CVE-2026-82259 in SvelteKit
Summary
by MITRE • 08/28/2026
SvelteKit versions from 2.49.0 through 2.53.2 (fixed in 2.53.3) contain a deserialization expansion issue in the experimental form remote function. When an application enables experimental.remoteFunctions and uses the form function to process the files array without validating files.length or individual file sizes, an attacker can submit relatively small inputs that expand into very large file arrays, leading to expensive processing and denial of service.
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Analysis
by VulDB Data Team • 08/28/2026
The vulnerability identified in SvelteKit versions ranging from 2.49.0 through 2.53.2 represents a significant risk related to resource exhaustion via deserialization expansion within the experimental remote function feature. This flaw specifically affects applications that have enabled the experimental.remoteFunctions configuration and utilize the form handling mechanism to process file uploads without implementing adequate validation checks on the number of files or their individual sizes. The core technical issue lies in how the framework processes incoming data structures, where a relatively small input payload can trigger an internal expansion logic that results in the creation of disproportionately large arrays within memory. This behavior is characteristic of issues classified under CWE-770: Allocation of Resources Without Limits or Throttling, as the application fails to enforce constraints on resource consumption during the parsing and processing phase.
From a technical perspective, the vulnerability exploits the gap between input size and internal data structure growth. When an attacker submits a crafted request containing specific file array parameters, the server-side logic expands these inputs into a much larger set of objects or references than originally intended by the developer. Because the application does not validate files.length or check individual file sizes before processing, it proceeds to allocate memory for this expanded dataset. This unbounded growth leads to excessive CPU usage and memory consumption as the framework attempts to serialize, deserialize, or otherwise manipulate the inflated data structures. The result is a denial of service condition where legitimate users are unable to access the application due to resource saturation on the server side.
The operational impact of this vulnerability is primarily focused on availability rather than confidentiality or integrity in its immediate execution phase. An attacker can trigger this state by sending multiple requests with optimized payloads, effectively creating a low-effort denial-of-service attack vector against any SvelteKit application that relies on the affected experimental feature without proper safeguards. This aligns with ATT&CK technique T1496: Resource Hijacking, where an adversary uses compromised resources to perform actions such as cryptocurrency mining or other computationally intensive tasks, although in this specific context, the primary outcome is service disruption rather than cryptomining. The lack of input validation allows for automated exploitation at scale, potentially overwhelming infrastructure if multiple vectors are targeted simultaneously.
Mitigation strategies must focus on both immediate patching and long-term architectural improvements. First and foremost, organizations running affected versions should upgrade to SvelteKit version 2.53.3 or later, where this deserialization expansion issue has been resolved by the development team. For applications that cannot immediately upgrade due to dependency constraints, it is critical to implement strict input validation at the application layer before any data reaches the experimental remote function handler. This includes enforcing maximum limits on the number of files in a single submission and validating individual file sizes against predefined thresholds. Additionally, developers should consider disabling the experimental.remoteFunctions feature if it is not strictly necessary for production workloads, thereby removing the attack surface entirely until robust validation patterns are established across all form processing endpoints.