Description

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.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.5",
  "pubDate": "2026-08-28T12:16:38.953Z",
  "pubdate": "2026-08-28T12:16:38.953Z",
  "executiveSummary": "SvelteKit versions 2.49.0 through 2.53.2 are susceptible to a denial-of-service (DoS) vulnerability arising from an uncontrolled resource consumption issue within the experimental form remote function feature.\nThe vulnerability is categorized as a deserialization-based expansion flaw. It enables an unauthenticated attacker to trigger excessive resource consumption by submitting specially crafted inputs that cause disproportionate memory or CPU utilization during the server-side processing of file arrays.\nThe core issue lies in the lack of input validation regarding the length of file arrays and individual file sizes within the remote function deserialization logic.\nBy manipulating the form submission payload, an attacker can induce rapid heap exhaustion or CPU saturation, rendering the application unavailable to legitimate users.\nThe impact is significant, as it permits service disruption without requiring high-level privileges or complex authentication, provided the experimental.remoteFunctions feature is explicitly enabled in the application configuration.\nThe vulnerability was remediated in version 2.53.3, which introduces necessary validation constraints on the input processing pipeline.",
  "technicalDetails": "The vulnerability stems from the internal handling of the experimental.remoteFunctions feature in SvelteKit, specifically within the logic responsible for processing the files array submitted via the form function.\nDuring the deserialization process, the framework fails to enforce strict bounds on the structure and content of the incoming data, specifically ignoring the total count of file objects and their respective payload sizes.\nAn attacker exploits this by transmitting a serialized request containing a maliciously crafted form payload. While the request itself may be relatively small in raw byte size—thereby bypassing rudimentary network-level size filters—the deserialization process expands these inputs into a significantly larger internal object representation.\nWhen the remote function logic parses these inputs, it attempts to instantiate or process a disproportionately large array of file references. Because the runtime environment attempts to allocate memory for every element defined in the expanded array, the application experiences rapid memory pressure.\nFurthermore, if the server-side logic iterates over these file arrays to perform validation or temporary storage, the CPU cycles required to traverse the artificially inflated data structure lead to resource exhaustion.\nStep-by-step exploitation flow: 1. The attacker identifies an endpoint utilizing the experimental.remoteFunctions capability. 2. The attacker crafts a request payload that exploits the deserialization logic to represent a high volume of file objects. 3. The request is transmitted to the server. 4. The server receives the request and begins the deserialization phase for the form data. 5. The lack of validation allows the memory footprint to expand exponentially relative to the input size. 6. The server-side process reaches its memory allocation limit or becomes blocked by intensive object processing, leading to a crash or severe performance degradation (denial of service).\nThis vulnerability is strictly limited to applications that have explicitly enabled the experimental.remoteFunctions flag, as the vulnerable codepath is not active by default in standard SvelteKit configurations.\nThe affected versions include 2.49.0 up to and including 2.53.2. Post-exploitation, the server may remain in an unstable state, requiring manual restarts or suffering from ongoing service unavailability until the malformed request queue is cleared or the server recovers from heap exhaustion."
}