CVE-2026-2790 is a security vulnerability identified in Mozilla Firefox's Networking: JAR component that enables a same-origin policy bypass. The same-origin policy is a fundamental security mechanism in web browsers that restricts how documents or scripts loaded from one origin can interact with resources from another origin. This vulnerability specifically affects Firefox versions earlier than 148 and Firefox ESR versions earlier than 140.8. The flaw resides in the handling of JAR files, which are archives that can contain multiple files and are sometimes used in web applications. Due to improper enforcement of the same-origin policy in this component, an attacker can craft malicious web content that accesses or manipulates data from other origins without authorization. This could lead to unauthorized reading or modification of sensitive information such as cookies, session tokens, or other private data accessible through the browser. Although no public exploits have been reported, the vulnerability poses a significant risk because it undermines a core security boundary in the browser. The absence of a CVSS score suggests that the vulnerability is newly disclosed and under evaluation. The vulnerability's exploitation does not require user authentication but likely requires user interaction such as visiting a malicious website. The scope of affected systems includes all users running vulnerable Firefox versions on desktop or potentially mobile platforms. Given Firefox's widespread use globally, this vulnerability has broad implications for user privacy and security.

The primary impact of CVE-2026-2790 is the compromise of the same-origin policy, which could allow attackers to bypass browser-enforced security boundaries. This can lead to unauthorized access to sensitive user data, including cookies, authentication tokens, and other private information stored or accessible via the browser. For organizations, this could result in data breaches, session hijacking, and unauthorized actions performed on behalf of users. The vulnerability could also facilitate further attacks such as cross-site scripting (XSS) or cross-site request forgery (CSRF) by enabling attackers to interact with multiple origins improperly. The impact extends to any environment where Firefox is used to access sensitive web applications, including corporate networks, government agencies, and critical infrastructure. The lack of known exploits in the wild reduces immediate risk but does not eliminate the threat, as attackers may develop exploits once the vulnerability details become widely known. The potential for widespread exploitation is significant due to Firefox's global user base, making timely patching critical to prevent data loss and maintain trust in web security.

To mitigate CVE-2026-2790, organizations and users should promptly update Mozilla Firefox to version 148 or later, or Firefox ESR to version 140.8 or later, once these versions are released with the security fix. Until patches are available, users should consider disabling or restricting the use of JAR files in the browser environment, if feasible, to reduce attack surface. Employing browser security policies such as Content Security Policy (CSP) can help limit the execution of untrusted scripts and resources. Network-level controls like web filtering and intrusion detection systems should be updated to detect and block suspicious traffic related to JAR file exploitation. Security teams should monitor threat intelligence sources for any emerging exploits and be prepared to respond quickly. Additionally, educating users about the risks of visiting untrusted websites and encouraging cautious browsing behavior can reduce the likelihood of exploitation. Organizations should also review and enhance their incident response plans to address potential browser-based attacks leveraging this vulnerability.