CVE-2025-4919 is a vulnerability in Mozilla Firefox and Thunderbird stemming from an out-of-bounds (OOB) memory access triggered during the optimization of linear sums in JavaScript engine internals. Specifically, the flaw arises when an attacker manipulates array index sizes to confuse the engine’s bounds checking, leading to either an out-of-bounds read or write on JavaScript objects. This type of vulnerability is classified under CWE-125 (Out-of-bounds Read) and CWE-787 (Out-of-bounds Write), both of which can lead to memory corruption. The vulnerability affects Firefox versions earlier than 138.0.4, Firefox ESR versions earlier than 128.10.1 and 115.23.1, and Thunderbird versions earlier than 128.10.2 and 138.0.2. The CVSS 3.1 base score is 8.8, indicating a high severity with the vector AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H, meaning it is remotely exploitable over the network with low attack complexity, no privileges required, but requires user interaction (such as visiting a malicious website). The impact includes full compromise of confidentiality, integrity, and availability, potentially allowing arbitrary code execution or sandbox escape. No public exploits are known at this time, but the vulnerability is published and enriched by CISA, signaling its seriousness. The absence of patch links suggests that fixes are either newly released or imminent. The vulnerability’s root cause lies in the JavaScript engine’s failure to correctly validate array indices during optimization passes, a critical flaw in memory safety.

For European organizations, the impact of CVE-2025-4919 is significant due to the widespread use of Mozilla Firefox and Thunderbird in both enterprise and government environments. Successful exploitation can lead to arbitrary code execution within the browser context, enabling attackers to steal sensitive data, deploy malware, or move laterally within networks. The vulnerability compromises confidentiality, integrity, and availability, posing risks to personal data protection under GDPR and potentially disrupting critical services. Organizations relying on Firefox for secure communications or Thunderbird for email may face data breaches or espionage attempts. The requirement for user interaction means phishing or malicious web content delivery is a likely attack vector, which is common in targeted attacks against European financial institutions, government agencies, and critical infrastructure. The vulnerability’s high severity and ease of exploitation increase the urgency for mitigation to prevent potential large-scale compromise or espionage campaigns targeting European entities.

1. Immediate upgrade to the latest patched versions of Firefox (≥138.0.4), Firefox ESR (≥128.10.1 and 115.23.1), and Thunderbird (≥128.10.2 and 138.0.2) as soon as they become available. 2. Until patches are applied, disable JavaScript execution in untrusted or high-risk browsing contexts using browser settings or extensions to reduce attack surface. 3. Employ network-level protections such as web filtering and intrusion prevention systems to block access to known malicious sites and suspicious payloads. 4. Educate users on phishing risks and the dangers of interacting with untrusted web content to reduce the likelihood of user interaction exploitation. 5. Use browser sandboxing and process isolation features to limit the impact of a successful exploit. 6. Monitor browser and email client logs for unusual behavior or crashes that may indicate exploitation attempts. 7. Coordinate with IT asset management to identify and prioritize vulnerable endpoints for patching, especially those in sensitive roles or handling critical data. 8. Consider deploying endpoint detection and response (EDR) solutions capable of detecting memory corruption exploits and anomalous process behavior related to browser exploitation.