CVE-2026-82333 in multer
Summary
by MITRE • 08/29/2026
multer is a middleware for handling multipart/form-data in Node.js. A small multipart request with two specially crafted text field names can make multer's field parser synchronously iterate a maximum-length sparse array, blocking the event loop so the process cannot handle other requests. A large numeric array index in the first field allocates a maximum-length sparse array, and a second field with a non-numeric key then triggers a full-length iteration inside the append-field dependency. All versions before 2.3.0 are affected, and this is a remotely triggerable denial of service. multer 2.3.0 adds an opt-in fieldArrayIndexLimit option that rejects oversized array indexes. Upgrade to multer 2.3.0 and set limits.fieldArrayIndexLimit to the largest array index your application needs to remediate.
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Analysis
by VulDB Data Team • 08/29/2026
The vulnerability identified in the multer middleware for Node.js represents a significant denial of service risk stemming from inefficient handling of multipart form data parsing logic. Multer serves as a critical component in many web applications, responsible for processing incoming requests that contain file uploads or complex form submissions using the multipart/form-data content type. The core issue lies within the internal field parser mechanism, specifically how it manages array indices when constructing JavaScript objects to hold submitted fields. This flaw allows an attacker to craft a maliciously small HTTP request containing two specific text fields with carefully chosen names that exploit the underlying behavior of sparse arrays in Node.js environments.
The technical root cause involves the interaction between multer and its dependency append-field, which handles the actual parsing of field values into JavaScript data structures. When the first field name contains a large numeric index, such as 10^9 or higher, the parser allocates a corresponding maximum-length sparse array to accommodate this potential indexing scheme. While allocating a sparse array is generally memory-efficient in modern engines due to lazy initialization, the subsequent processing step introduces the performance bottleneck. If a second field follows with a non-numeric key, the append-field dependency triggers an iteration over the entire length of that previously allocated sparse array rather than just iterating through defined elements. This synchronous full-length iteration consumes excessive CPU resources and blocks the Node.js event loop for a prolonged period.
The operational impact of this vulnerability is severe because it results in a remote denial of service condition without requiring authentication or complex exploitation chains. Since Node.js operates on a single-threaded event loop model, blocking that thread prevents the application from processing any other concurrent requests. An attacker can exploit this by sending multiple such crafted requests simultaneously, effectively freezing the web server and rendering it unresponsive to legitimate users. This type of attack is particularly dangerous in high-traffic environments where even brief periods of latency or downtime can lead to significant business disruption and loss of service availability for end-users.
This vulnerability aligns with CWE-400, which describes Uncontrolled Resource Consumption, as the application fails to limit the resources consumed by a single request due to unchecked array indexing logic. Furthermore, it falls under ATT&CK technique T1499, Endpoint Denial of Service, specifically within the context of resource exhaustion via inefficient algorithms or loops that degrade system performance. The lack of bounds checking on field names allows an adversary to manipulate internal data structures in a way that degrades service quality rather than causing immediate crashes, making it a stealthy yet impactful attack vector against Node.js-based web services.
Remediation requires upgrading the multer package to version 2.3.0 or later, which introduces protective measures against this specific class of attacks. The updated versions include an opt-in configuration option named fieldArrayIndexLimit that allows developers to define the maximum allowable numeric index for array fields. By setting limits.fieldArrayIndexLimit to a value appropriate for the application's legitimate use cases, any request attempting to create excessively large sparse arrays will be rejected before it can trigger the resource-intensive iteration loop. This proactive limit ensures that only reasonable field structures are processed, thereby preserving event loop responsiveness and maintaining service availability under normal and malicious load conditions alike.