CVE-2025-1010 is a use-after-free vulnerability identified in the Custom Highlight API of Mozilla Firefox and Thunderbird. Use-after-free (CWE-416) occurs when a program continues to use a pointer after the memory it points to has been freed, leading to undefined behavior including crashes or arbitrary code execution. This vulnerability affects Firefox versions earlier than 135, Firefox ESR versions earlier than 115.20 and 128.7, and Thunderbird versions earlier than 128.7 and 135. The flaw can be triggered remotely without any authentication or user interaction, as it involves the Custom Highlight API which processes user or web content inputs. Exploiting this vulnerability could allow an attacker to execute arbitrary code, escalate privileges, or cause denial of service by crashing the application. The CVSS v3.1 score of 9.8 reflects its critical nature with network attack vector, low attack complexity, no privileges required, no user interaction, and high impact on confidentiality, integrity, and availability. Although no exploits have been reported in the wild yet, the severity and ease of exploitation make it a significant threat. Mozilla has published the vulnerability details but patch links are not yet available, indicating the need for close monitoring of updates. Organizations using affected versions should prepare for immediate patch deployment once available. The vulnerability is particularly dangerous because Firefox and Thunderbird are widely used in enterprise and government environments for web browsing and email communication, making this a vector for remote compromise.

For European organizations, the impact of CVE-2025-1010 can be severe. Successful exploitation could lead to remote code execution, allowing attackers to gain control over affected systems, steal sensitive data, or disrupt operations. The vulnerability compromises confidentiality, integrity, and availability of systems running vulnerable Firefox or Thunderbird versions. This is especially critical for sectors such as finance, government, healthcare, and critical infrastructure where these applications are commonly used. The lack of required privileges or user interaction lowers the barrier for attackers, increasing the risk of widespread exploitation. Additionally, organizations relying on Firefox ESR versions for stability and long-term support may face delays in patching, prolonging exposure. The potential for denial of service through crashes could disrupt business continuity and user productivity. Given the widespread use of Firefox and Thunderbird across Europe, the threat could affect a broad range of organizations, from small businesses to large enterprises and public sector entities.

1. Immediately inventory all Firefox and Thunderbird installations to identify versions prior to Firefox 135, Firefox ESR 115.20 and 128.7, and Thunderbird 128.7 and 135. 2. Prioritize upgrading or patching affected applications as soon as Mozilla releases official patches. 3. Until patches are available, consider applying temporary mitigations such as disabling the Custom Highlight API if feasible or restricting access to untrusted web content. 4. Employ network-level protections such as web filtering and intrusion prevention systems to block or detect exploit attempts targeting this vulnerability. 5. Monitor endpoint logs and network traffic for unusual crashes or suspicious activity related to Firefox or Thunderbird processes. 6. Educate users about the risk and encourage cautious browsing behavior, especially avoiding untrusted or malicious websites. 7. Maintain up-to-date backups and incident response plans to quickly recover from potential exploitation. 8. Coordinate with IT and security teams to ensure rapid deployment of patches and continuous vulnerability management. 9. Engage with Mozilla security advisories and trusted threat intelligence sources for updates on exploit availability and mitigation guidance.