There is a strong parameters bypass vector in ActionPack.

Versions Affected: rails <= 6.0.3
Not affected: rails < 5.0.0
Fixed Versions: rails >= 5.2.4.3, rails >= 6.0.3.1

Impact

In some cases user supplied information can be inadvertently leaked from
Strong Parameters. Specifically the return value of each, or each_value,
or each_pair will return the underlying "untrusted" hash of data that was
read from the parameters. Applications that use this return value may be
inadvertently use untrusted user input.

Impacted code will look something like this:

def update
  # Attacker has included the parameter: `{ is_admin: true }`
  User.update(clean_up_params)
end

def clean_up_params
   params.each { |k, v|  SomeModel.check(v) if k == :name }
end

Note the mistaken use of each in the clean_up_params method in the above
example.

Workarounds

Do not use the return values of each, each_value, or each_pair in your
application.

References

• https://nvd.nist.gov/vuln/detail/CVE-2020-8164
• https://github.com/rubysec/ruby-advisory-db/blob/master/gems/actionpack/CVE-2020-8164.yml
• https://groups.google.com/forum/#!topic/rubyonrails-security/f6ioe4sdpbY
• https://hackerone.com/reports/292797
• https://groups.google.com/g/rubyonrails-security/c/f6ioe4sdpbY
• https://lists.debian.org/debian-lts-announce/2020/06/msg00022.html
• https://lists.debian.org/debian-lts-announce/2020/07/msg00013.html
• https://www.debian.org/security/2020/dsa-4766
• http://lists.opensuse.org/opensuse-security-announce/2020-09/msg00089.html
• http://lists.opensuse.org/opensuse-security-announce/2020-09/msg00093.html
• http://lists.opensuse.org/opensuse-security-announce/2020-09/msg00107.html