CVE-2025-40927 is a vulnerability in the Perl module CGI::Simple, specifically versions prior to 1.282, which suffers from an HTTP response splitting flaw (CWE-113). This vulnerability arises due to improper neutralization of CRLF (Carriage Return Line Feed) sequences in HTTP headers. Although CGI::Simple attempts some validation, it can be bypassed using URL-encoded newline characters (e.g., %0A), allowing an attacker to inject arbitrary HTTP headers or even HTML/JavaScript content into the server's response. This injection occurs through query parameters that are not properly sanitized. The flaw enables an attacker to break the HTTP response header, insert new headers or body content, and thus execute reflected cross-site scripting (XSS), open redirect attacks, cache poisoning, or header manipulation. This vulnerability is a continuation of an earlier issue documented in CVE-2010-4410, where the previous fix was incomplete, leaving the module still vulnerable. The lack of a CVSS score indicates this is a newly published vulnerability (2025-08-29) with no known exploits in the wild yet, but the technical risk remains significant given the nature of HTTP response splitting and its potential to facilitate multiple attack vectors.

For European organizations, the impact of CVE-2025-40927 can be substantial, especially for those relying on Perl-based web applications or frameworks that incorporate CGI::Simple for HTTP request handling. Successful exploitation can lead to reflected XSS attacks, which may compromise user sessions, steal credentials, or deliver malicious payloads to end users. Open redirect vulnerabilities can facilitate phishing campaigns by redirecting users to malicious sites under the guise of trusted domains. Cache poisoning can degrade the integrity and availability of web content, potentially causing users to receive malicious or outdated data. Header manipulation can interfere with security controls such as Content Security Policy (CSP) or HTTP Strict Transport Security (HSTS), weakening the overall security posture. Given the widespread use of Perl in legacy systems and some web applications across Europe, organizations in sectors such as finance, government, healthcare, and e-commerce could be particularly at risk. The vulnerability's exploitation does not require authentication, increasing the attack surface and risk of automated attacks or exploitation by unauthenticated remote attackers.

European organizations should immediately audit their use of CGI::Simple in Perl applications and determine if they are running versions prior to 1.282. The primary mitigation is to upgrade to the latest patched version of CGI::Simple once available. Until a patch is released, organizations should implement strict input validation and sanitization on all query parameters, specifically filtering out or encoding CRLF characters (%0A, %0D) to prevent header injection. Web application firewalls (WAFs) should be configured to detect and block HTTP requests containing suspicious CRLF sequences or unusual header injection patterns. Additionally, organizations should review HTTP response headers to ensure security headers like CSP, HSTS, and X-Content-Type-Options are properly set and cannot be overridden by injected headers. Logging and monitoring for anomalous HTTP responses or repeated injection attempts can help detect exploitation attempts early. Finally, developers should consider adopting safer HTTP handling libraries that are not vulnerable to response splitting.