XML-RPC: Prevent fatal error when system.multicall params are scalar

[tommusrhodus]

AI Description: Validates the per-call structure inside IXR_Server::multiCall() and returns a spec-compliant fault (-32602) for malformed entries, rather than passing non-array values to IXR_Server::call() where count() would TypeError on PHP 8+. Also adds a defensive guard inside call() mirroring the existing pattern in IXR_IntrospectionServer::call().

Human Discussion:

• Solves a 500 error being thrown in wp-includes/IXR/class-IXR-server.php, likely by malicious traffic.
• Expands upon the existing patch in 65124 to avoid creating issues downstream by simply checking $args is an array.
• Noting that pre PHP 8.0 this did not throw a fatal, but changes to count() mean that a fatal is now thown in 8.0+

Tested locally by creating an XML file with the following content:

<?xml version="1.0"?>
  <methodCall>
    <methodName>system.multicall</methodName>
    <params>
      <param><value><array><data>
        <value><struct>
          <member><name>methodName</name><value><string>system.listMethods</string></value></member>
          <member><name>params</name><value><string>malicious-non-array</string></value></member>
        </struct></value>
      </data></array></value></param>
    </params>
  </methodCall>

and then posting that to the local server:

curl -sS -X POST -H 'Content-Type: text/xml' \
    --data-binary @repro-ixr-multicall.xml \
    http://localhost:8889/xmlrpc.php -o /tmp/resp.xml -w '%{http_code}\n'

The return before this patch:

500
<?xml version="1.0" encoding="UTF-8"?>
<methodResponse>
  <fault>
    <value>
      <struct>
        <member>
          <name>faultCode</name>
          <value><int>500</int></value>
        </member>
        <member>
          <name>faultString</name>
          <value><string>&lt;p&gt;There has been a critical error on this website.&lt;/p&gt;&lt;p&gt;&lt;a href=&quot;https://wordpress.org/documentation/article/faq-troubleshooting/&quot;&gt;Learn more about troubleshooting WordPress.&lt;/a&gt;&lt;/p&gt;</string></value>
        </member>
      </struct>
    </value>
  </fault>
</methodResponse>

The return after this patch:

200
<?xml version="1.0" encoding="UTF-8"?>
<methodResponse>
  <params>
    <param>
      <value>
      <array><data>
  <value><struct>
  <member><name>faultCode</name><value><int>-32602</int></value></member>
  <member><name>faultString</name><value><string>server error. invalid method call structure</string></value></member>
</struct></value>
</data></array>
      </value>
    </param>
  </params>
</methodResponse>

Trac ticket: https://core.trac.wordpress.org/ticket/65124

Use of AI Tools

AI assistance: Yes
Tool(s): Claude
Model(s): Opus 4.7
Used for: Initial investigation route and test suggestions; final implementation and tests were reviewed and edited by me.

---

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[tommusrhodus]

Unit test failures seem unrelated and belong to different testing groups.