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. It stems from improper input validation (CWE-20) in the handling of external image sources. An attacker can exploit this flaw by supplying maliciously crafted external image URLs that cause the Next.js Image Optimization middleware to trigger file downloads with arbitrary content and filenames. This behavior can be abused to deliver malicious files or conduct phishing attacks by tricking users into downloading harmful payloads disguised as legitimate images. The vulnerability does not affect confidentiality directly but impacts integrity by enabling injection of arbitrary content and potentially user trust. Exploitation requires no privileges and no authentication but does require user interaction (e.g., clicking a link or loading a page with the malicious image). The vulnerability has been addressed in Next.js Middleware 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 no known exploits in the wild at the time of publication, but the ease of exploitation and potential for phishing make it a relevant threat to web applications using vulnerable Next.js versions.

For European organizations, this vulnerability poses a risk primarily to web applications built with the affected Next.js versions that utilize the Image Optimization feature. The impact includes the potential for attackers to deliver malicious files or phishing payloads through trusted web properties, undermining user trust and potentially leading to credential theft, malware infection, or further compromise. Organizations in sectors with high web traffic, such as e-commerce, finance, media, and public services, could see reputational damage and user impact if exploited. While the vulnerability does not directly compromise data confidentiality or availability, the indirect effects through phishing or malware delivery can lead to broader security incidents. Given the widespread adoption of Next.js in Europe, especially among startups and digital service providers, the threat is relevant and should be addressed promptly to prevent abuse.

Specific mitigation steps include: 1) Immediate upgrade of Next.js to versions 14.2.31 or 15.4.5 or later to apply the official patch. 2) Review and restrict the configuration of the Image Optimization feature to limit or validate allowed external image sources, employing allowlists where possible. 3) Implement Content Security Policy (CSP) headers to restrict the domains from which images and other resources can be loaded, reducing the risk of malicious external content. 4) Monitor web application logs for unusual requests or patterns involving image URLs that could indicate exploitation attempts. 5) Educate users and administrators about phishing risks related to file downloads triggered by web content. 6) Employ web application firewalls (WAFs) with rules tuned to detect and block suspicious image URL parameters or payloads targeting this vulnerability. These measures go beyond generic patching by adding layers of defense and detection tailored to the nature of this vulnerability.