CVE-2026-109869 in undici
Summary
by MITRE • 10/15/2026
undici is an HTTP client library for Node.js. In versions 6.23.0 through 6.29.0, 7.18.0 through 7.30.0, and 8.0.0 through 8.12.1, its fetch() implementation crashes the Node.js process when a malicious server returns a response with more than five content codings and closes the connection before sending the body. undici rejects such responses because of a content-encoding limit, but it created the internal response stream before that check and returned without attaching an error listener or aborting the request, so the early close surfaces a content-length mismatch on the orphaned stream, which Node.js treats as an unhandled error and terminates the process. A remote, unauthenticated server can trigger this with a single response, even when the application correctly catches the fetch() rejection, resulting in a denial of service. The issue is fixed in undici 6.29.1, 7.30.1, and 8.12.2, 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 within the undici HTTP client library for Node.js represents a critical denial-of-service condition arising from improper resource lifecycle management during response validation. Undici serves as a foundational component in many modern JavaScript ecosystems, handling network requests with high performance and efficiency. However, specific versions ranging from 6.23.0 through 6.29.0, 7.18.0 through 7.30.0, and 8.0.0 through 8.12.1 contain a flaw in the implementation of the fetch API method that leads to process termination under specific adversarial conditions. The core issue stems from an inconsistency between how undici validates incoming HTTP responses and how it manages internal streams associated with those responses. When a server returns a response containing more than five content codings, such as gzip or deflate encodings, undici is designed to reject the request based on its configured limits for nested decompression layers. This limit exists primarily to prevent resource exhaustion attacks related to excessive CPU and memory usage during decoding operations.
Despite correctly identifying that the incoming response violates the content-encoding policy, the library fails to properly clean up internal resources before rejecting the promise. Specifically, undici creates an internal response stream prior to performing the validation check for the number of content codings. When the violation is detected, the code path returns a rejection without attaching an error listener or explicitly aborting the underlying request connection. This architectural oversight leaves the internal stream in an orphaned state, detached from any active event handlers that would normally manage its lifecycle and handle potential errors gracefully. The situation becomes critical when the malicious server closes the TCP connection before transmitting the response body. Because the stream remains open but unmonitored, Node.js detects a mismatch between the declared Content-Length header and the actual data received, which is zero in this case due to the premature closure.
This content-length discrepancy triggers an unhandled exception within the Node.js runtime environment. Since no error listener was attached to handle the failure of the orphaned stream, the exception propagates up the call stack without being caught by application-level try-catch blocks surrounding the fetch invocation. Consequently, even if a developer has implemented robust error handling for network failures or HTTP errors, this specific internal crash bypasses those safeguards entirely. The result is an immediate and ungraceful termination of the Node.js process. This constitutes a severe denial-of-service vulnerability because it allows any remote, unauthenticated server to disrupt service availability with a single crafted response packet. An attacker does not need valid credentials or prior interaction beyond initiating a request; they simply need to host a malicious endpoint that responds with excessive content codings and immediately closes the connection.
From a classification perspective, this flaw aligns with CWE-400 Uncontrolled Resource Consumption, as it leads to resource exhaustion in the form of process termination rather than gradual degradation. It also relates to CWE-755 Improper Handling of Unexpected Exceptional Conditions, where the library fails to handle an exceptional state (the connection close) appropriately given its internal configuration limits. In terms of the MITRE ATT&CK framework, this vulnerability facilitates Denial of Service via Impactful Actions, specifically targeting Availability by crashing the host process. The attack vector is remote and network-based, requiring no user interaction or authentication, which significantly increases its severity rating in threat models that prioritize ease of exploitation and impact scope.
The operational impact extends beyond simple service interruption; it can lead to cascading failures in microservices architectures where a single downstream dependency crash might trigger timeouts or retries across multiple services, amplifying the denial-of-service effect. For applications relying on undici for critical data ingestion or API communication, this vulnerability poses a significant risk to uptime and reliability. There are no viable workarounds within the affected versions because the flaw is embedded in the core logic of how responses are processed and rejected. Developers cannot simply adjust configuration settings to mitigate the issue without upgrading the library itself. The only effective remediation strategy is to upgrade undici to version 6.29.1, 7.30.1, or 8.12.2, or any later release where this lifecycle management bug has been resolved. These patched versions ensure that internal streams are properly aborted and error handlers are attached before rejection occurs, thereby preventing the unhandled exception from reaching the Node.js runtime level. Organizations should prioritize patching these dependencies to maintain system stability against automated scanning tools that may probe for such known vulnerabilities in HTTP client implementations.