CVE-2025-55195 is a high-severity Prototype Pollution vulnerability (CWE-1321) found in the @std/toml module of the Deno Standard Library (denoland std) prior to version 1.0.9. The vulnerability arises when the library parses untrusted TOML data and merges it with an empty object that retains its prototype chain by default. This improper handling allows an attacker to inject or modify properties on the prototype chain, effectively polluting the prototype of base objects. Prototype Pollution can lead to serious security issues such as arbitrary code execution, denial of service, or bypassing security controls by manipulating object behavior at runtime. The vulnerability affects both Node.js runtime environments and browsers using the vulnerable library versions. The issue was patched in version 1.0.9 of the library. The CVSS v3.1 score is 7.3 (high), reflecting network vector (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), unchanged scope (S:U), and impacts on confidentiality, integrity, and availability (C:L/I:L/A:L). No known exploits are currently reported in the wild, but the ease of exploitation and impact potential make it a significant threat to applications relying on vulnerable versions of the Deno std library for TOML parsing.

For European organizations, this vulnerability poses a substantial risk especially to those developing or deploying applications using the Deno runtime environment or Node.js environments that incorporate the vulnerable @std/toml library versions. Prototype Pollution can lead to unauthorized modification of application logic, data leakage, or service disruption. This can affect web applications, backend services, or any software components parsing TOML configuration or data files from untrusted sources. Given the increasing adoption of Deno and Node.js in modern web development across Europe, exploitation could result in compromised systems, data breaches, or operational downtime. Critical sectors such as finance, healthcare, and government services that rely on secure and stable software infrastructure could face regulatory and reputational damage if impacted. Additionally, the vulnerability’s presence in browser environments increases the attack surface, potentially affecting client-side applications and end-users.

European organizations should immediately audit their software dependencies to identify usage of @std/toml versions prior to 1.0.9. Upgrading to version 1.0.9 or later is the primary and most effective mitigation. For environments where immediate upgrade is not feasible, implementing strict input validation and sanitization on TOML data sources can reduce risk. Employ runtime monitoring to detect anomalous prototype modifications or unexpected behavior in applications. Developers should avoid merging untrusted objects directly with objects that have prototypes; instead, use safer object creation patterns such as Object.create(null) to create prototype-less objects when merging untrusted data. Security teams should also review application logic for reliance on prototype properties that could be manipulated. Incorporating automated dependency scanning and integrating patch management processes will help prevent future exposure. Finally, organizations should monitor threat intelligence feeds for any emerging exploits targeting this vulnerability.