CVE-2025-1861 is a medium severity vulnerability affecting multiple recent versions of the PHP programming language (8.1.*, 8.2.*, 8.3.*, and 8.4.*). The issue arises from an incorrect calculation of buffer size when PHP parses HTTP redirect responses. Specifically, PHP limits the size of the 'Location' header buffer to 1024 bytes, whereas RFC 9110 recommends supporting up to 8000 bytes for the Location header value. This discrepancy can cause improper truncation of URLs during HTTP redirects, potentially redirecting users to unintended or malicious locations. The vulnerability is classified under CWE-131, which relates to incorrect calculation of buffer size, leading to buffer overflows or truncation errors. Although no known exploits are currently reported in the wild, the flaw could be leveraged by attackers to manipulate redirect behavior, possibly facilitating phishing attacks, session hijacking, or redirecting users to malicious sites without their knowledge. The CVSS 4.0 base score is 6.3, indicating a medium severity level, with an attack vector of network (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), and low impact on confidentiality (VC:L), with no impact on integrity or availability. The vulnerability does not require authentication or user interaction, making it easier to exploit remotely if an attacker can control or influence HTTP redirect responses processed by PHP applications. This issue primarily affects web applications and services built on vulnerable PHP versions that handle HTTP redirects, especially those that rely on the Location header for navigation or redirection logic. The lack of a patch link suggests that fixes may be forthcoming or pending release at the time of this report.

For European organizations, this vulnerability poses a risk primarily to web-facing applications and services that utilize affected PHP versions for HTTP redirect handling. Improper URL truncation can lead to redirection to malicious or unintended websites, potentially exposing users to phishing, malware distribution, or credential theft. This can undermine user trust, damage brand reputation, and lead to regulatory compliance issues under GDPR if personal data is compromised through subsequent attacks. Financial institutions, e-commerce platforms, government portals, and healthcare services in Europe that rely heavily on PHP-based web infrastructure are particularly at risk. The vulnerability could be exploited to bypass security controls or to redirect users to fraudulent sites, facilitating social engineering attacks. Although the direct impact on confidentiality, integrity, and availability is limited, the indirect consequences through user deception and subsequent exploitation can be significant. Given the widespread use of PHP in European web applications, the scope of affected systems is broad, increasing the potential impact across multiple sectors.

European organizations should prioritize upgrading PHP installations to the latest patched versions beyond 8.1.32, 8.2.28, 8.3.19, or 8.4.5 once available. Until patches are applied, organizations should implement strict validation and sanitization of HTTP redirect URLs within their application logic to ensure that truncated or malformed URLs do not lead to unsafe destinations. Web application firewalls (WAFs) can be configured to detect and block suspicious redirect patterns or unusually long Location headers that exceed expected lengths. Security teams should audit existing redirect implementations to identify any reliance on long Location headers and refactor code to handle redirects securely. Monitoring HTTP response headers for anomalies and logging redirect activities can help detect exploitation attempts. Additionally, educating developers about RFC 9110 compliance and secure redirect handling can prevent similar issues in future development. Organizations should also review third-party PHP modules or frameworks for vulnerability exposure and coordinate with vendors for timely updates.