CVE-2026-5680 in Undertow
Summary
by MITRE • 08/27/2026
A flaw was found in Undertow. A remote attacker could exploit this vulnerability by sending specially crafted WebSocket messages with permessage-deflate negotiated. This could lead to excessive memory consumption due to the PerMessageDeflateFunction.largerBuffer() method using exponential doubling, resulting in a Denial of Service (DoS) for the affected application.
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Analysis
by VulDB Data Team • 08/27/2026
The identified vulnerability resides within Undertow, a widely adopted high-performance web server and servlet container written in Java that powers numerous enterprise applications and microservices. The specific flaw is located in the implementation of WebSocket extensions, particularly concerning the permessage-deflate compression mechanism which is negotiated during the initial handshake to optimize data transfer by compressing payloads before transmission. This feature allows clients and servers to agree on a method for compressing message bodies using algorithms such as DEFLATE, thereby reducing bandwidth usage and improving latency for text-based communications. However, the underlying logic governing how memory buffers are allocated and expanded when handling compressed streams contains a critical design flaw that can be leveraged by malicious actors to destabilize the target system.
The technical root cause of this vulnerability lies in the behavior of the PerMessageDeflateFunction.largerBuffer() method within the Undertow codebase. When processing incoming WebSocket frames, if the decompressed data exceeds the current buffer capacity, the application must allocate a larger memory block to accommodate it. The flawed implementation employs an exponential doubling strategy for resizing these buffers rather than using incremental or capped growth algorithms. This means that each time the buffer needs to expand, its size is multiplied by two. While this approach can be efficient in benign scenarios with predictable data patterns, it becomes catastrophic when faced with adversarial input designed to trigger repeated reallocations without providing proportional useful decompressed output. An attacker can craft WebSocket messages where the compressed payload appears valid but results in a massive expansion ratio upon decompression, or simply forces multiple small allocations that rapidly escalate due to the doubling logic.
This mechanism directly leads to excessive memory consumption on the server side. As the application processes these specially crafted messages, it continuously allocates larger and larger chunks of heap memory without releasing them appropriately until the garbage collector intervenes, which may not happen quickly enough under sustained attack conditions. The rapid escalation of memory usage can exhaust the available Java Virtual Machine (JVM) heap space or even cause out-of-memory errors at the operating system level if direct buffers are involved. This resource exhaustion prevents the application from serving legitimate requests, effectively resulting in a Denial of Service condition. Since WebSocket connections often remain open for extended periods to facilitate real-time communication, an attacker can maintain this state continuously, ensuring that memory consumption remains high and the service remains degraded or completely unavailable to genuine users.
From a threat modeling perspective, this vulnerability aligns with CWE-400, which describes Uncontrolled Resource Consumption, specifically manifesting as resource exhaustion through inefficient allocation strategies. It also maps closely to ATT&CK technique T1496, Resource Hijacking, where an adversary uses computing resources in ways that degrade performance or cause denial of service for other users. The attack vector is remote and requires no authentication if the WebSocket endpoint is publicly accessible, making it a significant risk for any internet-facing application relying on Undertow for real-time features such as chat applications, live notifications, or financial trading platforms.
Mitigation strategies should focus primarily on upgrading to patched versions of Undertow where this exponential doubling logic has been corrected with more conservative buffer expansion policies or strict limits on maximum decompressed message sizes. Administrators and developers must ensure that their dependency management systems are configured to pull the latest secure releases from Maven Central or other artifact repositories. In addition to software updates, defense-in-depth measures should be implemented at the network perimeter using Web Application Firewalls (WAF) capable of inspecting WebSocket traffic for anomalous payload sizes or compression ratios. Rate limiting and connection throttling can also help mitigate the impact by restricting the number of concurrent connections or messages a single client IP address can send within a given timeframe, thereby reducing the effectiveness of automated exploitation attempts aimed at exhausting server resources.