CVE-2026-27125 is a vulnerability classified under CWE-915 (Improperly Controlled Modification of Dynamically-Determined Object Attributes) affecting the Svelte framework, specifically in server-side rendering prior to version 5.51.5. The root cause is that attribute spreading syntax (e.g., <div {...attrs}>) enumerates not only the object's own properties but also inherited properties from its prototype chain. In JavaScript, if Object.prototype has been polluted with additional properties, these unintended attributes can be included in the SSR output or cause rendering errors. This behavior is outside the control of Svelte itself but results from the environment's prototype pollution. The vulnerability does not affect client-side rendering, limiting the attack surface to SSR contexts. Exploitation requires the attacker to have low privileges and partial authentication, with no user interaction needed. The vulnerability can lead to unexpected or malicious attributes being injected into SSR HTML output, potentially causing application errors or unintended behavior. The CVSS 4.0 vector indicates network attack vector, high attack complexity, partial authentication required, no user interaction, low impact on confidentiality and integrity, no impact on availability, and scope change with high security requirements. No known exploits are reported in the wild. The issue is resolved in Svelte version 5.51.5 by restricting attribute spreading to only own properties, preventing prototype pollution effects.

The vulnerability can cause unexpected attributes to appear in server-side rendered HTML or cause SSR processes to throw errors, potentially disrupting web application functionality. While it does not directly lead to remote code execution or data leakage, the injection of unintended attributes could be leveraged in complex attack chains, such as injecting malicious attributes that affect client-side behavior or bypass security controls. Applications relying heavily on SSR with Svelte may experience degraded reliability or unexpected behavior, impacting user experience and trust. The requirement for partial authentication and the high attack complexity reduce the likelihood of widespread exploitation, but targeted attacks against specific applications or environments with polluted Object.prototype could be impactful. Organizations using vulnerable Svelte versions in production SSR environments face risks of service disruption and potential indirect security impacts. The vulnerability's scope is limited to SSR and does not affect client-side rendering, reducing the overall attack surface.

The primary mitigation is to upgrade Svelte to version 5.51.5 or later, where the vulnerability is fixed by ensuring attribute spreading enumerates only own properties, preventing prototype pollution effects. Organizations should audit their SSR environments for use of vulnerable Svelte versions and plan timely upgrades. Additionally, developers should avoid or carefully control environments where Object.prototype pollution is possible, implementing strict input validation and sanitization to prevent prototype pollution attacks upstream. Employing security-focused code reviews and static analysis tools to detect prototype pollution risks can further reduce exposure. Monitoring SSR output for unexpected attributes or errors may help detect exploitation attempts. Finally, applying runtime protections such as Content Security Policy (CSP) and Web Application Firewalls (WAF) can help mitigate potential downstream impacts of injected attributes.