CVE-2025-55173 is a medium-severity vulnerability affecting the Next.js framework, specifically its Image Optimization feature. Next.js is a widely used React framework for building full-stack web applications. The vulnerability exists in versions prior to 14.2.31 and from 15.0.0 up to but not including 15.4.5. The root cause is improper input validation (CWE-20) in the handling of external image sources. Under certain configurations, an attacker can supply a malicious external image URL that triggers the download of arbitrary files with attacker-controlled content and filenames. This behavior arises because the Image Optimization feature processes external images and can be manipulated to deliver files that are not legitimate images, effectively enabling content injection. The impact of this flaw is primarily the potential for phishing attacks or the delivery of malicious files disguised as images, which could deceive users into executing harmful payloads or disclosing sensitive information. The vulnerability does not directly compromise confidentiality or availability but undermines integrity by allowing unauthorized content injection. Exploitation requires no privileges but does require user interaction (e.g., clicking a link or loading a page). The vulnerability has been addressed in Next.js versions 14.2.31 and 15.4.5, and users are strongly advised to upgrade to these or later versions to mitigate the risk. There are currently no known exploits in the wild, but the ease of exploitation and potential for social engineering make this a relevant threat to web applications using affected Next.js versions.

For European organizations, the vulnerability poses a risk mainly to web-facing applications built with the affected Next.js versions. Attackers could exploit this flaw to deliver malicious files or phishing content to end users, potentially leading to credential theft, malware infections, or reputational damage. Sectors with high reliance on web applications for customer interaction, such as e-commerce, finance, and public services, are particularly at risk. The ability to inject arbitrary content undermines user trust and could facilitate targeted phishing campaigns leveraging the organization's brand. Although the vulnerability does not directly lead to data breaches or system compromise, the indirect consequences of successful phishing or malware delivery can be severe. Additionally, regulatory frameworks like GDPR impose strict requirements on protecting user data and preventing phishing attacks, so failure to address this vulnerability could result in compliance issues and fines.

European organizations should immediately audit their web applications to identify usage of Next.js versions prior to 14.2.31 or between 15.0.0 and 15.4.5. Upgrading to Next.js 14.2.31 or 15.4.5 (or later) is the primary and most effective mitigation. Where immediate upgrades are not feasible, organizations should consider disabling or restricting the Image Optimization feature, especially for external image sources, to prevent exploitation. Implementing strict Content Security Policies (CSP) that limit the domains from which images can be loaded can reduce exposure. Additionally, user education to recognize phishing attempts and suspicious downloads can mitigate the impact of social engineering attacks leveraging this vulnerability. Monitoring web traffic for unusual download patterns and employing web application firewalls (WAFs) with custom rules to detect and block suspicious image requests can provide additional layers of defense.