CVE-2025-0240 is a use-after-free vulnerability identified in Mozilla Firefox and Thunderbird prior to versions 134 and 128.6 ESR, respectively. The root cause is a compartment mismatch during the parsing of JavaScript JSON modules. Specifically, when a JavaScript module is parsed as JSON, the compartment or execution context boundaries are not properly enforced, leading to cross-compartment access. This improper handling can cause a use-after-free condition, where memory that has been freed is accessed again, potentially leading to memory corruption. Such memory corruption can be exploited to alter the integrity of the application’s execution, possibly enabling attackers to execute arbitrary code or cause application crashes. The vulnerability does not affect confidentiality or availability directly and does not require user interaction or elevated privileges to exploit, but it requires local access to the vulnerable application instance. The CVSS v3.1 score is 4.0, reflecting low attack vector (local), low complexity, no privileges required, no user interaction, unchanged scope, no confidentiality impact, limited integrity impact, and no availability impact. No public exploits have been reported yet, and no patches are linked at the time of this report. The vulnerability is classified under CWE-416 (Use After Free), a common memory corruption issue in software development.

For European organizations, the primary impact of CVE-2025-0240 lies in the potential compromise of application integrity within Firefox and Thunderbird clients. Since these applications are widely used for web browsing and email communication, exploitation could lead to targeted attacks that corrupt memory, potentially allowing attackers to execute arbitrary code or disrupt normal application behavior. This could result in unauthorized actions within the context of the affected application, such as executing malicious scripts or manipulating data handled by the browser or email client. However, the attack requires local access, limiting remote exploitation risks. Organizations with stringent security policies and endpoint protections may mitigate the risk, but those with less controlled environments or where users have local access to vulnerable versions remain exposed. The vulnerability could be leveraged in targeted attacks against high-value users or systems, especially in sectors like finance, government, or critical infrastructure where Firefox and Thunderbird are prevalent. The lack of confidentiality and availability impact reduces the risk of data leakage or denial of service but does not eliminate the threat of integrity compromise.

To mitigate CVE-2025-0240, European organizations should prioritize updating Mozilla Firefox and Thunderbird to versions 134 and 128.6 ESR or later once patches are released. Until updates are available, organizations should restrict local access to systems running vulnerable versions, limiting user permissions and enforcing strict endpoint security controls. Employ application whitelisting and behavior monitoring to detect anomalous activity that might indicate exploitation attempts. Disable or restrict the use of JavaScript JSON modules if feasible, or configure browser security settings to limit exposure to untrusted content. Regularly audit installed software versions across the enterprise to ensure compliance with security policies. Additionally, educate users about the risks of running outdated software and encourage prompt updates. Network segmentation can help contain potential exploitation impacts by isolating vulnerable endpoints. Finally, monitor security advisories from Mozilla and threat intelligence feeds for updates on exploit availability and patch releases.