CVE-2025-6424 identifies a use-after-free vulnerability within the FontFaceSet implementation of Mozilla Firefox and Thunderbird. Use-after-free (CWE-416) vulnerabilities occur when a program continues to use memory after it has been freed, potentially leading to memory corruption, crashes, or arbitrary code execution. In this case, the flaw allows an attacker to trigger a crash that can be exploited to execute arbitrary code remotely without any user interaction or privileges, as indicated by the CVSS vector (AV:N/AC:L/PR:N/UI:N). The vulnerability affects Firefox versions earlier than 140, Firefox ESR versions earlier than 115.25 and 128.12, as well as Thunderbird versions earlier than 140 and 128.12. The critical CVSS score of 9.8 reflects the high impact on confidentiality, integrity, and availability, with an attacker able to fully compromise affected systems remotely. Although no exploits have been reported in the wild yet, the nature of the vulnerability and its ease of exploitation make it a significant threat. The vulnerability was reserved and published in June 2025, but no official patches or exploit code links are currently available. This vulnerability is particularly dangerous because it does not require any user interaction or authentication, increasing the risk of widespread exploitation. The flaw resides in the handling of font face sets, a component that processes font data for rendering web content, which is a common operation in browsers and email clients like Thunderbird. Successful exploitation could allow attackers to execute arbitrary code, potentially leading to full system compromise, data theft, or disruption of services.

For European organizations, the impact of CVE-2025-6424 is substantial due to the widespread use of Firefox and Thunderbird in both public and private sectors. Exploitation could lead to unauthorized access to sensitive information, disruption of critical services, and potential lateral movement within networks. Sectors such as government, finance, healthcare, and critical infrastructure are particularly at risk due to the high value of their data and services. The vulnerability’s ability to be exploited remotely without user interaction increases the likelihood of automated attacks and wormable scenarios, potentially causing large-scale disruptions. Additionally, organizations relying on ESR versions for stability and long-term support may face delayed patching cycles, prolonging exposure. The compromise of email clients like Thunderbird could lead to interception or manipulation of sensitive communications. Overall, the vulnerability threatens confidentiality, integrity, and availability of systems, posing a severe risk to European digital infrastructure and data privacy compliance obligations such as GDPR.

1. Immediate patching: Organizations should prioritize updating Firefox and Thunderbird to versions 140 or later, or ESR versions 115.25 and 128.12 or later, as soon as patches are released. 2. Temporary workarounds: Until patches are available, consider disabling or restricting font-related features or sandboxing browser processes to limit exploitation impact. 3. Network controls: Implement network-level protections such as web filtering, intrusion detection/prevention systems (IDS/IPS), and firewall rules to block or monitor suspicious traffic targeting browsers. 4. Endpoint protection: Deploy advanced endpoint detection and response (EDR) solutions capable of detecting exploitation attempts and anomalous behavior related to memory corruption. 5. User awareness: Although no user interaction is required, educating users about the risks of visiting untrusted websites can reduce exposure. 6. Vulnerability management: Maintain an up-to-date inventory of affected software versions and monitor vendor advisories for patch releases. 7. Incident response readiness: Prepare for potential exploitation by having incident response plans and forensic capabilities in place to quickly identify and mitigate attacks. 8. Consider browser alternatives or hardened configurations in high-risk environments until patches are confirmed effective.