Nokogiri::XML::Schema trusts input by default, exposing risk of an XXE vulnerability

Description

In Nokogiri versions <= 1.11.0.rc3, XML Schemas parsed by Nokogiri::XML::Schema
are trusted by default, allowing external resources to be accessed over the
network, potentially enabling XXE or SSRF attacks.

This behavior is counter to
the security policy followed by Nokogiri maintainers, which is to treat all input
as untrusted by default whenever possible.

Please note that this security
fix was pushed into a new minor version, 1.11.x, rather than a patch release to
the 1.10.x branch, because it is a breaking change for some schemas and the risk
was assessed to be “Low Severity”.

Affected Versions

Nokogiri <= 1.10.10 as well as prereleases 1.11.0.rc1, 1.11.0.rc2, and 1.11.0.rc3

Mitigation

There are no known workarounds for affected versions. Upgrade to Nokogiri
1.11.0.rc4 or later.

If, after upgrading to 1.11.0.rc4 or later, you wish
to re-enable network access for resolution of external resources (i.e., return to
the previous behavior):

1. Ensure the input is trusted. Do not enable this option
   for untrusted input.
2. When invoking the Nokogiri::XML::Schema constructor,
   pass as the second parameter an instance of Nokogiri::XML::ParseOptions with the
   NONET flag turned off.

So if your previous code was:

# in v1.11.0.rc3 and earlier, this call allows resources to be accessed over the network
# but in v1.11.0.rc4 and later, this call will disallow network access for external resources
schema = Nokogiri::XML::Schema.new(schema)

# in v1.11.0.rc4 and later, the following is equivalent to the code above
# (the second parameter is optional, and this demonstrates its default value)
schema = Nokogiri::XML::Schema.new(schema, Nokogiri::XML::ParseOptions::DEFAULT_SCHEMA)

Then you can add the second parameter to indicate that the input is trusted by changing it to:

# in v1.11.0.rc3 and earlier, this would raise an ArgumentError
# but in v1.11.0.rc4 and later, this allows resources to be accessed over the network
schema = Nokogiri::XML::Schema.new(trusted_schema, Nokogiri::XML::ParseOptions.new.nononet)