CVE-2026-109932 in undici
Summary
by MITRE • 10/15/2026
undici is an HTTP client library for Node.js. In versions 5.15.0 through 6.29.0, 7.0.0 through 7.30.0, and 8.0.0 through 8.12.1, its WebSocket client, including the bundled WebSocket exposed by Node.js, can be driven to exhaust process memory by a malicious or compromised WebSocket server. For every PING frame received, the client automatically writes a PONG response and ignores the backpressure signal from the socket write, so a server that streams small PING frames while never reading the client's PONGs makes the outbound write queue and heap grow without bound until the Node.js process runs out of memory and terminates. The attack is remote and unauthenticated, needs only a normal WebSocket connection, and is asymmetric, since small inbound PINGs produce a much larger retained outbound queue, with availability the only impact. The issue is fixed in undici 6.29.1, 7.30.1, and 8.12.2, which pause WebSocket frame parsing when the automatic PONG response hits socket backpressure and resume on drain, bounding the outbound queue, and users should upgrade to one of those versions or later. There is no workaround.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified in undici, a widely used HTTP client library for Node.js, represents a significant availability risk stemming from improper handling of WebSocket protocol mechanics. This flaw affects multiple version ranges including 5.15.0 through 6.29.0, 7.0.0 through 7.30.0, and 8.0.0 through 8.12.1. The core issue resides within the library's WebSocket client implementation, which is also utilized by Node.js itself for its bundled WebSocket functionality. When a connection is established with a malicious or compromised server, the client fails to manage memory resources effectively during specific control frame exchanges, leading to an unbounded growth in process memory usage that can ultimately cause the application to crash due to out-of-memory conditions.
The technical mechanism of this exploitation relies on the behavior associated with WebSocket PING and PONG frames. According to the WebSocket specification, a server may send a PING frame to keep the connection alive or test latency, expecting the client to respond with a corresponding PONG frame. In vulnerable versions of undici, every received PING frame triggers an automatic write operation for the PONG response. Crucially, this implementation ignores the backpressure signal from the socket write stream. Backpressure is a standard Node.js mechanism that allows streams to pause reading when the destination cannot keep up with incoming data. By disregarding this signal, the client continues to queue outbound PONG frames in memory even if the network connection or server is slow or non-responsive regarding inbound reads of those responses.
An attacker can exploit this flaw by establishing a normal WebSocket connection and then streaming small PING frames continuously while deliberately not reading any of the returned PONG frames from the client side. This creates an asymmetric attack vector where minimal bandwidth consumption on the network results in substantial memory allocation on the victim's server. Each queued PONG frame consumes heap space, and as more PINGs are received, the outbound write queue expands without bound. Eventually, this uncontrolled accumulation of data exhausts the available process memory, causing the Node.js application to terminate abruptly. This constitutes a remote, unauthenticated denial-of-service attack that requires no special privileges or authentication beyond establishing a standard WebSocket connection.
The operational impact is strictly limited to availability, as there is no indication of code execution or information disclosure in this specific scenario. However, for services relying on persistent WebSocket connections such as real-time chat applications, live data feeds, or gaming servers, an unauthenticated crash can lead to significant service disruption and loss of connectivity for legitimate users. The attack is particularly insidious because it mimics normal keep-alive traffic patterns, making detection by standard network monitoring tools difficult without deep packet inspection focused on memory allocation metrics rather than just bandwidth usage.
To mitigate this vulnerability, organizations must upgrade undici to version 6.29.1, 7.30.1, or 8.12.2 and later. These patched versions introduce logic that pauses WebSocket frame parsing when the automatic PONG response encounters socket backpressure. The parser resumes only after the drain event occurs, ensuring that the outbound queue remains bounded and preventing uncontrolled memory growth. There is no viable workaround for older versions; therefore, upgrading the dependency is the only effective remediation strategy. Developers should also ensure their CI/CD pipelines enforce minimum version constraints to prevent regression into vulnerable states during future updates.
From a classification perspective, this vulnerability aligns with CWE-400 Uncontrolled Resource Consumption and specifically relates to improper handling of backpressure in stream-based I/O operations. In the context of the MITRE ATT&CK framework, this behavior facilitates Availability Impact through resource exhaustion techniques often categorized under Denial-of-Service attacks. Security teams should monitor for unusual memory spikes in Node.js processes associated with WebSocket endpoints as an indicator of potential exploitation attempts while patching efforts are underway.