Possible RCE escalation bug with Serialized Columns in Active Record

There is a possible escalation to RCE when using YAML serialized columns in
Active Record. This vulnerability has been assigned the CVE identifier
CVE-2022-32224.

Versions Affected: All.
Not affected: None
Fixed Versions: 7.0.3.1, 6.1.6.1, 6.0.5.1, 5.2.8.1

Impact

When serialized columns that use YAML (the default) are deserialized, Rails
uses YAML.unsafe_load to convert the YAML data in to Ruby objects. If an
attacker can manipulate data in the database (via means like SQL injection),
then it may be possible for the attacker to escalate to an RCE.

Impacted Active Record models will look something like this:

class User < ApplicationRecord
  serialize :options       # Vulnerable: Uses YAML for serialization
  serialize :values, Array # Vulnerable: Uses YAML for serialization
  serialize :values, JSON  # Not vulnerable
end

All users running an affected release should either upgrade or use one of the
workarounds immediately.

Releases

The FIXED releases are available at the normal locations.

The released versions change the default YAML deserializer to use
YAML.safe_load, which prevents deserialization of possibly dangerous
objects. This may introduce backwards compatibility issues with existing
data.

In order to cope with that situation, the released version also contains two
new Active Record configuration options. The configuration options are as
follows:

• config.active_record.use_yaml_unsafe_load

When set to true, this configuration option tells Rails to use the old
“unsafe” YAML loading strategy, maintaining the existing behavior but leaving
the possible escalation vulnerability in place. Setting this option to true
is not recommended, but can aid in upgrading.

• config.active_record.yaml_column_permitted_classes

The “safe YAML” loading method does not allow all classes to be deserialized
by default. This option allows you to specify classes deemed “safe” in your
application. For example, if your application uses Symbol and Time in
serialized data, you can add Symbol and Time to the allowed list as follows:

config.active_record.yaml_column_permitted_classes = [Symbol, Date, Time]

Workarounds

There are no feasible workarounds for this issue, but other coders (such as
JSON) are not impacted.