CVE-2026-12666

IBM MQ Classes for Java contains an XML external entity injection (XXE, CWE-611) flaw in the processing of MQRFH2 message headers, which can carry XML-formatted properties. An authenticated attacker triggers it by sending a message containing a crafted MQRFH2 header with a malicious external entity definition; when a Java client application parses the header, the entity is resolved. Successful exploitation lets the attacker read sensitive files accessible to the client JVM (information disclosure) or exhaust resources to cause a denial of service, reflected in the high CVSS 8.1 rating (network vector, low attack complexity, requires only low privileges). Any application using the affected Classes for Java releases — spanning MQ 9.1, 9.2, 9.3 (LTS and CD), 9.4 (LTS and CD), and 10.0.0.0 — is exposed. There is no known public proof of concept and no evidence of exploitation in the wild; the CVE is not in CISA's KEV catalog.

What to do: Upgrade the MQ Classes for Java client libraries (and the MQ installs that ship them) to the fixed builds specified in IBM's security bulletin — the first releases after each listed end version for 9.1–9.4 LTS/CD and 10.0. Inventory internal Java applications that put/get messages with MQRFH2 headers, since the flaw is client-side and queue-manager patching alone is insufficient. As interim mitigations, restrict MQ authentication to trusted applications, run client JVMs with least file-system privilege, and disable DTD/external-entity resolution in XML parsing where the application allows it.