CVE-2025-7339 is a vulnerability identified in the 'on-headers' middleware for Node.js, a module used to listen for when HTTP response headers are written. The issue affects versions prior to 1.1.0 of on-headers. The root cause lies in how the middleware handles the response.writeHead() method when an array is passed as an argument instead of the expected object. This misuse can lead to inadvertent modification of response headers, potentially causing unintended behavior in HTTP responses. The vulnerability is classified under CWE-241, which relates to improper handling of data types or unexpected input leading to incorrect program behavior. The CVSS v3.1 base score is 3.4, indicating a low severity level. The vector indicates that exploitation requires local access (AV:L), low attack complexity (AC:L), high privileges (PR:H), no user interaction (UI:N), unchanged scope (S:U), and results in low confidentiality and integrity impact (C:L/I:L) with no availability impact (A:N). There are no known exploits in the wild, and the issue can be mitigated by upgrading to version 1.1.0 of on-headers or by ensuring that response.writeHead() is called with an object rather than an array. This vulnerability primarily affects server-side JavaScript applications using the on-headers middleware, which is common in web servers and API backends built on Node.js. While the impact on confidentiality and integrity is low, the unintended header modifications could lead to subtle bugs or security misconfigurations in HTTP responses, such as incorrect caching headers or security headers, potentially weakening security postures indirectly.

For European organizations, the impact of CVE-2025-7339 is generally low but not negligible. Many European enterprises and service providers rely on Node.js for backend services, including web applications and APIs. If these applications use the on-headers middleware and pass arrays to response.writeHead(), they risk unintended header modifications that could lead to security misconfigurations or functional issues. This could indirectly expose sensitive information or weaken security controls such as Content Security Policy (CSP), CORS headers, or authentication-related headers. While the vulnerability does not directly allow remote code execution or denial of service, the subtle integrity and confidentiality impacts could be exploited in complex attack chains or combined with other vulnerabilities. Organizations handling sensitive data, such as financial institutions, healthcare providers, or government services, should be particularly cautious. The requirement for high privileges to exploit reduces the risk from external attackers but raises concerns about insider threats or compromised privileged accounts. Overall, the threat is more relevant in environments where strict HTTP header control is critical for security compliance or data protection under regulations like GDPR.

To mitigate CVE-2025-7339, European organizations should take the following specific actions: 1) Audit all Node.js applications and services to identify usage of the on-headers middleware, especially versions prior to 1.1.0. 2) Upgrade on-headers to version 1.1.0 or later, which contains the patch addressing this vulnerability. 3) Review application code to ensure that response.writeHead() is never called with an array argument; refactor code to always use an object for headers. 4) Implement automated code scanning or static analysis tools to detect improper usage patterns of response.writeHead() and middleware versions. 5) Conduct thorough testing of HTTP response headers after upgrades or code changes to verify that headers are correctly set and no unintended modifications occur. 6) Enforce strict access controls and monitoring on systems with privileged accounts to reduce the risk of exploitation requiring high privileges. 7) Incorporate this vulnerability into the organization's vulnerability management and patching processes to ensure timely updates. These steps go beyond generic advice by focusing on code-level review, automated detection, and privileged access management tailored to this specific vulnerability.