CVE-2026-76166 in Enterprise Application Platform
Summary
by MITRE • 08/19/2026
A flaw was found in mod_cluster's AdvertiseListenerImpl (org.jboss.modcluster core module). A single crafted UDP multicast datagram with a valid HTTP status line and a "Server:" header but without the "Date:", "Digest:", and "Sequence:" headers triggers a NullPointerException in verifyDigest() that is not caught by the worker thread's exception handler. This causes the advertise listener thread to terminate permanently. The failure is silent (isListening() continues to return true) and persists until the node is restarted. The crash occurs before the AdvertiseSecurityKey comparison, so deployments with a configured security key are still affected.
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Analysis
by VulDB Data Team • 08/19/2026
The vulnerability identified in mod_cluster's AdvertiseListenerImpl component within the org.jboss.modcluster core module represents a critical denial of service condition arising from improper exception handling during UDP multicast message processing. This flaw specifically targets the verifyDigest() method, which is responsible for validating incoming advertisement messages to ensure their integrity and authenticity before further processing by the worker threads. The root cause lies in the absence of robust defensive programming practices when parsing HTTP-like headers within these multicast datagrams. Specifically, the implementation fails to account for scenarios where certain mandatory or expected headers are missing from an otherwise valid-looking UDP packet. When a crafted datagram arrives containing a standard HTTP status line and a Server header but deliberately omits the Date, Digest, and Sequence headers, the internal logic attempts to access null values associated with these absent fields. This action triggers a NullPointerException that propagates up the call stack without being intercepted by the surrounding exception handlers designed for worker threads.
The operational impact of this vulnerability is severe due to its silent nature and persistent state. Unlike many other denial of service flaws where the service might crash visibly or become unresponsive in an obvious manner, mod_cluster's advertise listener thread terminates permanently while the system continues to report that it is actively listening via the isListening() method returning true. This creates a deceptive operational environment where administrators may believe the clustering mechanism is functioning correctly despite being effectively blind to new cluster members or unable to send advertisements to existing ones. The failure persists until the entire node running mod_cluster is restarted, leading to prolonged periods of degraded availability and potential split-brain scenarios in clustered environments if other nodes cannot discover this affected member properly.
From a security architecture perspective, it is crucial to note that this vulnerability bypasses authentication mechanisms based on configured security keys. Because the crash occurs during the initial parsing phase before the AdvertiseSecurityKey comparison logic is executed, any deployment utilizing strong cryptographic key verification for multicast advertisements remains vulnerable to this denial of service attack. An attacker does not need valid credentials or knowledge of the secret key to exploit this flaw; they only need network access to send a single UDP packet to the appropriate multicast address and port configured by mod_cluster. This significantly lowers the barrier to entry for malicious actors, allowing remote unauthenticated attackers to disrupt cluster communication with minimal effort.
This vulnerability aligns closely with CWE-248 Unhandled Exception in Common Weakness Enumeration standards, as it involves a failure to handle an exceptional condition that leads to system instability. Furthermore, from the MITRE ATT&CK framework perspective, this behavior can be categorized under Denial of Service techniques where availability is compromised through resource exhaustion or service disruption via malformed input. The specific vector relates to network-based attacks leveraging UDP multicast protocols, which are often less monitored than TCP traffic in many enterprise environments due to their connectionless nature and typical use for discovery services rather than data transfer.
Mitigation strategies should focus on both immediate remediation and long-term architectural improvements. The primary solution is to apply the vendor-provided patch that updates the AdvertiseListenerImpl class to include proper null checks and exception handling within the verifyDigest() method. Administrators must ensure their modcluster components are updated to versions where this specific parsing flaw has been addressed. In environments where immediate patching is not feasible, network-level mitigations can be employed. Implementing strict firewall rules that restrict UDP multicast traffic on the relevant ports to only trusted internal subnets or specific source IP addresses can significantly reduce the attack surface. Additionally, monitoring for unusual spikes in malformed UDP packets directed at modcluster ports may provide early warning indicators of an attempted exploitation. Long-term resilience requires adopting a defense-in-depth approach where input validation is rigorous across all network-facing components, and exception handling policies are standardized to ensure that no single parsing error can terminate critical background threads silently.