CVE-2026-12666 in IBM
Summary
by MITRE • 09/15/2026
IBM MQ 9.1.0.0 through 9.1.0.37 LTS, 9.2.0.0 through 9.2.0.43 LTS, 9.3.0.0 through 9.3.0.41 LTS, 9.3.0.0 through 9.3.5.1 CD, 9.4.0.0 through 9.4.0.25 LTS, 9.4.0.0 through 9.4.5.1 CD, and 10.0.0.0 Classes for Java could allow an authenticated attacker to obtain sensitive information or cause a denial of service due to XML external entity injection in MQRFH2 header processing.
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Analysis
by VulDB Data Team • 09/15/2026
The vulnerability identified affects IBM Message Queue versions ranging from 9.1.0.0 through 9.4.5.1, including both Long Term Support and Continuous Delivery releases across Java classes. This security flaw is rooted in the improper handling of XML data within the MQRFH2 header processing mechanism. The MQRFH2 header serves as a standard extension to the Message Descriptor structure in IBM MQ, allowing applications to pass application-specific properties with messages using an XML format known as Application Properties or AppProperties. When these headers are processed by the queue manager, they undergo parsing and validation routines designed to ensure data integrity and security. However, in the affected versions, this process fails to adequately sanitize input before it is interpreted by the underlying XML parser. This lack of rigorous input validation creates a pathway for an attacker to inject maliciously crafted XML content that exploits external entity references.
This specific technical flaw constitutes an XML External Entity injection vulnerability, which aligns with CWE-611 in the Common Weakness Enumeration taxonomy. The core issue lies in the configuration or implementation of the XML parser used during header processing. By default, many XML parsers allow the resolution and inclusion of external entities defined within a document's Document Type Definition. An authenticated attacker can leverage this capability by constructing a specially crafted MQRFH2 header that contains an entity reference pointing to an arbitrary resource on the network file system or local disk. When the IBM MQ server processes this message, it attempts to resolve the external entity, leading to unintended side effects depending on how the parser is configured and what resources are accessible from the server environment.
The operational impact of this vulnerability is twofold, encompassing both information disclosure and denial of service conditions. In terms of confidentiality, an attacker can utilize the injection vector to read sensitive files from the system where IBM MQ is hosted. By directing the external entity resolution to local paths such as etc/passwd on Linux systems or critical configuration files on Windows environments, the contents are returned within the XML parsing error messages or response structures, effectively allowing data exfiltration beyond the intended scope of the message queue application. This capability transforms a standard messaging protocol into an arbitrary file read mechanism, potentially exposing credentials, internal network topology details, and other confidential organizational data stored in accessible files.
Regarding availability, the vulnerability can be exploited to cause a denial of service against the IBM MQ infrastructure. An attacker may craft payloads that trigger infinite loops or excessive resource consumption during entity resolution, such as by referencing entities that recursively expand or point to non-responsive network locations. This behavior consumes significant CPU and memory resources on the queue manager host, potentially leading to system instability or complete unresponsiveness for legitimate users attempting to send or receive messages. The impact is particularly severe because it requires only authentication, which may be relatively easy to obtain if weak credentials are in use or if lateral movement has already occurred within the network perimeter.
From a threat modeling perspective, this vulnerability maps directly to ATT&CK technique T1059, specifically sub-techniques related to command and script interpreters or system utilities that facilitate data exfiltration via XML parsing artifacts. It also relates to T1213, Data from Information Repositories, as the attacker extracts data stored in local files through the messaging interface. The attack vector is classified as network-based with authentication requirements, placing it within ATT&CK tactic TA0008 for Lateral Movement or TA0009 for Collection depending on the specific exploitation path taken by the adversary.
Mitigation strategies must prioritize immediate patching to eliminate the root cause of the flaw. IBM has released updates in later versions of MQ 9.1, 9.2, 9.3, and 9.4 that address this issue by hardening the XML parser configuration or implementing stricter validation rules for MQRFH2 headers. Organizations should apply the latest available fix levels as soon as possible to close this attack surface. In environments where immediate patching is not feasible due to operational constraints, network segmentation can provide a layer of defense by restricting access to IBM MQ ports from untrusted networks and ensuring that only authorized application servers can connect to the queue managers. Additionally, implementing strict input validation at the application level before messages are enqueued can help prevent malicious payloads from reaching the vulnerable parser component.
Security monitoring should be enhanced to detect anomalous patterns associated with XML external entity attacks. This includes looking for unusually large MQRFH2 headers or specific error signatures in logs that indicate failed XML parsing attempts involving external references. Enabling detailed audit logging on IBM MQ instances can assist forensic analysis if an incident occurs, providing visibility into which accounts and source IPs were involved in the exploitation attempt. Furthermore, adopting a principle of least privilege for service accounts running IBM MQ processes ensures that even if file read operations are successful, the attacker cannot access highly sensitive system files due to restricted operating system permissions. Regular vulnerability assessments focused on messaging infrastructure components are essential to maintain resilience against evolving XML-based attack vectors.