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The SafeYAML gem provides an alternative implementation of

suitable for accepting user input in Ruby applications. Unlike Ruby's built-in implementation of
, SafeYAML's version will not expose apps to arbitrary code execution exploits (such as the ones discovered in Rails in early 2013).

If you encounter any issues with SafeYAML, check out the 'Common Issues' section below. If you don't see anything that addresses the problem you're experiencing, by all means, create an issue!


Add this line to your application's Gemfile:

gem "safe_yaml"


If all you do is add SafeYAML to your project, then

will operate in "safe" mode, which means it won't deserialize arbitrary objects. However, it will issue a warning the first time you call it because you haven't explicitly specified whether you want safe or unsafe behavior by default. To specify this behavior (e.g., in a Rails initializer):
SafeYAML::OPTIONS[:default_mode] = :safe # or :unsafe

Another important option you might want to specify on startup is whether or not to allow symbols to be deserialized. The default setting is

, since symbols are not garbage collected in Ruby and so deserializing them from YAML may render your application vulnerable to a DOS (denial of service) attack. To allow symbol deserialization by default:
SafeYAML::OPTIONS[:deserialize_symbols] = true

For more information on these and other options, see the "Usage" section down below.

What is this gem for, exactly?

Suppose your application were to use a popular open source library which contained code like this:

class ClassBuilder
  def []=(key, value)
    @class ||=

@class.class_eval <

Now, if you were to use

on user input anywhere in your application without the SafeYAML gem installed, an attacker who suspected you were using this library could send a request with a carefully-crafted YAML string to execute arbitrary code (yes, including
system("unix command")
) on your servers.

This simple example demonstrates the vulnerability:

yaml = <
> YAML.load(yaml)
I'm in yr system!
=> #<0x007fdbbe2e25d8>>

With SafeYAML, the same attacker would be thwarted:

> require "safe_yaml"
=> true
> YAML.load(yaml, :safe => true)
=> {"foo; end; puts %(I'm in yr system!); def bar"=>"baz"}


When you require the safe_yaml gem in your project,

is patched to accept one additional (optional)
parameter. This changes the method signature as follows:

  • for Syck and Psych prior to Ruby 1.9.3:
    YAML.load(yaml, options={})
  • for Psych in 1.9.3 and later:
    YAML.load(yaml, filename=nil, options={})

The most important option is the

option (default:
), which controls whether or not to deserialize arbitrary objects when parsing a YAML document. The other options, along with explanations, are as follows.
  • :deserialize_symbols
    ): Controls whether or not YAML will deserialize symbols. It is probably best to only enable this option where necessary, e.g. to make trusted libraries work. Symbols receive special treatment in Ruby and are not garbage collected, which means deserializing them indiscriminately may render your site vulnerable to a DOS attack.
  • :whitelisted_tags
    : Accepts an array of YAML tags that designate trusted types, e.g., ones that can be deserialized without worrying about any resulting security vulnerabilities. When any of the given tags are encountered in a YAML document, the associated data will be parsed by the underlying YAML engine (Syck or Psych) for the version of Ruby you are using. See the "Whitelisting Trusted Types" section below for more information.
  • :custom_initializers
    : Similar to the
    option, but allows you to provide your own initializers for specified tags rather than using Syck or Psyck. Accepts a hash with string tags for keys and lambdas for values.
  • :raise_on_unknown_tag
    ): Represents the highest possible level of paranoia. If the YAML engine encounters any tag other than ones that are automatically trusted by SafeYAML or that you've explicitly whitelisted, it will raise an exception. This may be a good choice if you expect to always be dealing with perfectly safe YAML and want your application to fail loudly upon encountering questionable data.

All of the above options can be set at the global level via

. You can also set each one individually per call to
; an option explicitly passed to
will take precedence over an option specified globally.

What if I don't want to patch

Excellent question! You can also get the methods

without touching the
module at all like this:
require "safe_yaml/load" # instead of require "safe_yaml"

This way, you can use

to parse YAML that you don't trust, without affecting the rest of an application (if you're developing a library, for example).

Supported Types

The way that SafeYAML works is by restricting the kinds of objects that can be deserialized via

. More specifically, only the following types of objects can be deserialized by default:
  • Hashes
  • Arrays
  • Strings
  • Numbers
  • Dates
  • Times
  • Booleans
  • Nils

Again, deserialization of symbols can be enabled globally by setting

SafeYAML::OPTIONS[:deserialize_symbols] = true
, or in a specific call to
YAML.load([some yaml], :deserialize_symbols => true)

Whitelisting Trusted Types

SafeYAML supports whitelisting certain YAML tags for trusted types. This is handy when your application uses YAML to serialize and deserialize certain types not listed above, which you know to be free of any deserialization-related vulnerabilities.

The easiest way to whitelist types is by calling

, which can accept a variable number of safe types, e.g.:
SafeYAML.whitelist!(Foo, Bar)

You can also whitelist YAML tags via the

# Using Syck
SafeYAML::OPTIONS[:whitelisted_tags] = [",2002:object:OpenStruct"]

Using Psych

SafeYAML::OPTIONS[:whitelisted_tags] = ["!ruby/object:OpenStruct"]

And in case you were wondering: no, this feature will not allow would-be attackers to embed untrusted types within trusted types:

yaml = <
> YAML.safe_load(yaml)
=> #"baz"}>

Known Issues

If you add SafeYAML to your project and start seeing any errors about missing keys, or you notice mysterious strings that look like

(i.e., start with a colon), it's likely you're seeing errors from symbols being saved in YAML format. If you are able to modify the offending code, you might want to consider changing your YAML content to use plain vanilla strings instead of symbols. If not, you may need to set the
option to
, either in calls to
or---as a last resort---globally, with

Also be aware that some Ruby libraries, particularly those requiring inter-process communication, leverage YAML's object deserialization functionality and therefore may break or otherwise be impacted by SafeYAML. The following list includes known instances of SafeYAML's interaction with other Ruby gems:

  • ActiveRecord: uses YAML to control serialization of model objects using the
    class method. If you find that accessing serialized properties on your ActiveRecord models is causing errors, chances are you may need to:
    1. set the
      option to
    2. whitelist some of the types in your serialized data via
      or the
      option, or
    3. both
  • delayed_job: Uses YAML to serialize the objects on which delayed methods are invoked (with
    ). The safest solution in this case is to use
    to whitelist the types you need to serialize.
  • Guard: Uses YAML as a serialization format for notifications. The data serialized uses symbolic keys, so setting
    SafeYAML::OPTIONS[:deserialize_symbols] = true
    is necessary to allow Guard to work.
  • sidekiq: Uses a YAML configiuration file with symbolic keys, so setting
    SafeYAML::OPTIONS[:deserialize_symbols] = true
    should allow it to work.

The above list will grow over time, as more issues are discovered.


SafeYAML will follow semantic versioning so any updates to the first major version will maintain backwards compatability. So expect primarily bug fixes and feature enhancements (if anything!) from here on out... unless it makes sense to break the interface at some point and introduce a version 2.0, which I honestly think is unlikely.


SafeYAML requires Ruby 1.8.7 or newer and works with both Syck and Psych.

If you are using a version of Ruby where Psych is the default YAML engine (e.g., 1.9.3) but you want to use Syck, be sure to set

YAML::ENGINE.yamler = "syck"
before requiring the safe_yaml gem.

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