CVE-2025-10529 is a medium-severity vulnerability identified in Mozilla Firefox and Thunderbird products prior to versions 143 and ESR versions prior to 140.3. The vulnerability is classified as a Same-Origin Policy (SOP) bypass affecting the Layout component of these applications. The Same-Origin Policy is a critical security mechanism implemented in web browsers to restrict how documents or scripts loaded from one origin can interact with resources from another origin. A bypass of this policy can allow malicious web content to access or manipulate data from other origins, potentially leading to information disclosure or integrity violations. This vulnerability is tracked under CWE-942, which relates to improper enforcement of restrictions on web-origin interactions. The CVSS v3.1 base score is 6.5, indicating a medium severity level, with the vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N. This means the vulnerability can be exploited remotely over the network without any privileges or user interaction, with low attack complexity. The impact affects confidentiality and integrity but does not affect availability. No known exploits are reported in the wild as of the published date (September 16, 2025), and no patches or mitigation links are provided yet. The vulnerability affects Firefox and Thunderbird versions before 143 and ESR versions before 140.3, but exact affected versions are unspecified. The vulnerability could allow an attacker to bypass SOP restrictions via the Layout component, potentially enabling unauthorized access to sensitive data or manipulation of content across origins within the browser environment.

For European organizations, this vulnerability poses a significant risk due to the widespread use of Mozilla Firefox and Thunderbird as primary web browsers and email clients in both private and enterprise environments. The SOP bypass could allow attackers to steal sensitive information such as session tokens, personal data, or confidential business information by circumventing browser security boundaries. This could lead to data breaches, unauthorized data manipulation, or further exploitation through chained attacks. Organizations handling sensitive or regulated data (e.g., GDPR-protected personal data) could face compliance violations and reputational damage if exploited. The lack of required user interaction and the ability to exploit remotely increase the risk of automated or large-scale attacks. However, the absence of known exploits in the wild and the medium severity rating suggest that immediate widespread exploitation is not confirmed but remains a credible threat. The impact on email clients (Thunderbird) also raises concerns about potential compromise of email confidentiality and integrity, which is critical for secure communications in European businesses and government entities.

1. Immediate upgrade to Mozilla Firefox version 143 or later and Thunderbird version 143 or later, or ESR versions 140.3 or later, as soon as official patches or updates are released by Mozilla. 2. Until patches are available, consider deploying network-level controls such as web filtering and intrusion detection systems to monitor and block suspicious web traffic that could exploit this vulnerability. 3. Implement Content Security Policy (CSP) headers on internal web applications to restrict cross-origin resource access and reduce the risk of SOP bypass exploitation. 4. Educate users about the risks of visiting untrusted websites and encourage cautious browsing behavior, especially avoiding suspicious links or sites. 5. For organizations using Firefox or Thunderbird in managed environments, use centralized update management tools to ensure timely deployment of security updates. 6. Monitor Mozilla security advisories and CVE databases for updates or exploit reports related to CVE-2025-10529 to adjust defenses accordingly. 7. Consider sandboxing or isolating critical browser sessions to limit the impact of potential SOP bypass exploitation.