CVE-2026-82257 in SvelteKit
Summary
by MITRE • 08/28/2026
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.
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Analysis
by VulDB Data Team • 08/28/2026
The identified security flaw resides within SvelteKit versions prior to 2.69.1 and specifically targets remote form functions that process file input fields accepting arbitrary user-controlled path names. This vulnerability stems from a failure in proper validation of file paths provided by the client during multipart form data submissions. When an application processes these inputs, it inadvertently allows attackers to inject maliciously crafted strings into internal object structures used for handling file operations or metadata storage. Because JavaScript objects are inherently mutable and prototype-based, this lack of sanitization enables what is classified as a Prototype Pollution attack. In this context, the attacker manipulates specific keys within the data payload that map directly to properties on the Object.prototype chain rather than just local variables associated with the current request scope.
The technical mechanism involves sending specially crafted HTTP requests where file input fields contain path names designed to overwrite standard prototype methods such as toString or hasOwnProperty. By polluting these foundational object properties, an attacker can alter the behavior of all objects in the application that inherit from Object.prototype without needing direct access to those specific instances. This manipulation is particularly dangerous because it affects core JavaScript functionality rather than just application-specific data structures. The vulnerability exploits the way SvelteKit processes multipart form data, where file names or associated metadata are parsed and stored in a manner that does not sufficiently isolate user input from internal object construction logic.
The operational impact of this prototype pollution can be severe depending on how the affected application utilizes standard JavaScript methods. If critical functions like toString are overwritten with non-function values or malicious code, any subsequent operation relying on these methods will fail or behave unpredictably. This can lead to denial of service conditions where legitimate users experience broken functionality due to unhandled exceptions in server-side logic that assumes standard prototype behavior remains intact. Furthermore, if the application relies on property checks using hasOwnProperty for security decisions such as access control validation or data integrity verification, these checks may be bypassed entirely, potentially leading to privilege escalation or unauthorized data exposure depending on the specific implementation details of the surrounding codebase.
This vulnerability is categorized under CWE-1325: Improperly Controlled Modification of Object Prototype Attributes and aligns with MITRE ATT&CK technique T1068 Exploitation for Privilege Escalation, as it allows an unauthenticated or low-privilege user to modify core system behaviors. To mitigate this risk, developers must upgrade immediately to SvelteKit version 2.69.1 or later where the issue has been resolved through stricter input validation and isolation of prototype chains during form data processing. Additionally, implementing defensive coding practices such as using Object.create(null) for objects that store user-supplied keys can prevent accidental pollution of the global prototype chain regardless of framework updates. Security teams should also audit existing codebases for similar patterns where file uploads or multipart forms directly influence object property assignments without sanitization checks against reserved JavaScript identifiers.