CVE-2025-5266 is a vulnerability identified in Mozilla Firefox and Thunderbird browsers prior to versions 139 and ESR 128.11. The issue arises from how script elements that load cross-origin resources generate load and error events, which can be observed by an attacker to leak information. This leakage enables cross-site leaks (XS-Leaks) attacks, a class of side-channel attacks where an attacker can infer sensitive information about a user's browsing context or data by monitoring browser behavior and event timing related to cross-origin resource loading. The vulnerability specifically allows attackers to detect whether certain resources exist or have loaded successfully by exploiting the generation of these events, which can reveal confidential information without compromising the integrity or availability of the system. The CVSS score of 4.3 reflects a medium severity, indicating that the attack vector is network-based with low attack complexity, no privileges required, but user interaction is necessary. The vulnerability affects all Firefox versions below 139 and Thunderbird versions below 139 and ESR versions below 128.11. No patches or exploits are currently reported, but the vulnerability is publicly disclosed and should be addressed promptly. The underlying weakness corresponds to CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor).

For European organizations, this vulnerability poses a risk primarily to confidentiality. Attackers could leverage XS-Leaks to infer sensitive information such as user activity, presence of specific resources, or other data that could aid in further attacks or espionage. Organizations relying on Firefox or Thunderbird for communication or web access may inadvertently expose sensitive internal or user data. While the vulnerability does not affect system integrity or availability, the leakage of confidential information can have serious consequences, including data privacy violations under GDPR, reputational damage, and potential regulatory penalties. The impact is heightened for sectors handling sensitive or classified information, such as government agencies, financial institutions, and critical infrastructure operators. Since exploitation requires user interaction, phishing or social engineering campaigns could be used to trigger the attack. The lack of known exploits in the wild currently reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits following public disclosure.

The primary mitigation is to update Mozilla Firefox and Thunderbird to versions 139 or later, or ESR 128.11 or later, as soon as these patches become available. Until updates are applied, organizations should implement strict Content Security Policies (CSP) to restrict cross-origin resource loading and reduce the attack surface for XS-Leaks. Disabling or limiting the execution of untrusted scripts and cross-origin resource requests can help mitigate information leakage. Educating users to avoid interacting with suspicious links or sites can reduce the risk of triggering the vulnerability. Network-level controls such as web filtering and monitoring for anomalous cross-origin requests may provide additional protection. Security teams should monitor Mozilla advisories for patch releases and any emerging exploit reports. Finally, reviewing and minimizing the exposure of sensitive information in web resources can reduce the potential impact of XS-Leaks.