CVE-2025-1736 is a medium-severity vulnerability affecting multiple recent versions of PHP (8.1.* before 8.1.32, 8.2.* before 8.2.28, 8.3.* before 8.3.19, and 8.4.* before 8.4.5). The vulnerability stems from improper input validation (CWE-20) related to user-supplied HTTP headers. Specifically, PHP does not sufficiently validate end-of-line (EOL) characters in headers sent by users. This can lead to two main issues: certain headers may be blocked from being sent, or headers may be misinterpreted by the server or downstream components. The improper handling of EOL characters can cause header injection or header splitting scenarios, which can be exploited to manipulate HTTP responses or requests. Although no known exploits are currently reported in the wild, the vulnerability has a CVSS 4.0 base score of 6.3, indicating a medium severity level. The attack vector is network-based (AV:N), but requires high attack complexity (AC:H) and partial attack prerequisites (AT:P), with no privileges or user interaction needed. The impact is limited to confidentiality (VC:L) with no integrity or availability impact. This vulnerability affects PHP installations that handle user-supplied headers, which is common in web applications and APIs. Improper header validation can lead to security issues such as HTTP response splitting, cache poisoning, or bypassing security controls relying on headers. The vulnerability is present in core PHP versions widely used in web hosting environments, CMS platforms, and custom web applications.

For European organizations, this vulnerability poses a moderate risk primarily to web-facing applications and services running vulnerable PHP versions. Exploitation could allow attackers to manipulate HTTP headers, potentially enabling attacks like HTTP response splitting, web cache poisoning, or cross-site scripting (XSS) via header injection. These attacks can compromise confidentiality by leaking sensitive information or bypassing security controls such as Content Security Policy (CSP) or authentication mechanisms relying on headers. While the vulnerability does not directly impact integrity or availability, the indirect effects of successful exploitation could lead to data exposure or session hijacking. Given the widespread use of PHP in Europe, especially in sectors like e-commerce, government portals, and financial services, the vulnerability could affect a broad range of organizations. However, the high attack complexity and partial prerequisites reduce the likelihood of widespread exploitation. Organizations relying on PHP-based CMS platforms (e.g., WordPress, Drupal) or custom PHP applications should be particularly vigilant. The absence of known exploits in the wild suggests that immediate risk is moderate but patching is advised to prevent future exploitation.

European organizations should prioritize updating PHP to the fixed versions: 8.1.32 or later, 8.2.28 or later, 8.3.19 or later, and 8.4.5 or later. Since no official patch links are provided in the data, organizations should monitor the PHP Group's official releases and security advisories for updates. In the interim, developers should audit and sanitize all user-supplied headers rigorously, ensuring that EOL characters (CR, LF) are properly validated and escaped or removed before being processed or sent. Web application firewalls (WAFs) can be configured to detect and block suspicious header injection patterns. Additionally, organizations should review HTTP header handling logic in custom PHP code and third-party libraries to ensure compliance with secure coding practices. Logging and monitoring HTTP traffic for anomalous header patterns can help detect attempted exploitation. Finally, organizations should conduct penetration testing focused on HTTP header injection and response splitting to identify vulnerable endpoints.