Description

SvelteKit versions before 2.69.1 contain a prototype pollution vulnerability in remote form functions with file input fields that accept arbitrary user-controlled path names. Attackers can manipulate the deletion path to remove methods on the prototype, potentially disabling application functionality.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "4.3",
  "pubDate": "2026-08-28T12:16:38.607Z",
  "pubdate": "2026-08-28T12:16:38.607Z",
  "executiveSummary": "SvelteKit versions prior to 2.69.1 are susceptible to a prototype pollution vulnerability originating from the handling of remote form functions that incorporate file input fields.\nThis vulnerability allows an attacker to supply maliciously crafted input to manipulate object properties.\nThe primary impact involves the ability of an attacker to influence the prototype chain, specifically by manipulating deletion paths to remove critical methods from the global Object prototype.\nSuch actions can lead to a state of denial-of-service, where fundamental application functionality becomes disabled due to missing methods on objects used by the server-side runtime.\nThe vulnerability is localized to the way SvelteKit processes remote form submissions containing file data.\nRisk is significant, as successful exploitation results in application instability and potential loss of service integrity.",
  "technicalDetails": "The vulnerability resides in the SvelteKit mechanism responsible for parsing and processing form data in remote form actions, specifically when those forms include file input fields.\nThe root cause is an improper validation of user-controlled path names passed during the file processing routine. When an attacker provides a crafted path, the underlying logic fails to prevent the manipulation of object prototypes.\nIn JavaScript, prototype pollution occurs when an application merges or assigns properties to an object without sanitizing the keys, allowing an attacker to inject properties into Object.prototype.\nIn this specific instance, the exploitation flow involves the attacker submitting a remote form that includes file inputs. By manipulating the path parameters associated with these files, the attacker targets internal object paths used by the framework.\nThe manipulation of these paths permits the attacker to trigger a deletion operation on the prototype chain. By targeting essential methods, the attacker can effectively 'nullify' or delete functions that the SvelteKit runtime relies on for standard operation.\nOnce these methods are removed from the prototype, any object instance within the application context that expects those methods will fail to execute them, leading to runtime exceptions, incorrect logic flows, or a complete crash of the affected server-side process.\nThe attack is performed via standard HTTP form submission methods facilitated by SvelteKit's remote form functionality. No specific authentication or high-level privileges are explicitly required beyond the ability to interact with the vulnerable form submission endpoint.\nThe payload behavior involves supplying a JSON-like structure or path string as part of the form field input that traverses the prototype chain (e.g., using __proto__ or similar property accessors).\nPost-exploitation, the server-side environment remains in a corrupted state for the lifecycle of the process, as the modifications to the Object prototype persist, leading to persistent denial of service across subsequent requests until the application instance is restarted."
}