CVE-2025-27820 is a high-severity vulnerability affecting Apache HttpClient version 5.4.0, part of the Apache HttpComponents project maintained by the Apache Software Foundation. The vulnerability arises from a flaw in the Public Suffix List (PSL) validation logic, which is responsible for correctly identifying domain boundaries to enforce security policies such as cookie management and hostname verification. Specifically, the bug disables domain checks, allowing an attacker to bypass PSL validation. This can lead to improper handling of cookies and failure to verify hostnames correctly during HTTPS connections. The consequence is a potential integrity compromise where an attacker could manipulate cookies or spoof hostnames, possibly enabling session hijacking or man-in-the-middle attacks without triggering expected security controls. The vulnerability does not impact confidentiality directly and does not affect availability. It requires no privileges or user interaction to exploit and can be triggered remotely over the network. The issue was discovered by the Apache HttpClient team and fixed in version 5.4.3. The CVSS v3.1 score is 7.5, reflecting a high severity due to the ease of exploitation and impact on integrity. The weakness is categorized under CWE-295, which relates to improper certificate validation, underscoring the risk of trust boundary violations in TLS connections.

For European organizations, this vulnerability poses a significant risk, especially for those relying on Apache HttpClient 5.4.0 in their web applications, microservices, or backend systems that perform HTTP communications. The compromised PSL validation can allow attackers to bypass domain restrictions, leading to cookie theft or session fixation attacks, which can result in unauthorized access to sensitive data or systems. This is particularly critical for sectors handling personal data under GDPR, such as finance, healthcare, and e-commerce, where session integrity is paramount. Additionally, hostname verification bypass can facilitate man-in-the-middle attacks, undermining the trustworthiness of encrypted communications. The impact is heightened in environments where Apache HttpClient is used in automated workflows or service-to-service communications, as attackers could impersonate legitimate services. Although no known exploits are currently reported in the wild, the vulnerability's nature and ease of exploitation warrant immediate attention to prevent potential targeted attacks against European enterprises.

European organizations should promptly upgrade Apache HttpClient to version 5.4.3 or later, where the PSL validation bug is fixed. Until the upgrade can be applied, organizations should implement strict network-level controls to limit exposure of vulnerable services, such as firewall rules restricting outbound HTTP connections to trusted domains only. Application-level mitigations include enforcing additional hostname verification logic outside of HttpClient, such as custom validation callbacks or using alternative HTTP client libraries with robust domain validation. Security teams should audit their codebases and dependencies to identify usage of Apache HttpClient 5.4.0 and assess the risk exposure. Monitoring network traffic for unusual cookie or hostname anomalies can help detect exploitation attempts. Finally, organizations should ensure their incident response plans include scenarios involving session hijacking and man-in-the-middle attacks to respond swiftly if exploitation occurs.