CVE-2026-27125 is a vulnerability classified under CWE-915 (Improperly Controlled Modification of Dynamically-Determined Object Attributes) affecting the Svelte framework, specifically in its server-side rendering (SSR) implementation prior to version 5.51.5. The root cause is that during SSR, when attribute spreading syntax (e.g., <div {...attrs}>) is used, the framework enumerates not only the object's own properties but also inherited properties from its prototype chain. If the Object.prototype has been polluted—meaning additional properties have been added to the global prototype—these unintended properties can be included in the SSR output. This can result in unexpected or malicious attributes being rendered into HTML, potentially causing rendering errors or injection of unintended data. The vulnerability does not affect client-side rendering, as the issue is specific to SSR attribute enumeration. Exploitation requires an environment where Object.prototype is polluted, which is outside Svelte's control, and partial authentication privileges. The CVSS 4.0 score is 5.3 (medium), reflecting moderate impact on confidentiality and integrity with a high scope. The vulnerability was publicly disclosed on February 20, 2026, and fixed in Svelte version 5.51.5. No known exploits have been reported in the wild.

The vulnerability can lead to the injection of unexpected attributes into SSR-generated HTML, which may cause application errors or potentially expose sensitive data if the polluted prototype properties contain confidential information. This undermines the integrity of the rendered content and could disrupt application availability due to rendering failures. While client-side rendering remains unaffected, server-side rendered applications using vulnerable Svelte versions are at risk. Attackers with low privileges and partial authentication could exploit polluted prototype chains to influence SSR output, potentially leading to security issues such as information leakage or denial of service. The scope is high because it affects all SSR outputs in vulnerable versions, impacting any organization using Svelte for server-side rendering. Although no active exploits are known, the risk remains significant for web applications relying on SSR with Svelte versions prior to 5.51.5.

The primary mitigation is to upgrade all Svelte instances to version 5.51.5 or later, where the vulnerability is fixed by restricting attribute spreading to own properties only. Additionally, organizations should audit their JavaScript environments to prevent pollution of Object.prototype, which is a prerequisite for exploitation. Implementing strict Content Security Policies (CSP) can help mitigate the impact of unexpected attributes if they lead to injection attacks. Developers should review SSR code for usage of attribute spreading and consider sanitizing or validating input objects before spreading attributes. Monitoring server logs for SSR errors or unexpected HTML attributes can help detect exploitation attempts. Finally, applying runtime protections that detect prototype pollution attempts can reduce the risk of this vulnerability being exploited.