CVE-2025-57820 is a high-severity vulnerability affecting versions of the Svelte utility library 'devalue' prior to 5.3.2. The vulnerability is classified as CWE-1321, which involves improperly controlled modification of object prototype attributes, commonly known as prototype pollution. The root cause lies in the devalue.parse function, which accepts a string representation of an object. This function fails to properly validate input strings that include a '__proto__' property or non-numeric indices. As a result, an attacker can craft malicious input that modifies the prototype of JavaScript objects, thereby altering the behavior of all objects inheriting from that prototype. Prototype pollution can lead to a wide range of security issues, including arbitrary code execution, denial of service, or bypassing security controls, depending on how the polluted prototype is leveraged within the application. The vulnerability does not require authentication, user interaction, or privileges to exploit, and it can be triggered remotely by supplying crafted input to the vulnerable devalue.parse function. The issue was addressed in version 5.3.2 of the devalue library, which includes proper validation to prevent prototype pollution by disallowing '__proto__' properties and ensuring indices are numeric. No known exploits are currently reported in the wild, but the high CVSS 4.0 score of 7.9 reflects the potential impact and ease of exploitation. Given that Svelte and its associated libraries are widely used in modern web development for building reactive user interfaces, this vulnerability poses a significant risk to applications that incorporate vulnerable versions of devalue, especially those that parse untrusted input.

For European organizations, the impact of this vulnerability can be substantial, particularly for those relying on Svelte-based web applications or services that utilize the devalue library for serialization or parsing tasks. Prototype pollution can compromise application integrity and confidentiality by enabling attackers to manipulate application logic, escalate privileges, or execute arbitrary code. This can lead to data breaches, service disruptions, or unauthorized access to sensitive information. The vulnerability's remote exploitability without authentication increases the attack surface, making it attractive for threat actors targeting European enterprises, including e-commerce platforms, financial services, healthcare providers, and public sector applications that use Svelte frameworks. Additionally, compromised applications may violate GDPR requirements concerning data protection and security, potentially resulting in regulatory penalties and reputational damage. The lack of known exploits in the wild provides a window for proactive mitigation, but the high severity necessitates urgent attention to prevent exploitation as attackers may develop exploits rapidly once the vulnerability is publicly disclosed.

European organizations should take the following specific and practical steps to mitigate this vulnerability: 1) Identify all applications and services using the devalue library, particularly versions prior to 5.3.2, through software inventory and dependency analysis tools. 2) Immediately upgrade the devalue library to version 5.3.2 or later, which contains the fix for prototype pollution. 3) Audit application code to ensure that untrusted input is never directly passed to devalue.parse without validation or sanitization. Implement input validation layers that explicitly reject or sanitize '__proto__' properties and non-numeric indices before parsing. 4) Employ runtime application self-protection (RASP) or Web Application Firewall (WAF) rules that detect and block payloads attempting prototype pollution patterns, such as those containing '__proto__' keys. 5) Conduct thorough testing and code reviews focusing on deserialization and parsing logic to detect similar prototype pollution risks elsewhere in the codebase. 6) Monitor security advisories and threat intelligence feeds for any emerging exploits targeting this vulnerability and prepare incident response plans accordingly. 7) Educate development teams about secure coding practices related to object prototype manipulation and the risks of prototype pollution to prevent recurrence.