CVE-2026-101025 in JavaScript
Summary
by MITRE • 10/15/2026
Description
NGINX JavaScript (njs) has a vulnerability where the built-in njs engine leaks a libxml2 parser context when xml.parse() fails. The failure path did not register cleanup and the context was not released for the lifetime of the process. An unauthenticated attacker may send requests with malformed XML and can cause unbounded memory growth in long-lived NGINX worker processes.
Impact
This vulnerability may allow remote attackers to send requests that trigger repeated parsing of malformed XML and may lead to NGINX worker termination or denial of service. There is no control plane exposure; this is a data plane issue only.
Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified in the NGINX JavaScript engine, commonly referred to as njs, represents a critical resource management flaw within the server's ability to process Extensible Markup Language data through its embedded scripting capabilities. This issue specifically manifests when the built-in xml.parse function encounters an error while processing incoming XML content. Under normal operational conditions, the libxml2 library is utilized by njs to parse and interpret XML structures for execution or transformation purposes. However, in scenarios where the input XML is malformed or structurally invalid, triggering a parsing failure, the internal cleanup routines fail to execute correctly. This oversight results in the persistent retention of the parser context allocated during the initial attempt to process the data. Instead of being released back into the system memory pool upon error, this context remains locked within the worker process for its entire lifespan.
From a technical perspective, this flaw constitutes an improper resource cleanup mechanism that leads directly to unbounded memory growth. Each time a request containing malformed XML is processed by a vulnerable version of NGINX with njs enabled, a new libxml2 parser context is allocated but never freed due to the missing error-handling path. In high-traffic environments or targeted attack scenarios where an adversary sends repeated requests with invalid XML payloads, these leaked contexts accumulate rapidly within the memory space of long-lived NGINX worker processes. As the process continues to run without restarts, the cumulative effect of these unreleased resources causes a steady and significant increase in memory consumption. This behavior is characteristic of a resource exhaustion vulnerability where system stability degrades over time rather than failing immediately upon exploitation.
The operational impact of this vulnerability centers on denial of service conditions affecting NGINX instances that rely on njs for request processing or transformation tasks. As the worker processes consume increasing amounts of available memory, they eventually approach their allocated limits or trigger operating-level out-of-memory protections. This typically results in the termination of the affected worker process by the system kernel to prevent broader instability. When multiple workers are impacted simultaneously due to sustained attack traffic, the overall capacity of the NGINX instance diminishes significantly, potentially leading to a complete service outage for downstream applications relying on this reverse proxy or application server layer. It is important to note that this vulnerability affects only the data plane and does not expose any control plane interfaces, meaning it cannot be used to gain administrative access but solely to disrupt availability through resource depletion.
This flaw aligns with Common Weakness Enumeration category CWE-401, which describes a missing release of memory after successful allocation, as well as CWE-789, concerning the uncontrolled consumption of resources when incorrect input is received. In terms of adversarial tactics and techniques, this vulnerability maps to MITRE ATT&CK technique T1499.002, specifically Endpoint Denial of Service via Resource Exhaustion by Application Layer Flooding. Attackers can leverage standard HTTP methods such as POST or PUT to deliver the malformed XML payloads without requiring any form of authentication, making it a viable vector for remote unauthenticated attackers seeking to degrade service availability. The lack of input validation in the error handling path allows this exploitation to occur seamlessly within normal traffic patterns unless specific mitigations are applied.
Mitigation strategies primarily involve updating the NGINX and njs software components to versions that include patches addressing this memory leak. Administrators should ensure that all instances running vulnerable configurations are upgraded promptly to eliminate the root cause of the resource retention issue. In environments where immediate patching is not feasible, temporary mitigations may include implementing strict input validation at a preceding network layer or load balancer to reject malformed XML requests before they reach the NGINX worker processes. Additionally, configuring worker process restarts on a scheduled basis can help mitigate the impact by periodically releasing accumulated memory leaks, although this approach introduces service interruptions and is not a substitute for proper software remediation. Continuous monitoring of worker process memory usage metrics should also be implemented to detect anomalous growth patterns that may indicate active exploitation attempts against unpatched systems.