML

YAML is a popular human readable data serialization format used by many Ruby programs for configuration and database persistence of Ruby object trees.

Similar to Marshal, it is able to deserialize into arbitrary Ruby classes. For example, the following YAML data will create an ERB object when deserialized:

!ruby/object:ERB
src: puts `uname`

Because of this, many of the security considerations applying to Marshal are also applicable to YAML. Do not use YAML to deserialize untrusted data.

Symbols

Symbols are often seen as syntax sugar for simple strings, but they play a much more crucial role. The MRI Ruby implementation uses Symbols internally for method, variable and constant names. The reason for this is that symbols are simply integers with names attached to them, so they are faster to look up in hashtables.

Starting in version 2.2, most symbols can be garbage collected; these are called mortal symbols. Most symbols you create (e.g. by calling to_sym) are mortal.

Immortal symbols on the other hand will never be garbage collected. They are created when modifying code:

• defining a method (e.g. with define_method),

• setting an instance variable (e.g. with instance_variable_set),

• creating a variable or constant (e.g. with const_set)

C extensions that have not been updated and are still calling SYM2ID will create immortal symbols. Bugs in 2.2.0: send and __send__ also created immortal symbols, and calling methods with keyword arguments could also create some.

Don’t create immortal symbols from user inputs. Otherwise, this would allow a user to mount a denial of service attack against your application by flooding it with unique strings, which will cause memory to grow indefinitely until the Ruby process is killed or causes the system to slow to a halt.

While it might not be a good idea to call these with user inputs, methods that used to be vulnerable such as to_sym, respond_to?, method, instance_variable_get, const_get, etc. are no longer a threat.

Regular expressions

Ruby’s regular expression syntax has some minor differences when compared to other languages. In Ruby, the ^ and $ anchors do not refer to the beginning and end of the string, rather the beginning and end of a line.

This means that if you’re using a regular expression like /^[a-z]+$/ to restrict a string to only letters, an attacker can bypass this check by passing a string containing a letter, then a newline, then any string of their choosing.

If you want to match the beginning and end of the entire string in Ruby, use the anchors \A and \z.

eval

Never pass untrusted or user controlled input to eval.

Unless you are implementing a REPL like irb or pry, eval is almost certainly not what you want. Do not attempt to filter user input before passing it to eval - this approach is fraught with danger and will most likely open your application up to a serious remote code execution vulnerability.

send

‘Global functions’ in Ruby (puts, exit, etc.) are actually private instance methods on Object. This means it is possible to invoke these methods with send, even if the call to send has an explicit receiver.

For example, the following code snippet writes “Hello world” to the terminal:

1.send(:puts, "Hello world")

You should never call send with user supplied input as the first parameter. Doing so can introduce a denial of service vulnerability:

foo.send(params[:bar]) # params[:bar] is "exit!"

If an attacker can control the first two arguments to send, remote code execution is possible:

# params is { :a => "eval", :b => "...ruby code to be executed..." }
foo.send(params[:a], params[:b])

When dispatching a method call based on user input, carefully verify that the method name. If possible, check it against a whitelist of safe method names.

Note that the use of public_send is also dangerous, as send itself is public:

1.public_send("send", "eval", "...ruby code to be executed...")

DRb

As DRb allows remote clients to invoke arbitrary methods, it is not suitable to expose to untrusted clients.

When using DRb, try to avoid exposing it over the network if possible. If this isn’t possible and you need to expose DRb to the world, you must configure an appropriate security policy with DRb::ACL.