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 [email protected] (the PGP public key), which is a private mailing list. Reported problems will be published after fixes.

$SAFE

Ruby provides a mechanism to restrict what operations can be performed by Ruby code in the form of the $SAFE variable.

However, $SAFE does not provide a secure environment for executing untrusted code.

If you need to execute untrusted code, you should use an operating system level sandboxing mechanism. On Linux, ptrace or LXC can be used to sandbox potentially malicious code. Other similar mechanisms exist on every major operating system.

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.

Once a symbol is created, the memory used by it is never freed. If you convert user input to symbols with to_sym or intern, it is possible for an attacker to mount a denial of service attack against your application by flooding it with unique strings. Because each string is kept in memory until the Ruby process exits, this will cause memory consumption to grow and grow until Ruby runs out of memory and crashes.

Be careful with passing user input to methods such as send, instance_variable_get or _set, const_get or _set, etc. as these methods will convert string parameters to symbols internally and pose the same DoS potential as direct conversion through to_sym/intern.

The workaround to this is simple - don't convert user input to symbols. You should attempt to leave user input in string form instead.

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.