CVE-2026-53723: guzzlehttp/guzzle-services' XML Request Serialization Vulnerable to XML Injection via CDATA Terminator

guzzlehttp/guzzle-services does not safely serialize scalar XML element values containing the CDATA terminator ]]>. The XML request serializer writes values containing <, >, or & with XMLWriter::writeCData($value). If attacker-controlled input contains ]]>, the CDATA section closes early and the remainder is interpreted as XML markup. This is an outgoing request-body integrity issue, not a response parsing issue. The attacker does not need to control the service description or schema.

Users are affected when all of the following are true:

1. The application uses guzzlehttp/guzzle-services to serialize outgoing requests.
2. A request parameter or additionalParameters schema uses location: xml.
3. The value is serialized as XML element text, not an XML attribute.
4. The value can contain attacker-controlled, user-controlled, tenant-controlled, or otherwise untrusted input.
5. The value is not constrained by a safe enum, pattern, or custom filter that excludes ]]>.
6. The downstream service parses the generated XML structurally and may act on unexpected, duplicated, or injected elements.

Applications that serialize untrusted input into location: xml request parameters can emit XML containing attacker-controlled elements outside the intended text node. Depending on the receiving service, this can alter operation semantics, smuggle privileged fields, bypass modeled parameter boundaries, or create conflicting duplicated elements. Fixed service descriptions are sufficient if they contain an XML element parameter populated from attacker-controlled input.

Users are not directly affected if they only use Guzzle Services to deserialize HTTP response bodies. Response XML parsing uses the response XML location visitor and does not invoke the vulnerable request XML serializer. Response bodies matter only in a second-order flow, such as parsing attacker-controlled response XML, storing or forwarding a parsed string value, and later using it as a location: xml request parameter.

Example fixed service description:

'DisplayName' => ['location' => 'xml', 'type' => 'string']

If an attacker-controlled display name is:

Alice]]></DisplayName><Role>admin</Role><DisplayName><![CDATA[

the vulnerable serializer can emit an injected element outside the intended DisplayName text node:

<Request><DisplayName><![CDATA[Alice]]></DisplayName><Role>admin</Role><DisplayName><![CDATA[]]></DisplayName></Request>

If the downstream service treats <Role> as meaningful, the attacker has set a field the modeled DisplayName parameter was not intended to set.