CVE-2025-1861 is a medium-severity vulnerability affecting multiple recent versions of PHP (8.1.*, 8.2.*, 8.3.*, and 8.4.*) related to the incorrect calculation of buffer size when parsing HTTP redirect responses. Specifically, PHP limits the size of the 'Location' header value to 1024 bytes, whereas RFC 9110 recommends supporting up to 8000 bytes. This discrepancy can lead to truncation of the URL in the Location header during HTTP redirects, causing the application to redirect users to an incorrect or unintended location. The root cause is classified under CWE-131, which involves incorrect calculation of buffer size, potentially leading to buffer overflows or data truncation issues. Although this vulnerability does not appear to allow direct code execution or privilege escalation, the incorrect redirect behavior can be exploited to redirect users to malicious sites, facilitating phishing attacks, session hijacking, or other social engineering exploits. The vulnerability is exploitable remotely without authentication or user interaction, as it occurs during automatic HTTP redirect handling. The CVSS 4.0 base score is 6.3 (medium), reflecting network attack vector, low complexity, no privileges required, no user interaction, and limited confidentiality impact. No known exploits are currently reported in the wild, and no official patches have been linked yet, indicating the need for monitoring and timely updates once available.

For European organizations, this vulnerability poses a risk primarily to web applications and services built on affected PHP versions that handle HTTP redirects. The incorrect truncation of redirect URLs can lead to users being redirected to malicious or unintended destinations, increasing the risk of phishing, credential theft, or malware delivery. This can undermine user trust and potentially lead to data breaches if attackers exploit the redirect to capture sensitive information. Organizations relying heavily on PHP-based web infrastructure, including e-commerce, government portals, and financial services, may face reputational damage and regulatory scrutiny under GDPR if user data is compromised. Although the vulnerability does not directly compromise system integrity or availability, the indirect effects of successful phishing or redirection attacks can be significant. The lack of required authentication or user interaction for exploitation increases the attack surface, making automated scanning and exploitation feasible by attackers. Given the widespread use of PHP in Europe, especially in SMEs and public sector web services, the impact could be broad if not mitigated promptly.

Monitor official PHP Group announcements and security advisories for patches addressing CVE-2025-1861 and apply updates promptly once available. Implement strict validation and sanitization of redirect URLs within application logic to ensure redirects only occur to trusted domains or paths, mitigating risks from truncated or malformed Location headers. Use web application firewalls (WAFs) configured to detect and block suspicious or unusually long redirect URLs that exceed expected lengths or patterns. Conduct security testing and code reviews focusing on HTTP redirect handling to identify and remediate any custom implementations that might be affected by this buffer size limitation. Where feasible, limit the use of automatic HTTP redirects or replace them with explicit user confirmations to reduce the risk of unintended redirections. Maintain comprehensive logging and monitoring of redirect events to detect anomalous redirect patterns that could indicate exploitation attempts. Educate users and administrators about the risks of phishing via malicious redirects and encourage vigilance when encountering unexpected redirects.