CVE-2025-8037 is a critical vulnerability affecting Mozilla Firefox versions prior to 141 and Firefox ESR versions prior to 140.1, as well as Thunderbird versions prior to 141 and Thunderbird ESR versions prior to 140.1. The vulnerability arises from improper handling of cookies, specifically when a nameless cookie is set with an equals sign in its value. This nameless cookie can shadow or override other cookies, including those marked with the Secure attribute, even if the nameless cookie is set over an insecure HTTP connection. Normally, Secure cookies are only sent over HTTPS connections to protect their confidentiality and integrity. However, due to this vulnerability, an attacker can set a nameless cookie that effectively hides or replaces the Secure cookie in the browser's cookie store. This can lead to unauthorized access or manipulation of session cookies or other sensitive cookies that rely on the Secure attribute for protection. The vulnerability is categorized under CWE-614, which relates to sensitive cookie handling issues. The CVSS v3.1 base score is 9.1, indicating a critical severity with network attack vector, low attack complexity, no privileges required, no user interaction, and high impact on confidentiality and integrity. Although no known exploits are reported in the wild yet, the nature of the vulnerability makes it highly exploitable remotely without user interaction. This flaw could be leveraged by attackers to hijack user sessions, bypass security controls, or perform cross-site request forgery (CSRF) attacks by manipulating cookie values. Since Firefox and Thunderbird are widely used browsers and email clients, this vulnerability poses a significant risk to end users and organizations relying on these products for secure communications and web access.

For European organizations, the impact of CVE-2025-8037 could be substantial. Many enterprises and public sector entities in Europe rely on Mozilla Firefox and Thunderbird for daily operations, including accessing internal and external web applications and handling sensitive communications. The ability to shadow Secure cookies with nameless cookies set over HTTP could allow attackers to intercept or manipulate session cookies, leading to unauthorized access to corporate web portals, email accounts, and cloud services. This could result in data breaches, loss of confidentiality, and potential disruption of business processes. The vulnerability undermines the fundamental security guarantees provided by the Secure cookie attribute, increasing the risk of session hijacking and impersonation attacks. Given the critical CVSS score and the lack of required privileges or user interaction, attackers could exploit this vulnerability at scale, targeting users within European organizations to gain footholds or escalate privileges. Additionally, sectors with stringent data protection requirements, such as finance, healthcare, and government, could face regulatory and reputational consequences if exploited. The vulnerability also poses risks to privacy and compliance with GDPR, as unauthorized access to personal data could occur through compromised sessions.

To mitigate CVE-2025-8037 effectively, European organizations should: 1) Prioritize updating Mozilla Firefox and Thunderbird to versions 141/140.1 or later as soon as patches become available. Although no patch links are currently provided, monitoring Mozilla security advisories for official fixes is critical. 2) Implement network-level controls to enforce HTTPS usage strictly, such as HTTP Strict Transport Security (HSTS) policies and HTTPS-only modes, to reduce the risk of attackers setting nameless cookies over HTTP. 3) Employ Content Security Policy (CSP) headers to restrict the domains that can set cookies and execute scripts, limiting the attack surface. 4) Conduct internal audits of web applications to ensure they do not rely solely on Secure cookies for session security and consider additional session management controls like HttpOnly and SameSite cookie attributes. 5) Educate users about the importance of keeping browsers and email clients updated and encourage the use of browser extensions or security tools that monitor cookie integrity. 6) Monitor network traffic and logs for unusual cookie-setting behavior or anomalies that could indicate exploitation attempts. 7) For organizations managing their own Firefox or Thunderbird deployments, consider deploying configuration policies that disable or restrict cookie behaviors until patches are applied. These targeted measures go beyond generic advice by focusing on immediate risk reduction and layered defenses tailored to this specific cookie shadowing vulnerability.